C programming language provides following types of loop to handle looping requirements.
Loop Type / Descriptionwhile loop / Repeats a statement or group of statements while a given condition is true. It tests the condition before executing the loop body.
for loop / Execute a sequence of statements multiple times and abbreviates the code that manages the loop variable.
do...while loop / Like a while statement, except that it tests the condition at the end of the loop body
nested loops / You can use one or more loop inside any another while, for or do..while loop.
While Loop
Awhileloop statement in C programming language repeatedly executes a target statement as long as a given condition is true.
Syntax:
The syntax of awhileloop in C programming language is:
while(condition)
{
statement(s);
}
Herestatement(s)may be a single statement or a block of statements. Theconditionmay be any expression, and true is any nonzero value. The loop iterates while the condition is true.
When the condition becomes false, program control passes to the line immediately following the loop.
Here key point of thewhileloop is that the loop might not ever run. When the condition is tested and the result is false, the loop body will be skipped and the first statement after the while loop will be executed.
Example:
#include <stdio.h>
#include <conio.h>
int main ()
{
/* local variable definition */
int a = 10;
/* while loop execution */
while( a < 20 )
{
printf("value of a: %d\n", a);
a++;
}
getch();}
When the above code is compiled and executed, it produces following result:
value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15
value of a: 16
value of a: 17
value of a: 18
value of a: 19
For Loop
Aforloop is a repetition control structure that allows you to efficiently write a loop that needs to execute a specific number of times.
Syntax:
The syntax of aforloop in C programming language is:
for ( init; condition; increment )
{
statement(s);
}
Here is the flow of control in a for loop:
1. Theinitstep is executed first, and only once. This step allows you to declare and initialize any loop control variables. You are not required to put a statement here, as long as a semicolon appears.
2. Next, theconditionis evaluated. If it is true, the body of the loop is executed. If it is false, the body of the loop does not execute and flow of control jumps to the next statement just after the for loop.
3. After the body of the for loop executes, the flow of control jumps back up to theincrementstatement. This statement allows you to update any loop control variables. This statement can be left blank, as long as a semicolon appears after the condition.
4. The condition is now evaluated again. If it is true, the loop executes and the process repeats itself (body of loop, then increment step, and then again condition). After the condition becomes false, the for loop terminates.
Example:
#include <stdio.h>
#include <conio.h>
int main ()
{
/* for loop execution */
for( int a = 10; a < 20; a = a + 1 )
{
printf("value of a: %d\n", a);
}
getch();
}
When the above code is compiled and executed, it produces following result:
value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15
value of a: 16
value of a: 17
value of a: 18
value of a: 19
Do – While Loop
Unlikeforandwhileloops, which test the loop condition at the top of the loop, thedo...whileloop in C programming language checks its condition at the bottom of the loop.
Ado...whileloop is similar to a while loop, except that a do...while loop is guaranteed to execute at least one time.
Syntax:
The syntax of ado...whileloop in C programming language is:
do
{
statement(s);
}while( condition );
Notice that the conditional expression appears at the end of the loop, so the statement(s) in the loop execute once before the condition is tested.
If the condition is true, the flow of control jumps back up to do, and the statement(s) in the loop execute again. This process repeats until the given condition becomes false.
Example:
#include <stdio.h>
int main ()
{
/* local variable definition */
int a = 10;
/* do loop execution */
do
{
printf("value of a: %d\n", a);
a = a + 1;
}while( a < 20 );
return 0;
}
When the above code is compiled and executed, it produces following result:
value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15
value of a: 16
value of a: 17
value of a: 18
value of a: 19
Nested Loops
C programming language allows to use one loop inside another loop. Following section shows few examples to illustrate the concept.
Syntax:
The syntax for anested for loopstatement in C is as follows:
for ( init; condition; increment )
{
for ( init; condition; increment )
{
statement(s);
}
statement(s);
}
The syntax for anested while loopstatement in C programming language is as follows:
while(condition)
{
while(condition)
{
statement(s);
}
statement(s);
}
The syntax for anested do...while loopstatement in C programming language is as follows:
do
{
statement(s);
do
{
statement(s);
}while( condition );
}while( condition );
A final note on loop nesting is that you can put any type of loop inside of any other type of loop. For example a for loop can be inside a while loop or vice versa.
Example:
The following program uses a nested for loop to find the prime numbers from 2 to 100:
#include <stdio.h>
int main ()
{
/* local variable definition */
int i, j;
for(i=2; i100; i++) {
for(j=2; j <= (i/j); j++)
if(!(i%j)) break; // if factor found, not prime
if(j (i/j)) printf("%d is prime\n", i);
}
return 0;
}
When the above code is compiled and executed, it produces following result:
2 is prime
3 is prime
5 is prime
7 is prime
11 is prime
13 is prime
17 is prime
19 is prime
23 is prime
29 is prime
31 is prime
37 is prime
41 is prime
43 is prime
47 is prime
53 is prime
59 is prime
61 is prime
67 is prime
71 is prime
73 is prime
79 is prime
83 is prime
89 is prime
97 is prime
Break Statement
Thebreakstatement in C programming language has following two usage:
1. When thebreakstatement is encountered inside a loop, the loop is immediately terminated and program control resumes at the next statement following the loop.
