Repetition Repetition allows you to repeat an operation or a series of operations many times. This is called looping and is one of the basic structured programming concepts. An action or a series of actions The above flowchart would be considered an infinite loop because its repeats the action over and over again and never stops. To make a loop end we must have a condition that controls the loop. In other words, the loop must be designed so that before or after each iteration, it checks to see if it is done. Condition Body (action) false true Pretest loop Posttest loop true false

Repetition Two general categories of loops: event-controlled loop - this is where an event will stop the loop from executing (e.g. reading the end of a file.) counter-controlled (conditional) loop - this is used if the number of times an action to be repeated is known in advance. Condition Action false true Pretest loop Posttest loop true false Initialize Loop update Initialize Loop update In a pretest loop, the body can be done zero or more times. In a posttest loop, the body must be done at least once. Before a loop can start, a variable must be initialized. The value of a variable or expression is tested. Then something must happen inside the loop to update the variable, otherwise you could have an infinite loop.

Repetition Loops in C There are three loop statements in C: the while, the for, and the do…while. The first two are pretest loops and the do…while is a posttest loop. All of them can be used for even-controlled and counter-controlled loops. The while and do…while are most commonly used for event-controlled loops and the for is usually used for counter-controlled loops. All three of these loop constructs continue looping when the condition is true and terminate when it is false. The while loop The while loop uses an expression to control the loop. Since it is a pretest loop, it tests the expression before every iteration of the loop. Syntax: expression statements false true while (expression) statement no semicolon If the expression evaluates to true ( any nonzero value) the statements are executed and if it evaluates to false ( 0 ) the loop body is skipped and execution continues after the loop construct.

Repetition Example of the while statement: count_emp = 0; while( count_emp < 7) { printf(“Enter Hours >”); scanf(“%d”, &hours); printf(“Enter rate >”); scanf(“%lf”, &rate); pay = hours * rate; printf(“Pay is $%6.2f\n”, pay); count_emp = count_emp + 1; } printf(“\nAll employees processed\n”); loop control variable initialized before entering while statement loop repetition condition count_emp is tested before the start of each loop repetition. If the condition is true the loop body is executed again loop body executed once for each value of count_emp that is less than 7 control variable is incremented ( updated ) in the body of the loop when count_emp becomes 7, the condition evaluates to false and control passes to the statement that follows the loop body

Repetition The for loop The for statement is a pretest loop that uses three expressions. The first expression contains any initialization statements, the second contains the terminating expression (test ) and the third contains the updating expression. The for loop is used when your loop is to be executed a known number of times. You can do the same thing with a while loop. But a for loop is easier to read and more natural for counting loops. Syntax: statement false true for ( expr 1; expr 2; expr 3 ) statement expr3 expr1 expr2 expr 1 expr 2 expr 3 statement true false Expanded flowchart Flowchart

Repetition Example of the for statement: total_pay = 0.0 ; for( count_emp = 0 ; count_emp < number_emp ; count_emp++ ) { printf(“Enter Hours >”); scanf(“%d”, &hours); printf(“Enter rate >”); scanf(“%lf”, &rate); pay = hours * rate; printf(“Pay is $%6.2f\n”, pay); total_pay = total_pay + pay; } printf(“\nAll employees processed\n”); printf(“Total payroll is $%8.2f\n”, total_pay); exp 1 initialization expression - initializes the loop control variable - only done once exp 2 loop repetition condition count_emp is tested before the start of each loop repetition if the condition is true the loop body is executed again if the condition is false, the loop is exited and the next program statement after the for statement is executed loop body if the condition is true, the statements in the loop body are executed. After the last statement in the loop body is executed, the update expression is executed. expr 3 update expression the value of the control variable is changed and then the repetition condition is tested again 1st 2nd 3rd 4th semicolons

Repetition Example 2 of the for statement:

Repetition The do…while loop The do…while statement is a posttest loop. Like the while and for loops, it also uses an expression to control the loop, but it tests the expression after the execution of the body. Syntax: expression statements false do statement while (expression) ; true semicolon

Repetition Example of the do…while statement:

Repetition The Comma expression A comma expression is a complex expression made up of two expressions separated by commas. Although it can be used in many places, it is generally used only in for statements. The expressions are evaluated left to right. The value and type of the expression are the value and type of the right expression - the other expression is included for its side effect. The comma expression has the lowest priority of all expressions, priority 1. Example: for ( sum = 0, i = 1; i <= 20; i++ ) { scanf ( “%d”, &a ); sum += a ; } Comma expressions can be nested. When they are, all expression values other than the last are discarded. Results: while loop: Loop Count: 11 Number of tests: 11 do…while loop: Loop Count: 11 Number of tests: 10

Repetition The nested Loop A nested loop is a loop within a loop. Within each iteration of the first or outer loop and inner loop is executed one or more times. Example: #include void main(void) { int i, j, k, m = 0; for ( i = 1; i <= 5 ; i += 2 ) for ( j = 1 ; j <= 4 ; j++ ) { k = i + j ; printf( “ i = %3d, j = %3d, k = %3d\n”, i, j, k); } m = k + i; } Output: i = 1, j = 1, k = 2 i = 1, j = 2, k = 3 i = 1, j = 3, k = 4 i = 1, j = 4, k = 5 i = 3, j = 1, k = 4 i = 3, j = 2, k = 5 i = 3, j = 3, k = 6 i = 3, j = 4, k = 7 i = 5, j = 1, k = 6 i = 5, j = 2, k = 7 i = 5, j = 3, k = 8 i = 5, j = 4, k = 9

Repetition Other statements related to looping ( the break & continue) The break statement causes a loop to terminate. It is the same as setting the loop’s limit test to false. break can be used in any of the loop statements - while, for and do…while and in the selection switch statement. Good structured programming limits its use to the switch statement. The continue statement does not terminate the loop, but simply transfers to the testing statement in the while and do…while statements and transfers to the updating expression in a for statement. The use of the continue statement is also considered unstructured programming.