1. C Programming

1.1 What is C?

  • C is a programming language developed at AT & T’s Bell Laboratories of USA in 1972 by Dennis Ritchie.
  • Any programming Language can be divided in to two categories.

    • Problem oriented (High level language)
    • Machine oriented (Low level language)

    But C is considered as a Middle level Language.

  • C is modular, portable, reusable.

1.2 Feature of C Program

  • Structured language
    • It has the ability to divide and hide all the information and instruction.
    • Code can be partitioned in C using functions or code block.
    • C is a well structured language compare to other.
  • General purpose language
    • Make it ideal language for system programming.
    • It can also be used for business and scientific application.
    • ANSI established a standard for c in 1983.
    • The ability of c is to manipulate bits,byte and addresses.
    • It is adopted in later 1990.
  • Portability
    • Portability is the ability to port or use the software written .
    • One computer C program can be reused.
    • By modification or no modification.
  • Code Re-usability & Ability to customize and extend
    • A programmer can easily create his own function
    • It can can be used repeatedly in different application
    • C program basically collection of function
    • The function are supported by 'c' library
    • Function can be added to 'c' library continuously
  • Limited Number of Key Word
    • There are only 32 keywords in 'C'
    • 27 keywords are given by ritchie
    • 5 is added by ANSI
    • The strength of 'C' is lies in its in-built function
    • Unix system provides as large number of C function
    • Some function are used in operation .
    • Other are for specialized in their application

1.3 C program structure

pre-processor directives  
global declarations

main()  
{
    local variable deceleration
    statement sequences
    function invoking
}

1.4 C Keywords

Keywords are the words whose meaning has already been explained to the C compiler. There are only 32 keywords available in C. The keywords are also called ‘Reserved words’.

auto        double      int         struct  
break       else        long        switch  
case        enum        register    typedef  
char        extern      return      union  
const       float       short       unsigned  
continue    for         signed      void  
default     goto        sizeof      volatile  
do          if          static      while  

1.5 C Character Set

A character denotes any alphabet, digit or special symbol used to represent information. Following are the valid alphabets, numbers and special symbols allowed in C.
* Alphabets - A, B, ….., Y, Z a, b, ……, y, z * Digits - 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 * Special symbols - ~ ‘ ! @ # % ^ & * ( ) _ - + = | \ { } [ ] : ; " ' < > , . ? /

1.6 Rules for Writing, Compiling and Executing the C program

  • C is case sensitive means variable named "COUNTER" is different from a variable named "counter".
  • All keywords are lowercased.
  • Keywords cannot be used for any other purpose (like variable names).
  • Every C statement must end with a ;. Thus ;acts as a statement terminator.
  • First character must be an alphabet or underscore, no special symbol other than an underscore, no commas or blank spaces are allowed with in a variable, constant or keyword.
  • Blank spaces may be inserted between two words to improve the readability of the statement. However, no blank spaces are allowed within a variable, constant or keyword.
  • Variable must be declared before it is used in the program.
  • File should be have the extension .c
  • Program need to be compiled before execution.

1.7 Data types & Placeholders

  • C has 5 basic built-in data types.
  • Data type defines a set of values that a variable can store along with a set of operations that can be performed on it.
  • A variable takes different values at different times.
  • General form for declaring a variable is: type name;
  • An example for using variables comes below:
#include<stdio.h> 
main()  
{ 
    int sum; 
    sum=12; 
    sum=sum+5; 
    printf("Sum is %d",sum); 
}

printf function will print the following: Sum is 17 In fact %d is the placeholder for integer variable value that its name comes after double quotes.
* Common data types are: * int - integer * char - character * long - long integer * float - float number * double - long float * Other placeholders are:

Placeholders        Format  
%c                  Character
%d                  Signed decimal integer
%i                  Signed decimal integer
%e                  Scientific notation[e]
%E                  Scientific notation[E]
%f                  Decimal floating point
%o                  unsigned octal
%s                  String of character
%u                  unsigned decimal integer
%x                  unsigned Hexadecimal (lower)
%X                  unsigned Hexadecimal (upper)
%p                  dispaly a pointer
%%                  print a %

1.8 Control characters (Escape sequences)

Certain non printing characters as well as the backslash () and the apostrophe('), can be expressed in terms of escape sequence.
* \a - Bell * \n - New line
* \r - Carriage return * \b - Backspace
* \f - Formfeed * \t - Horizontal tab * \" - Quotation mark
* \v - Vertical tab
* \' - Apostrophe * \\ - Backslash * \? - Question mark * \0 - Null

1.9 Receiving input values from keyboard

scanf function used to receiving input from keyboard.
General form of scanf function is :

scanf("Format string",&variable,&variable,...); 

Format string contains placeholders for variables that we intend to receive from keyboard. A & sign comes before each variable name that comes in variable listing. Character strings are exceptions from this rule. They will not come with this sign before them.

