LOOPING

INTRODUCTION : - There may be a situation when WE need to execute a block of code several number of  times. In general statements are executed sequentially: The first statement in a function is  executed first, followed by the second, and so on. Programming languages provide various control structures that allow for more complicated  execution paths. A loop statement allows us to execute a statement or group of statements multiple times and following is the general from of a loop statement in most of the programming languages: C programming language provides following types of loop to handle looping requirements. Click the following links to check their detail. Loop Type Description while 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 WE can use one or more loop inside any another while, for or do..while loop. ------------------------------------------------------------------------------------------------------------------------------------------------ THE FOR LOOP  : A for loop is a repetition control structure that allows WE to efficiently write a loop that needs to execute a specific number of times. Syntax: The syntax of a for loop in C programming language is: for ( initialisation; condition; increment ) { statement(s); } Here is the flow of control in a for loop: The initialisation step is executed first, and only once. This step allows US to declare and  initialize any loop control variables. WE ARE  not required to put a statement here, as  long as a semicolon appears. Next, the condition is 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. After the body of the for loop executes, the flow of control jumps back up to the increment statement. This statement allows US to update any loop control variables. This statement can be left blank, as long as a semicolon appears after the condition. 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. Flow Diagram: Example: #include <stdio.h> int main () { int i; /* for loop execution */ for( i = 0; i < 10; i = i + 1)/*we can also write i++ */ { printf("value of i: %d\n", i); } return 0; } When the above code is compiled and executed, it produces following result: value of i: 0 value of i: 1 value of i: 2 value of i: 3 value of i: 4 value of i: 5 value of i: 6 value of i: 7 value of i: 8 value of i: 9 THE WHILE LOOP: A while loop statement in C programming language repeatedly executes a target statement as long as a given condition is true. Syntax: The syntax of a while loop in C programming language is: while(condition) { statement(s); } Here statement(s) may be a single statement or a block of statements. The condition may 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. Flow Diagram: Here key point of the while loop 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> int main () { /* local variable definition */ int i = 0; /* while loop execution */ while( i < 10 ) { printf("value of i: %d\n", i); i++; } return 0; } When the above code is compiled and executed, it produces following result: value of a: 0 value of a: 1 value of a: 2 value of a: 3 value of a: 4 value of a: 5 value of a: 6 value of a: 7 value of a: 8 value of a: 9 THE DO - WHILE LOOP : Unlike for and while loops, which test the loop condition at the top of the loop, the do...while loop in C programming language checks its condition at the bottom of the loop. A do...while loop is similar to a while loop, except that a do...while loop is guaranteed to execute at least one time. Syntax: The syntax of a do...while loop 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. Flow Diagram: 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 ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Loop Control Statements: Loop control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed. C supports the following control statements.  Control Statement Description break statement Terminates the loop or switch statement and transfers execution to the statement immediately following the loop or switch. continue statement Causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating. goto statement Transfers control to the labeled statement. Though it is not advised to use goto statement in our program. THE BREAK STATEMENT : The break statement in C programming language has following two usage: When the break statement is encountered inside a loop, the loop is immediately terminated and program control resumes at the next statement following the loop. It can be used to terminate a case in the switch statement (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 a break statement in C is as follows: break; Flow Diagram: 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 THE  CONTINUE STATEMENT : The continue statement in C programming language works somewhat like the break statement. Instead of forcing termination, however, continue forces the next iteration of the loop to take place, skipping any code in between. For the for loop, continue statement causes the conditional test and increment portions of the loop to execute. For the while and do...while loops, continue statement causes the program control passes to the conditional tests. Syntax: The syntax for a continue statement in C is as follows: continue; Flow Diagram: 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 The Infinite Loop: A loop becomes infinite loop if a condition never becomes false. The for loop is traditionally used for this purpose. Since none of the three expressions that form the for loop are required, WE can make an endless loop by leaving the conditional expression empty. #include <stdio.h> int main () { for( ; ; ) { printf("This loop will run forever.\n"); } return 0; } When the conditional expression is absent, it is assumed to be true. WE may have an initialization and increment expression, but C programmers more commonly use the for( ; ; ) construct to signify an infinite loop. NOTE: WE can terminate an infinite loop by pressing Ctrl + C  OR (ctrl+pause break ) keys.