2. It can be used to terminate a case in theswitchstatement (covered in the next chapter).
If you are using nested loops ( ie. one loop inside another loop), the break statement will stop the execution of the innermost loop and start executing the next line of code after the block.
Syntax:
The syntax for abreakstatement in C is as follows:
break;
Example:
#include <stdio.h>
int main ()
{
/* local variable definition */
int a = 10;
/* while loop execution */
while( a < 20 )
{
printf("value of a: %d\n", a);
a++;
if( a > 15)
{
/* terminate the loop using break statement */
break;
}
}
return 0;
}
When the above code is compiled and executed, it produces following result:
value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15
Continue Statement
Thecontinuestatement in C programming language works somewhat like thebreakstatement. Instead of forcing termination, however, continue forces the next iteration of the loop to take place, skipping any code in between.
For theforloop,continuestatement causes the conditional test and increment portions of the loop to execute. For thewhileanddo...whileloops,continuestatement causes the program control passes to the conditional tests.
Syntax:
The syntax for acontinuestatement in C is as follows:
continue;
Example:
#include <stdio.h>
int main ()
{
/* local variable definition */
int a = 10;
/* do loop execution */
do
{
if( a == 15)
{
/* skip the iteration */
a = a + 1;
continue;
}
printf("value of a: %d\n", a);
a++;
}while( a < 20 );
return 0;
}
When the above code is compiled and executed, it produces following result:
value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 16
value of a: 17
value of a: 18
value of a: 19
goto Statement
Agotostatement in C programming language provides an unconditional jump from the goto to a labeled statement in the same function.
NOTE:Use ofgotostatement is highly discouraged in any programming language because it makes difficult to trace the control flow of a program, making the program hard to understand and hard to modify. Any program that uses a goto can be rewritten so that it doesn't need the goto.
Syntax:
The syntax for agotostatement in C is as follows:
goto label;
..
.
label: statement;
Herelabelcan be any plain text except C keyword and it can be set anywhere in the C program above or below togotostatement.
Example:
#include <stdio.h>
int main ()
{
/* local variable definition */
int a = 10;
/* do loop execution */
LOOP:do
{
if( a == 15)
{
/* skip the iteration */
a = a + 1;
goto LOOP;
}
printf("value of a: %d\n", a);
a++;
}while( a < 20 );
return 0;
}
When the above code is compiled and executed, it produces following result:
value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 16
value of a: 17
value of a: 18
value of a: 19
Functions
A function is a group of statements that together perform a task. Every C program has at least one function which ismain(), and all the most trivial programs can define additional functions.
You can divide up your code into separate functions. How you divide up your code among different functions is up to you, but logically the division usually is so each function performs a specific task.
A functiondeclarationtells the compiler about a function's name, return type, and parameters. A functiondefinitionprovides the actual body of the function.
The C standard library provides numerous built-in functions that your program can call. For example, functionstrcat()to concatenate two strings, functionmemcpy()to copy one memory location to another location and many more functions.
A function is known with various names like a method or a sub-routine or a procedure etc.
Defining a Function:
The general form of a function definition in C programming language is as follows:
return_type function_name( parameter list )
{
body of the function
}
A function definition in C programming language consists of afunction headerand afunction body. Here are all the parts of a function:
· Return Type: A function may return a value. Thereturn_typeis the data type of the value the function returns. Some functions perform the desired operations without returning a value. In this case, the return_type is the keywordvoid.
· Function Name:This is the actual name of the function. The function name and the parameter list together constitute the function signature.
· Parameters:A parameter is like a placeholder. When a function is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument. The parameter list refers to the type, order, and number of the parameters of a function. Parameters are optional; that is, a function may contain no parameters.
· Function Body:The function body contains a collection of statements that define what the function does.
Example:
Following is the source code for a function calledmax(). This function takes two parameters num1 and num2 and returns the maximum between the two:
/* function returning the max between two numbers */
int max(int num1, int num2)
{
/* local variable declaration */
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
Function Declarations:
A functiondeclarationtells the compiler about a function name and how to call the function. The actual body of the function can be defined separately.
A function declaration has the following parts:
return_type function_name( parameter list );
For the above defined function max(), following is the function declaration:
int max(int num1, int num2);
Parameter names are not important in function declaration only their type is required, so following is also valid declaration:
int max(int, int);
Function declaration is required when you define a function in one source file and you call that function in another file. In such case you should declare the function at the top of the file calling the function.
Calling a Function:
While creating a C function, you give a definition of what the function has to do. To use a function, you will have to call that function to perform the defined task.
When a program calls a function, program control is transferred to the called function. A called function performs defined task and when its return statement is executed or when its function-ending closing brace is reached, it returns program control back to the main program.
To call a function you simply need to pass the required parameters along with function name and if function returns a value then you can store returned value. For example:
#include <stdio.h>
/* function declaration */
int max(int num1, int num2);
int main ()
{
/* local variable definition */
int a = 100;
int b = 200;
int ret;
/* calling a function to get max value */
ret = max(a, b);
printf( "Max value is : %d\n", ret );
return 0;
}
/* function returning the max between two numbers */
int max(int num1, int num2)