Note: You are not allowed to insert any additional characters in format string other than placeholders and some special characters. Entering even a space or other undesired character will cause your program to work incorrectly and the results will be unexpected. So make sure you just insert placeholder characters in scanf format string. The following example receives multiple variables from keyboard.

float a;  
int n;  
scanf("%d%f",&n,&a);  

Pay attention that scanf function has no error checking capabilities built in it. Programmer is responsible for validating input data (type, range etc.) and preventing errors



2. Expression & Operators Precedence

Following table summarizes the rules for precedence and associativity of all operators, including those that we have not yet discussed. Operators on the same line have the same precedence; rows are in order of decreasing precedence, so, for example, *, /, and % all have the same precedence, which is higher than that of binary + and -. The ``operator'' () refers to function call. The operators -> and . are used to access members of structures;

Description Operators Associativity
Function Expression () Left to Right
Array Expression [] Left to Right
Structure Operator -> Left to Right
Structure Operator . Left to Right
Unary minus - Right to Left
Increment/Decrement ++, -- Right to Left
One’s compliment ~ Right to Left
Negation ! Right to Left
Address of & Right to Left
Value of address `*` Right to Left
Type cast (type) Right to Left
Size in bytes sizeof Right to Left
Multiplication `*` Left to Right
Division / Left to Right
Modulus % Left to Right
Addition + Left to Right
Subtraction - Left to Right
Left shift << Left to Right
Right shift >> Left to Right
Less than < Left to Right
Less than or equal to <= Left to Right
Greater than > Left to Right
Greater than or equal to >= Left to Right
Equal to == Left to Right
Not equal to != Left to Right
Bitwise AND & Left to Right
Bitwise exclusive OR ^ Left to Right
Bitwise inclusive OR | Left to Right
Logical AND && Left to Right
Logical OR || Left to Right
Conditional ?: Right to Left
Assignment =, *=, /=, %=, +=, -=, &=, ^=, |=, <<=, >>= Right to Left
Comma , Right to Left

Unary & +, -, and * have higher precedence than the binary forms.



3. The Decision Control Structure

C has three major decision making instructions—the if statement, the if-else statement, and the switch statement.

3.1 The if Statement

C uses the keyword if to implement the decision control instruction. The general form of if statement looks like this:

//for single statement
if(condition)  
    statement;

//for multiple statement
if(condition)  
{ 
    block of statement; 
}

The more general form is as follows:

//for single statement
if(expression)  
    statement;

//for multiple statement
if(expression)  
{ 
    block of statement; 
}

Here the expression can be any valid expression including a relational expression. We can even use arithmetic expressions in the ifstatement. For example all the following if statements are valid

if (3 + 2 % 5)  
    printf("This works");

The expression (3 + 2 % 5) evaluates to 5 and since 5 is non-zero it is considered to be true. Hence the printf("This works"); gets executed.

3.2 The if-else Statement

The if statement by itself will execute a single statement, or a group of statements, when the expression following if evaluates to true. It does nothing when the expression evaluates to false. Can we execute one group of statements if the expression evaluates to true and another group of statements ifthe expression evaluates to false? Of course! This is what is the purpose of the else statement that is demonstrated as

if (expression)  
{ 
    block of statement;
}
else  
    statement;

Note * The group of statements after the if upto and not including the else is called an if block. Similarly,the statements after the else form the else block. * Notice that the else is written exactly below the if. The statements in the if block and those in the else block have been indented to the right. * Had there been only one statement to be executed in the if block and only one statement in the else block we could have dropped the pair of braces. * As with the if statement, the default scope of else is also the statement immediately after the else. To override this default scope a pair of braces as shown in the above "Multiple Statements within if" must be used.

3.3 Nested if-elses

If we write an entire if-else construct within either the body of the if statement or the body of an else statement. This is called "nesting" of ifs. This is demonstrated as -

if (expression1)  
    statement;
else  
{
    if (expression2)
        statement;  
    else
    { 
        block of statement;
    }
}

3.4 The if-else Ladder/else-if Clause

The else-if is the most general way of writing a multi-way decision.

if(expression1)  
    statement; 
else if(expression2)  
    statement; 
else if(expression3)  
    statement; 
else if(expression4)  
{ 
    block of statement; 
} 
else  
    statement;

The expressions are evaluated in order; if an expressionis true, the "statement" or "block of statement" associated with it is executed, and this terminates the whole chain. As always, the code for each statement is either a single statement, or a group of them in braces. The last else part handles the "none of the above" or default case where none of the other conditions is satisfied.

3.5 Switch Statements or Control Statements

The switch statement is a multi-way decision that tests whether an expression matches one of a number of constant integer values, and branches accordingly. The switch statement that allows us to make a decision from the number of choices is called a switch, or more correctlya switch-case-default, since these three keywords go together to make up the switch statement.

switch (expression)  
{
    case constant-expression: 
        statement1;
        statement2;
        break;
    case constant-expression: 
        statement;
        break;
    ...
    default: 
        statement;
}
  • In switch…case command, each case acts like a simple label. A label determines a point in program which execution must continue from there. Switch statement will choose one of case sections or labels from where the execution of the program will continue. The program will continue execution until it reaches break command.
  • break statements have vital rule in switch structure. If you remove these statements, program execution will continue to next case sections and all remaining case sections until the end of switch block will be executed (while most of the time we just want one case section to be run).
  • default section will be executed if none of the case sections match switch comparison.

