Branching and Loops

BRANCHING AND LOOPS

Introduction• C Language supports the following decision-making

statements:• if statement• switch statement• Conditional operator statement• while statement• for statement

In this lecture we study do while, for and switch statements.

The For Loop and Comma OperatorThe comma operator extends the flexibility of the for loop by enabling you to include more than one initialization or update expression in a single for loop specification

a program that prints first-class postage rates

Comma Operator

The program uses a for loop to print the "Powers of 2" table for the power 0 to 20, both positive and negative.

The program evaluates the value successively by multiplying 2 by itself n times and .Note that we have declared p as a long int and q as a double.

Zeno Meets the for Loop• The Greek philosopher Zeno once argued that an arrow

will never reach its target. First, he said, the arrow covers half the distance to the target. Then it has to cover half of the remaining distance. Then it still has half of what's left to cover, ad infinitum. Because the journey has an infinite number of parts, Zeno argued, it would take the arrow an infinite amount of time to reach its journey's end.

• Let's take a quantitative approach and suppose that it takes the arrow 1 second to travel the first half. Then it would take 1/2 second to travel half of what was left, 1/4 second to travel half of what was left next, and so on. You can represent the total time by the following infinite series:

1 + 1/2 + 1/4 + 1/8 + 1/16 +....

The short program finds the sum of the first few terms

1 + 1/2 + 1/4 + 1/8 + 1/16 +....

Nested Loops• A nested loop is one loop inside another.• A common use for nested loops is to display data in rows and columns.

A Nested VariationThe inner loop may behave differently each cycle depending on the outer loop.

An Exit-Condition Loop: do whileThe while loop and the for loop are both entry-condition loops.

First do the loop and the check an exit condition.

Structure of a do while loopThe general form of the do while loopdo

statement

while ( expression );

You should restrict the use of do while loops to cases that require at least one iteration:do {

prompt for password read user input } while (input not equal to password); Avoid a do while structure of the type shown in the following pseudocode:do {

ask user if he or she wants to continue some clever stuff

} while (answer is yes);

Nested do while loopA program to print the multiplication table from 1 x 1 to 12 x 10

This program contains two do.... while loops in nested form. The outer loop is controlled by the variable row and executed 12 times. The inner loop is controlled by the variable column and is executed 10 times, each time the outer loop is executed. That is, the inner loop is executed a total of 120 times, each time printing a value in the table.

Summary: The do while Statement• The do while statement creates a loop that repeats until

the test expression becomes false or zero.do statement while (expression); Example:do scanf("%d", &number); while (number != 20);

Which Loop?• First, decide whether you need an entry-condition loop or an exit-condition

loop. • Assume that you need an entry-condition loop. Should it be a for or a while?

for ( ;test; ) while (test)

initialize; while (test) { body; update; }

for (initialize; test; update) body;

• A while loop is natural for the following conditionwhile (scanf("%ld", &num) == 1)

• The for loop is a more natural choice for loops involving counting with an index:for (count = 1; count <= 100; count++)

Loop flow controls: break; and continue;• C uses two different orders to control loop’s flow

• break – escapes from the nearest outer loop• continue –

• inside “while” and “do” loop: switches program execution to test condition, • inside “for” loop: switches program execution to “for” loop step and then to

condition test (also applies for nearest outer loop)

Possible algorithm enhancements (decreasing number of loop’s iterations/repeats):

• It is enough to loop n/2 times, better, only till sqrt(n).

• Test if the number is dividable with 2, and if it isn’t, test inside loop if the number is dividable with odd numbers bigger than 2.

The example illustrates the use of the break statement in a C program.• The program reads a list of positive values and calculates their average. The for loop is

written to read 1000 values. However, if we want the program to calculate the average of any set of values less than 1000, then we must enter a 'negative' number after the last value in the list, to mark the end of input.

The example illustrates the use of continue statement.

The program evaluates the square root of a series of numbers and prints the results. The process stops when the number 9999 is typed in.

Branching condition order switch – caseswitch( expression ){

case const_expression1: orders1;case const_expression2: orders2; …

default : ordersN;}

Used instead of multisided if selection

Pay attention: if the keyword break isn’t stated inside case block; program continues to next case block in the list!

Example

If break; was to be left-out from every case block, for given grade 3 (example), the result would be false and following:

Review questionsWhat errors can you find?

Review questionsWhat will each of the following programs print?

Review questionsGiven the input "Go west, young man!", what would each of the following programs produce for output?