loops

By Nita Arora (KHMS) 1

Repetition(Looping Concept)


After studying this chapter you will be able to:After studying this chapter you will be able to:

OOBJECTIVESBJECTIVES

Understand the basic components of a loop: initialization, control

expression, and update.

Understand and use pretest, post-test, and count-controlled loops.

Differentiate between event-controlled and counter-controlled loops.

Write loops in C++ using while, for, and do...while loops.

Understand the limitations and use of break and continue statements in

loops.


Concept of a Loop


The two most important features of a computer are:

• The ability to compare values to make decisions, and

• The ability to repeat a series of operations in a controlled way.


Figure : The concept of a loop


Pretest and Post-Test

Loops


•Loops need to be coded in such a way as that they may be ended in a controlled manner.

•The decision making process make take place at either the ‘top’ of the loop or at the ‘bottom’.

•When the test takes place at the top of the loop, it is referred to as a ‘Pretest Loop’.

•When the test takes place at the bottom of the loop, it is then a ‘Post-test Loop’.


Pretest Loop

In each iteration, the loop control expression is tested first. If it is true, the loop action(s) is

executed; if it is false, the loop is terminated.

Post-test Loop

In each iteration, the loop action(s) are executed. Next, the loop control expression is tested. If it is

true, a new iteration is started; otherwise, the loop terminates.

Note:Note:


Figure : Pretest and post-test loops

“WHILE” test at the top…. “WHILE” test at the bottom

CONDITION

CONDITION


Figure : Two different strategies for starting exercise

CONDITION

CONDITION

False

True


Figure : Minimum number of iterations in two loops

It is important that you understand this critical difference between the two basic types…


Initialization and Updating


Figure : Loop initialization and updating

The initialization may optionally be part of the loop structure.


Figure : Initialization and updating for exercise


Event-Controlled and

Counter-Controlled Loops


Figure : Event-controlled loop concept


Figure : Counter-controlled loop concept


Loopsin C++


Figure : C++ loop constructs

• All three C/C++ loop constructs involve a test.

• The only difference is when the test occurs – either before or after doing something..


Figure : The while statement

while the expression is true, do …..


Figure : Compound while statement


Figure : for statement

The for is just a special case of a while loop.


The for loop is constructed in this way…

for ( initialization statement(s),

while test expression,

update statement(s) )

{

body of loop…

}


A 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 the for loop is easier to

read and more natural for counting loops.

Note:Note:


Figure : Compound for statement


Figure : Comparing for and while loops


In C/C++ the for loop counter is almost always initialized to a ZERO. So, if you want to do something 5 times, the for loop would be coded like this… for (int i = 0; i < 5; i++) {

do something… }It is also legal to have the test expression limit be a variable, as in: for (int i = 0; i < Num; i++)


Figure : Format of the do…while statement


Pre-test and post-test loops


Assignment

1. Write a program (WAP) in C++ to accept a number and print its table upto 10 terms.

2. WAP in C++ to print the following output using for loops:1223334444

3. WAP in C++ to accept a number and calculate its factorial.eg. If number = 5

Factorial of 5 = 120 should be displayed


The End

loops

Education

loop initialization

c loop

loop actions

loop constructs

body of loop

apretest loop

loop structure

aposttest loop