3.6 switch Versus if-else Ladder

There are some things that you simply cannot do with a switch.
These are:

  • A float expression cannot be tested using a switch
  • Cases can never have variable expressions (for example it is wrong to say case a +3 :)
  • Multiple cases cannot use same expressions.


4. The Loop Control Structure

Sometimes we want some part of our code to be executed more than once. We can either repeat the code in our program or use loops instead. It is obvious that if for example we need to execute some part of code for a hundred times it is not practical to repeat the code. Alternatively we can use ourrepeating code inside a loop.

There are three methods for, while and do-while which we can repeat a part of a program.

4.1 while loop

while loop is constructed of a condition or expression and a single command or a block of commands that must run
in a loop.

//for single statement  
while(expression)  
    statement;

//for multiple statement   
while(expression)  
{ 
    block of statement 
} 

The statements within the while loopwould keep on getting executed till the condition being tested remains true. When the condition becomes false, the control passes to the first statement that follows the body of the while loop.

The general form of while is as shown below:

initialise loop counter;  
while (test loopcounter using a condition)  
{ 
    do this; 
    and this; 
    increment loopcounter;
} 

4.2 for loop

for loop is something similar to while loop but it is more complex. for loop is constructed from acontrol statement that determines how many times the loop will run and a command section. Command section is either a single command or a block of commands.

//for single statement  
for(control statement)  
    statement;  

//for multiple statement
for(control statement)  
{ 
    block of statement 
} 

Control statement itself has three parts:

for ( initialization; test condition; run every time command )
  • Initialization part is performed only once at for loop start. We can initialize a loop variable here.
  • Test condition is the most important part of the loop. Loop will continue to run if this condition is valid (true). If the condition becomes invalid (false) then the loop will terminate.
  • Run every time command section will be performed in every loop cycle. We use this part to reach the final condition for terminating the loop. For example we can increase or decrease loop variable’s value in a way that after specified number of cycles the loop condition becomes invalid and for loop can terminate.

4.3 do-while loop

The while and for loops test the termination condition at the top. By contrast, the third loop in C, the do-while, tests at the bottom after making each pass through the loop body; the body is always executed at least once.
The syntax of the do is

do  
{
    statements;
}while (expression);

The statement is executed, then expression is evaluated. If it is true, statement is evaluated again, and so on. When the expression becomes false, the loop terminates. Experience shows that do-while is much less used than while and for. A do-whileloop is used to ensure that the statements within the loop are executed at least once.

4.4 Break and Continue statement

We used break statement in switch...case structures in previouslly. We can also use
"break" statement inside loops to terminate a loop and exit it (with a specific condition).

In above example loop execution continues until either num>=20 or entered score is negative.

while (num<20)  
{ 
    printf("Enter score : "); 
    scanf("%d",&scores[num]); 
    if(scores[num]<0) 
        break; 
} 

Continue statement can be used in loops. Like breakcommand continue changes flow of a program. It does not terminate the loop however. It just skips the rest of current iteration of the loop and returns to starting point of the loop.

#include<stdio.h> 
main()  
{ 
    while((ch=getchar())!='\n') 
    { 
        if(ch=='.') 
            continue; 
        putchar(ch); 
    }  
} 

In above example, program accepts all input but omits the '.' character from it. The text will be echoed as you enter it but the main output will be printed after you press the enter key (which is equal to inserting a "\n" character) is pressed. As we told earlier this is because getchar() function is a buffered input function.

4.5 Goto and labels

C provides the infinitely-abusable goto statement, and labels to branch to. Formally, the goto statement is never necessary, and in practice it is almost always easy to write code without it. We have not used goto in this book.

Nevertheless, there are a few situations where gotos may find a place. The most common is to abandon processing in some deeply nested structure, such as breaking out of two or more loops at once. The break statement cannot be used directly since it only exits from the innermost loop. Thus:

for ( ... )  
{           
    for ( ... ) 
    {               
        ...               
        if (disaster)
        {                   
            goto error;
        }           
    }
}       
...   
error:  
/* clean up the mess */

This organization is handy if the error-handling code is non-trivial, and if errors can occur in several places.

A label has the same form as a variable name, and is followed by a colon. It can be attached to any statement in the same function as the goto. The scope of a label is the entire function.

Note - By uses of goto, programs become unreliable, unreadable, and hard to debug. And yet many programmers find gotoseductive.



5. Arrays

Arrays are structures that hold multiple variables of the same data type. The first element in the array is numbered 0, so the last element is 1 less than the size of the array. An array is also known as a subscripted variable. Before using an array its type and dimension must be declared.

