Loops

After studying this chapter you will be able to: O BJECTIVES  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.

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

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:

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…

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

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:

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

Assignment Write a program (WAP) in C++ to accept a number and print its table upto 10 terms. WAP in C++ to print the following output using for loops: 1 22 333 4444 WAP in C++ to accept a number and calculate its factorial. eg. If number = 5 Factorial of 5 = 120 should be displayed

Loops

Loops

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Loops