Decision Making And Looping 5b6s13

DECISION MAKING AND LOOPING

DECISION MAKING AND LOOPING INTRODUCTION • It is possible to execute a segment of a program repeatedly by introducing a counter and later testing it using the if statement • We need to initialize and increment a counter and test its value at an appropriate place in the program for the completion of the loop

LOOPING • Looping method in C enable us to develop concise programs containing repetitive processes without the use of goto statements • In looping, the sequence of statements are executed until some conditions for termination of the loop are satisfied

PROGRAM LOOP • A program loop consists of two segments – Body of the loop – Control statement

• The control statements tests certain conditions and then directs the repeated execution of the statements contained in the body of the loop • Depending on the position of the control statement in the loop, a control structure may be classified either as – The entry-controlled loop – The exit-controlled loop

PROGRAM LOOP • Entry-Controlled Loop – The control conditions are tested before the start of the loop execution – If the conditions are not satisfied, then the body of the loop will not be executed

• Exit-Controlled Loop – The test is performed at the end of the body of the loop – Therefore, the body is executed unconditionally for the first time

PROGRAM LOOP

PROGRAM LOOP • The test conditions should be carefully stated on order to perform the desired number of loop executions • Based on the test conditions – Control transfers out of the loop – Control sets up an infinite loop and the body is executed over and over again

• A looping process, include the following four steps – – – –

Setting and initialization of a counter Execution of the statements in the loop Test for a specified condition for execution of the loop Incrementing the counter

• The test determine whether the loop has been repeated the specified number of times or to determine whether a particular condition has been met

PROGRAM LOOP • The C language provides three looping statements – The while statement – The do statement – The for statement

THE WHILE STATEMENT • Simplest looping structure • Entry-controlled loop statement • Syntax: while (test condition) { Body of the loop }

• The test-condition is evaluated and if the condition is true, then the body of the loop is executed • After execution of the body, the test condition is once again evaluated and if it is true, the body is executed once again

THE WHILE STATEMENT • This process of repeated execution of the body continues until the test condition finally becomes false and the control is transferred out of the loop • On exit, the program continues with the statement immediately after the body of the loop • The body may have one or more statements • The braces are needed only if the body contains two or more statements • It is a good practice to use braces even if the body have one statement

THE WHILE STATEMENT

To calculate X

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THE DO STATEMENT • On some occasions, it might be necessary to execute the body of the loop before the test is performed • Such situations are handled with the help of do statement • Syntax: do { Body of the loop }

while (test-condition); • The program proceeds to evaluate the body of the loop first • At the end of the loop, the test condition in the while statement is evaluated

THE DO STATEMENT • If the condition is true, the program continues to evaluate the body of the loop once again • This process continues as long as the condition is true • When the condition becomes false, the loop will be terminated and the control goes to the statement that appears immediately after the while statement • The test condition is evaluated at the bottom of the loop • Exit-controlled loop • The body of the loop is always executed at least once

DO...WHILE PROGRAM main() { int i=1 do { printf("\nProgram for do...while loop"); i++; } while(i<=5); }

OUTPUT: Program for loop Program for loop Program for loop Program for loop Program for loop

do...while do...while do...while do...while do...while

DO...WHILE PROGRAM main() { int i=7 do { printf("\nProgram for do...while loop"); i++; } while(i<=5); } OUTPUT: Program for do...while loop

THE FOR STATEMENT • Entry-controlled loop • Syntax: for (initialization; test-condition; increment) { body of the loop }

• The execution of the for statement is as follows: – STEP 1: • Initialization of the control variables is done first, using assignment statements. Example: i = 1 and count = 0 • The variables used are known as loop-control variables

THE FOR STATEMENT – STEP 2:

• The value of the control variables is tested using the test-condition. • The test condition is a relational expression that determines when the loop will exit. Example: i < 10 • If the condition is true, the body of the loop is executed; otherwise the loop is terminated • the execution continues with the statement that immediately follows the loop

– STEP 3:

• When the body of the loop is executed, the control is transferred back to the for statement after evaluating the last statement in the loop. • The control variable is incremented using an assignment statement and the new value of the control variable is again tested to see whether it satisfies the loop condition. Example: i = i + 1 • The process continues till the value of the control variable fails to satisfy the test condition

THE FOR STATEMENT int count; for (count = 1 ; count <= 10 ; + +count) printf ("\n%d", count);

THE FOR STATEMENT • Features of for loop for(int i = 1, j = 2 ; i<=5 ; i++, j = j+2) printf("\n %d", i*j);

– More than one variable can be initialized at a time – The increment section may also have more than one part – The test-condition may have any compound relation and the testing need not be limited only to the loop control variables for(int i = 1 ; i <= count && sum < 10; i++) sum += i;

– Unique aspect: one or more sections of for statement can be omitted for( ;; ) statement;

THE FOR STATEMENT • Nesting of for loops – One for statement within another for statement for (i=1;i<10;++i) { …. for (j=1;j!=5;++j) { …. } …. }

COMPARISON OF THREE LOOPS

JUMPS IN LOOPS • Sometimes, when executing a loop it becomes desirable to skip a part of the loop or to leave the loop as soon as a certain condition occurs • Example: searching for a number in a list • C permits a jump from one statement to another within a loop as well as a jump out of a loop

JUMPING OUT OF A LOOP • Early exit from a loop can be accomplished by using the break statement or the goto statement • break used in switch statement and goto used in if…else construct • These can be used in while, do, or for loop

JUMPING OUT OF A LOOP • break statement – When break statement is encountered inside a loop, the loop is immediately exited and the program continues with the statement immediately following the loop – When the loops are nested , the break would exit from the loop containing it – The break will exit only single loop

• goto statement – A goto statement can transfer the control to any place in a program, it is useful to provide branching within a loop – Important use of goto is to exit from a deeply nested loops when an error occurs

SKIPPING A PART OF A LOOP • Continue statement – Unlike the break which causes the loop to be terminated, the continue causes the loop to be continued with the next iteration after skipping any statements in between SKIP THE FOLLOWING AND CONTINUE WITH THE NEXT ITERATION

• The format is: continue;

AVOIDING GOTO • Reasons to avoid goto – When goto is used, many compilers generate a less efficient code – Using many goto makes a program logic complicated and renders the program unreadable

CONCISE TEST EXPRESSION • Test expressions used in if, for, while and do statements that are evaluated and compared with zero for making decisions • Every integer expression has a true/false value, we need not make explicit comparisons with zero • The expression x is true whenever x is not zero, and false when x is zero • Applying ! Operator, we can write concise test expressions without using any relational operators – if( expression == 0 ) is equivalent to if(! expression) – if( expression != 0 ) is equivalent to if( expression)

• Example: if(m%5 == 0 && n%5 ==0) is same as if(! (m%5) && ! (n %5))

SELECTING A LOOP • Analyze the problem and see whether it required a pre-test or post-test loop • If it requires a post-test loop, then use do while • If it requires a pre-test loop, then can use for or while • Decide whether the loop termination requires counter-based control or sentinel-based control • Use for loop if the counter-based control is necessary • Use while loop if the sentinel-based control is required • Note that both the loops can be implemented by all the three control structures

THE END