5.1 Array Declaration

Like other variables an array needs to be declared so that the compiler will know what kind of an array and how large an array we want.

int marks[30] ;

Here, int specifies the type of the variable, just as it does with ordinary variables and the word marks specifies the name of the variable. The [30] however is new. The number 30 tells how many elements of the type int will be in our array. This number is often called the "dimension" of the array. The bracket ( [ ] ) tells the compiler that we are dealing with an array.

Let us now see how to initialize an array while declaring it. Following are a few examples that demonstrate this.

int num[6] = { 2, 4, 12, 5, 45, 5 } ;  
int n[] = { 2, 4, 12, 5, 45, 5 } ;  
float press[] = { 12.3, 34.2 -23.4, -11.3 } ;  

5.2 Accessing Elements of an Array

Once an array is declared, let us see how individual elements in the array can be referred. This is done with subscript, the number in the brackets following the array name. This number specifies the element’s position in the array. All the array elements are numbered, starting with 0. Thus, marks [2] is not the second element of the array, but the third.

int valueOfThirdElement = marks[2];

5.3 Entering Data into an Array

Here is the section of code that places data into an array:

for(i = 0;i <= 29;i++)  
{ 
    printf("\nEnter marks "); 
    scanf("%d", &marks[i]); 
}

The for loop causes the process of asking for and receiving a student’s marks from the user to be repeated 30 times. The first time through the loop, i has a value 0, so the scanf() function will cause the value typed to be stored in the array element marks[0], the first element of the array. This process will be repeated until i becomes 29. This is last time through the loop, which is a good thing, because there is no array element like marks[30].

In scanf() function, we have used the "address of" operator (&) on the element marks[i] of the array. In so doing, we are passing the address of this particular array element to the scanf() function, rather than its value; which is what scanf() requires.

5.4 Reading Data from an Array

The balance of the program reads the data back out of the array and uses it to calculate the average. The for loop is much the same, but now the body of the loop causes each student’s marks to be added to a running total stored in a variable called sum. When all the marks have been added up, the result is divided by 30, the number of students, to get the average.

for ( i = 0 ; i <= 29 ; i++ )  
    sum = sum + marks[i] ; 
avg = sum / 30 ;  
printf ( "\nAverage marks = %d", avg ) ;  

5.5 Example

Let us try to write a program to find average marks obtained by a
class of 30 students in a test.

#include<stdio.h>  
main()  
{ 
    int avg, i, sum=0; 
    int marks[30] ; /*array declaration */ 
    for ( i = 0 ; i <= 29 ; i++ ) 
    { 
        printf ( "\nEnter marks " ) ; 
        scanf ( "%d", &marks[i] ) ; /* store data in array */ 
    } 
    for ( i = 0 ; i <= 29 ; i++ ) 
        sum = sum + marks[i] ; /* read data from an array*/ 
    avg = sum / 30 ; 
    printf ( "\nAverage marks = %d", avg ) ; 
} 


6 Strings

6.1 What is String?

Strings are arrays of characters. Each member of array contains one of characters in the string.

Example

#include<stdio.h> 
main()  
{ 
    char name[20]; 
    printf("Enter your name : "); 
    scanf("%s",name); 
    printf("Hello, %s , how are you ?\n",name); 
} 

Output Results:

Enter your name : Vineet  
Hello, Vineet, how are you ?  

If user enters "Vineet" then the first member of array will contain 'V' , second cell will contain 'i' and so on. C determines end of a string by a zero value character. We call this character as NULL character and show it with \0 character. (It's only one character and its value is 0, however we show it with two characters to remember it is a character type, not an integer).

Equally we can make that string by assigning character values to each member.

name[0]='B';  
name[1]='r';  
name[2]='i';  
name[3]='a';  
name[4]='n';  
name[5]='\0';  

As we saw in above example placeholder for string variables is %s. Also we will not use a & sign for receiving string values.

6.2 Point to Note

While entering the string using scanf() we must be cautious about
two things:
* The length of the string should not exceed the dimension of the character array. This is because the C compiler doesn’t perform bounds checking on character arrays. * scanf() is not capable of receiving multi-word strings. Therefore names such as "Vineet Choudhary" would be unacceptable. The way to get around this limitation is by using the function gets().The usage of functions gets() and its counter part puts() is shown below.

#include<stdio.h> 
main( )  
{ 
    char name[25] ; 
    printf ("Enter your full name ") ; 
    gets (name) ; 
    puts ("Hello!") ; 
    puts (name) ; 
} 

And here is the output...

Enter your name Vineet Choudhary  
Hello!  
Vineet Choudhary  

The program and the output are self-explanatory except for the fact that, puts() can display only one string at a time (hence the use of two puts() in the program above). Also, on displaying a string, unlike printf(), puts() places the cursor on the next line. Though gets() is capable of receiving only one string at a time, the plus point with gets() is that it can receive a multi-word string.

If we are prepared to take the trouble we can make scanf() accept multi-word strings by writing it in this manner:

char name[25] ;  
printf ("Enter your full name ") ;  
scanf ("%[^\n]s", name) ;  

Though workable this is the best of the ways to call a function, you would agree.

6.3 Standard Library String Functions

With every C compiler a large set of useful string handling library functions are provided in string.h file.

  • strlen - Finds length of a string
  • strlwr - Converts a string to lowercase
  • strupr - Converts a string to uppercase
  • strcat - Appends one string at the end of another
  • strncat - Appends first n characters of a string at the end of another
  • strcpy - Copies a string into another
  • strncpy - Copies first n characters of one string into another
  • strcmp - Compares two strings
  • strncmp - Compares first n characters of two strings
  • strcmpi - Compares two strings without regard to case ("i" denotes that this function ignores case)
  • stricmp - Compares two strings without regard to case (identical to strcmpi)
  • strnicmp - Compares first n characters of two strings without regard to case
  • strdup - Duplicates a string
  • strchr - Finds first occurrence ofa given character in a string
  • strrchr - Finds last occurrence ofa given character in a string
  • strstr - Finds first occurrence of a given string in another string
  • strset - Sets all characters ofstring to a given character
  • strnset - Sets first n characters ofa string to a given character
  • strrev - Reverses string


7. Functions

7.1 What is a Function?

A function is combined of a block of code that can be called or used anywhere in the program by calling the name. Body of a function starts with { and ends with } . This is similar to the main function. Example below shows how we can write a simple function.

#include<stdio.h> 

/*Function prototypes*/ 
myfunc();

main()  
{ 
    myfunc();  
}

/*Function Defination*/ 
myfunc()  
{ 
    printf("Hello, this is a test\n"); 
} 

In above example we have put a section of program in a separate function. Function body can be very complex though. After creating a function we can call it using its name. Functions can also call each other. A function can even call itself.

By the way pay attention to function prototypes section. In some C compilers we are needed to introduce the functions we are creating in a program above the program. Introducing a function is being called function prototype.

Note: * Any C program contains at least one function. * If a program contains only one function, it must be main(). * If a C program contains more than one function, then one (and only one) of these functions must be main(), because program execution always begins with main(). * There is no limit on the number of functions that might be present in a C program. * Each function in a program is called in the sequence specified by the function calls in main(). * After each function has done its thing, control returns to main(). When main()runs out of function calls, the program ends.

7.2 Why Use Functions

Why write separate functions at all? Why not squeeze the entire
logic into one function, main( )? Two reasons:

  • Writing functions avoids rewriting the same code over and over.
  • Using functions it becomes easier to write programs and keep track of what they are doing. If the operation of a program can be divided into separate activities, and each activity placed in a different function, then each could be written and checked more or less independently. Separating the code into modular functions also makes the program easier to design and understand.

What is the moral of the story? Don’t try to cram the entire logic in one function. It is a very bad style of programming. Instead, break a program into small units and write functions for each of these isolated subdivisions. Don’t hesitate to write functions that are called only once. What is important is that these functions perform some logically isolated task.

7.3 Function arguments

Functions are able to accept input parameters in the form of variables. These input parameter variables
can then be used in function body.

#include<stdio.h> 
/* use function prototypes */ 
sayhello(int count);  
main()  
{ 
    sayhello(4);  
} 

sayhello(int count)  
{ 
    int c; 
    for(c=0;c<count;c++) 
        printf("Hello\n"); 
} 

In above example we have called sayhello() function with the parameter 4. This function receives an input value and assigns it to count variable before starting execution of function body. sayhello() function will then print a hello message count times on the screen.

7.4 Function return values

In mathematics we generally expect a function to return a value. It may or may not accept arguments but it always returns a value.

This return value has a type as other values in C. It can be int, float, char or anything else. Type of this return value determines type of your function.

Default type of function is int or integer. If you do not indicate type of function that you use, it will be of type int. As we told earlier every function must return a value. We do this with return command.

int sum()  
{ 
    int a,b,c; 
    a=1; 
    b=4; 
    c=a+b; 
    reurn c; 
} 

Above function returns the value of variable c as the return value of function. We can also use expressions in return command. For example we can replace two last lines of function with return a+b; If you forget to return a value in a function you will get a warning message in most of C compilers. This message will warn you that your function must return a value. Warnings do not stop program execution but errors stop it.

In our previous examples we did not return any value in our functions. For example you must return a value in main() function.

main()  
{ 
    . 
    . 
    . 
    return 0; 
} 

Default return value for an int type function is 0. If you do not insert return 0 or any other value in your main() function a 0 value will be returned automatically. If you are planning to return an int value in your function, it is seriously preferred to mention the return value in your function header and make.

7.5 void return value

There is another void type of function in C language. Void type function is a function that does not return a value. You can define a function that does not need a return value as void.

void test ()  
{ 
    /* fuction code comes here but no return value */ 
}

void functions cannot be assigned to a variable because it does not return value. So you cannot write:

a=test(); 

Using above command will generate an error.

7.6 Recursive Function

In C, it is possible for the functions to call themselves. A function is called recursive if a statement within the body of a function calls the same function.

Following is the recursive function to calculate the factorial value.

#include<stdio.h>
int rec(int);  
main()  
{ 
    int a, fact ; 
    printf("\nEnter any number "); 
    scanf ("%d", &a); 
    fact = rec(a); 
    printf ("Factorial value = %d", fact); 
} 
int rec (int x)  
{ 
    int f; 
    if (x == 1) 
        return (1); 
    else 
        f = x * rec (x - 1) ; 
    return (f) ; 
}

Output-

Enter any number 5  
Factorial value = 120  

Let us understand this recursive factorial function.

what happens is,

rec(5) returns(5 * rec(4),  
 which returns (4 * rec(3), 
  which returns (3 * rec(2), 
   which returns (2 * rec(1), 
    which returns (1)))))

Foxed? Well, that is recursion for you in its simplest garbs. I hope you agree that it’s difficult to visualize how the control flows from one function call to another. Possibly Following figure would make things a bit clearer.

7.7 Multiple Parameters

In fact you can use more than one argument in a function. Following example will show you how you can do this.

#include<stdio.h> 
int min(int a,int b);  
main()  
{ 
    int m; 
    m=min(3,6); 
    printf("Minimum is %d",m);  
    return 0; 
} 
int min(int a,int b)  
{ 
    if(a<b) 
        return a; 
    else 
        return b; 
} 

As you see you can add your variables to arguments list easily.

7.8 Call by value

C programming language function calls, use call by value method. Let’s see an example to understand this subject better.

#include<stdio.h> 
void test(int a);  
main()  
{ 
    int m; 
    m=2; 
    printf("\nM is %d",m); 
    test(m); 
    printf("\nM is %d\n",m); 
    return 0; 
} 
void test(int a)  
{ 
    a=5; 
} 

In main() function, we have declared a variable m. We have assigned value 2 to m. We want to see if function call is able to change value of m as we have modified value of incoming parameter inside test() function.

Program output result is:

M is 2  
M is 2  

So you see function call has not changed the value of argument. This s is because function-calling method only sends value of variable m to the function and it does not send variable itself. Actuallyit places value of variable m in a memory location called stack and then function retrieves the value without having access to main variable itself. This is why we call this method of calling "call by value".

7.9 Call by reference

There is another method of sending variables being called "Call by reference". This second method enables function to modify value of argument variables used in function call.

We will first see an example and then we will describe it.

#include<stdio.h> 
void test(int *ptr);  
main()  
{ 
    int m; 
    m=2; 
    printf("\nM is %d",m); 
    test(&m); 
    printf("\nM is %d\n",m); 
    return 0; 
} 
void test(int *ptr)  
{ 
    printf("\nModifying the value inside memory address%ld",&m); 
    *ptr=5; 
} 

To be able to modify the value of a variable used as an argument in a function, function needs to know memory address of the variable (where exactly the variable is situated in memory).

In C programming language & operator gives the address at which the variable is stored. For example if m is a variable of type int then &m will give us the starting memory address of our variable.We call this resulting address a pointer.

ptr=&m;

In above command ptr variable will contain memory address of variable m. This method is used in some of standard functions in C. For example scanf function uses this method to be able to receive values from console keyboard and put it in a variable. In fact it places received value in memory location of the variable used in function. Now we understand the reason why we add & sign before variable names in scanf variables.

scanf(“%d”,&a);

Now that we have memory address of variable we must use an operator that enables us to assign a value or access to value stored in that address.

As we told, we can find address of variable a using & operator.

ptr=&a; 

Now we can find value stored in variable a using * operator:

val=*ptr; /* finding the value ptr points to */

We can even modify the value inside the address:

*ptr=5; 

Let’s look at our example again. We have passed pointer (memory address) to function. Now function is able to modify value stored inside variable. If yourun the program, you will get these results:

M is 2  
Modifying the value inside memory address 2293620  
M is 5  

So you see this time we have changed value of an argument variable inside the called function (by modifying the value inside the memory location of our main variable).



8. Pointers

8.1 What is a Pointer?

A pointer is a variable that contains the address of a variable. The main thing is that once you can talk about the address of a variable, you'll then be able to goto that address and retrieve the data stored in it.

8.2 C Pointer Syntax

Pointers require a bit of new syntax because when you have a pointer, you need the ability to both request the memory location it stores and the value stored at that memory location. Moreover, since pointers are some what special, you need to tell the compiler when you declare your pointer variable that the variable is a pointer, and tell the compiler what type of memory it points to.

The pointer declaration looks like this:

<variable_type> *<name>;

For example, you could declarea pointer that stores the address of an integer with the following syntax:

int *points_to_integer;

Notice the use of the *. This is the key to declaring a pointer; if you add it directly before the variable name, it will declare the variable to be a pointer.

8.3 Pointer Notation

Consider the declaration,

int i = 3; 

This declaration tells the C compiler to:
* Reserve space in memory to hold the integer value. * Associate the name i with this memory location. * Store the value 3 at this location.

We may represent i’s location in memory by the following memory map.

We see that the computer has selected memory location 65524 as the place to store the value 3. The location number 65524 is not a number to be relied upon, because some other time the computer may choose a different location for storing the value 3. The important point is, i’s address in memory is a number. We can print this address number through the following program:

#include<stdio.h>
main()  
{ 
    int i = 3 ; 
    printf("\nAddress of i = %u",&i); 
    printf("\nValue of i = %d",i); 
} 

The output of the above program would be:

Address of i = 65524  
Value of i = 3  

Look at the first printf() statement carefully. & used in this statement is C’s "address of" operator. The expression &i returns the address of the variable i, which in this case happens to be 65524. Since 65524 represents an address, there is no question of a sign being associated with it. Hence it is printed out using %u, which is a format specifier for printing an unsigned integer. We have been using the & operator all the time in the scanf() statement.

The other pointer operator available in C is *, called "value at address" operator. It gives the value stored at a particular address. The "value at address" operator is also called "indirection" operator.

Observe carefully the output of the following program:

#include<stdio.h>
main()  
{ 
    int i = 3 ; 
    printf("\nAddress of i = %u",&i); 
    printf("\nValueof i = %d",i); 
    printf("\nValue of i = %d",*(&i)); 
}

The output ofthe above programwould be:

Address of i = 65524  
Value of i = 3  
Value of i = 3  

Note that printing the value of *(&i) is same as printing the value of i. The expression &i gives the address of the variable i. This address can be collected in a variable, by saying,

j = &i ;

But remember that j is not an ordinary variable like any other integer variable. It is a variable that contains the address of other variable (i in this case). Since j is a variable the compiler must provide it space in the memory. Once again, the following memory map would illustrate the contents of i and j.

As you can see, i's value is 3 and j's value is i's address. But wait, we can’t use j in a program without declaring it. And since j is a variable that contains the address of i, it is declared as,

int *j ;

This declaration tells the compiler that j will be used to store the address of an integer value. In other words j points to an integer. How do we justify the usage of * in the declaration,

int *j ;

Let us go by the meaning of *. It stands for "value at address". Thus, int *j would mean, the value at the address contained in j is an int.

8.4 Example

Here is a program that demonstrates the relationships we have been discussing.

#include<stdio.h>
main( )  
{ 
    int i = 3 ; 
    int *j ; 
    j = &i ; 
    printf ("\nAddress of i = %u",&i) ; 
    printf ("\nAddress of i = %u",j) ; 
    printf ("\nAddress of j = %u",&j) ; 
    printf ("\nValue of j = %u",j) ; 
    printf ("\nValue of i = %d",i) ; 
    printf ("\nValue of i = %d",*(&i)) ; 
    printf ("\nValue of i = %d",*j) ; 
} 

The output of the above program would be:

Address of i = 65524  
Address of i = 65524  
Address of j = 65522  
Value of j = 65524  
Value of i = 3  
Value of i = 3  
Value of i = 3  


9. Structures

A structure is a collection of one or more variables, possibly of different types, grouped together under a single name for convenient handling.

9.1 Declaring a Structure

The general form of a structure declaration statement is given below:

struct <structure name>  
{ 
    structure element 1; 
    structure element 2; 
    structure element 3; 
    ...... 
    ......  
    structure element n;
};

Once the new structure data type has been defined one or more variables can be declared to be of that type.

For example the variables b1, b2, b3 can be declared to be of the type struct book,

struct book  
{ 
    char name; 
    float price; 
    int pages; 
}; 

as,

struct book b1, b2, b3 ; 

This statement sets aside space in memory. It makes available space to hold all the elements in the structure—in this case, 7 bytes — one for name, four for price and two for pages. These bytes are always in adjacent memory locations.

Like primary variables and arrays, structure variables can also be initialized where they are declared. The format used is quite similar to that used to initiate arrays.

struct book  
{ 
    char name[10]; 
    float price; 
    int pages; 
};

struct book b1 = { "Basic", 130.00, 550 } ;  
struct book b2 = { "Physics", 150.80, 800 } ;  

9.2 Accessing Structure Elements

In arrays we can access individual elements of an array using a subscript. Structures use a different scheme. They use a dot (.) operator. So to refer to pages of the structure defined in book structure we have to use,

b1.pages 

Similarly, to refer to price we would use,

b1.price 

Note that before the dot there must always be a structure variable and after the dot there must always be a structure element.

9.3 Example

The following example illustrates the use of this data type.

#include<stdio.h> 
main()  
{ 
    struct book 
    { 
        char name; 
        float price; 
        int pages; 
    }; 
    struct book b1, b2, b3 ;

    printf("\nEnter names, prices & no. of pages of 3 books\n"); 
    scanf("%c %f %d", &b1.name, &b1.price, &b1.pages); 
    scanf("%c %f %d", &b2.name, &b2.price, &b2.pages); 
    scanf("%c %f %d", &b3.name, &b3.price, &b3.pages);

    printf("\n\nAnd this is what you entered"); 
    printf("\n%c %f %d", b1.name, b1.price, b1.pages); 
    printf("\n%c %f %d", b2.name, b2.price, b2.pages); 
    printf("\n%c %f %d", b3.name, b3.price, b3.pages); 
} 

And here is the output...

Enter names, prices and no. of pages of 3 books  
A 100.00 354  
C 256.50 682  
F 233.70 512 

And this is what you entered  
A 100.000000 354  
C 256.500000 682  
F 233.700000 512  

9.4 Summary

  • A structure is usually used when we wish to store dissimilar data together.
  • Structure elements can be accessed through a structure variable using a dot (.) operator.
  • Structure elements can be accessed through a pointer to a structure using the arrow (->) operator.
  • All elements of one structure variable can be assigned to another structure variable using the assignment (=) operator.
  • It is possible to pass a structure variable to a function either by value or by address.
  • It is possible to create an array of structures


10. Files

10.1 File Pointers

There are many ways to use files in C. The most straight forward use of files is via a file pointer.

FILE *fp; 

fp is a pointer to a file.

The type FILE, is not a basic type, instead it is defined in the header file stdio.h, this file must be included in your program.

10.2 Opening a File

fp = fopen(filename, mode);

The filename and mode are both strings.

The mode can be
* r - read * w - write, overwrite file if it ex ists * a - write, but append instead of overwrite * r+ - read & write, do not destroy file if it exists * w+ - read & write, but overwrite file if it exists * a+ - read & write, but append instead of overwrite * b - may be appended to any of the above to force the file to be opened in binary mode rather than text mode * fp = fopen("data.dat","a"); - will open the disk file data.dat for writing, and any information written will be appended to the file.

The following useful table from the ANSI C Rationale lists the different actions and requirements of the different modes for opening a file:

fopen returns NULL if the file could not be opened in the mode requested. The returned value should be checked before any attempt is made to access the file. The following code shows how the value returned by fopen might be checked. When the file cannot be opened a suitable error message is printed and the program halted. In most situations this would be inappropriate, instead the user should be given the chance of re-entering the file name.

#include <stdio.h>
int main()  
{
    char filename[80];
    FILE *fp;
    printf("File to be opened? ");
    scanf("%79s", filename);
    fp = fopen(filename,"r");
    if (fp == NULL)
    {
        fpri ntf(stderr, "Unable to open file %s\n", filename);
        return 1; /* Exit to operating system */
    }
    //code that accesses the contents of the file
    return 0;
}

Sequential file access is performed with the following library functions.

  • fprintf(fp, formatstring , ...) - print to a file
  • fscanf(fp, formatstring , ...) - read from a file
  • getc(fp) - get a character from a file
  • putc(c, fp) - put a character in a file
  • ungetc(c, fp) - put a character back onto a file (only one character is guaranteed to be able to be pushed back)
  • fopen( filename , mode) - open a file
  • fclose(fp) - close a file

The standard header file stdio.h defines three file pointer constants, stdin ,stdout and stderr for the standard input, output and error streams. It is considered good practice to write error messages to the standard error stream.

Use the fprintf() function to do this:

fprintf(stderr,"ERROR: unable to open file %s\n", filename);

The functions fscanf() and getc() are used for sequential access to the file, that is subsequent reads will read the data following the data just read, and eventually you will reach the end of the file. If you want to move forwards and backwards through the file, or jump to a specific offset from the beginning, end or current location use the fseek() function (though the file mustbe opened in binary mode). The function ftell() reports the current offset from the beginning of the file.

If you wish to mix reading and writing to the same file, each switch from reading to writing (or vice versa) mustbe preceded by a call to fseek(), fsetpos(), rewind() or fflush(). If it is required to stay at the same position in the file then use fflush() or fseek(fp,0L,SEEK_CUR) which will move 0 bytes from the current position!



11. Command Line Arguments

The arguments that we pass on to main() at the command prompt are called command line arguments. The full declaration of main looks like this:

int main (int argc, char *argv[])

The function main() can have two arguments, traditionally named as argc and argv. Out of these, argv is an array of pointers to strings and argc is an int whose value is equal to the number of strings to which argv points. When the program is executed, the strings on the command line are passed to main(). More precisely, the strings at the command line are stored in memory and address of the first string is stored in argv[0], address of the second string is stored in argv[1] and so on. The argument argc is set to the number of strings given on the command line.

For example, in our sample program, if at the command prompt we give,

filecopy PR1.C PR2.C

then,

argc would contain 3 * argv[0] - would contain base address of the string "filecopy" * argv[1] - would contain base address of the string "PR1.C" * argv[2] - would contain base address of the string "PR2.C"