Chapter 8 Repetition Statements

Spring 2014

Chapter 8

Repetition Statements

Introduction

Iteration - process of looping or the repetition of one or more statements

Loop body - the statement, or statements, that will be repeated

8.1 General Repetition Concepts

Two different categories of loops• Pre-test: Test condition first - complete the

body of the loop if condition is true

• Post-test: Test condition after body has executed once - body will only be repeated if the condition is true

8.1 General Repetition Concepts

Start Loop Start Loop

EvaluateCondition

Body of Loop

Body of Loop

EvaluateCondition

End Loop End Loop

True

False

True

False

Pre-TestLoop

Post-TestLoop

8.1 General Repetition Concepts

Looping structures must have the following:• Variable with an initial value that controls

whether the body of the loop is executed (i.e., control variable)

• Conditional expression involving control variable

• Statement within the body where control variable is modified each time the body of the loop is executed

8.1 General Repetition Concepts

Infinite loop – a loop that continuously executes - the program or loop has to be terminated by the user or programmer• Usually an error situation

• Most common cause - failure to manipulate the control variable

• In Windows - pressing Ctrl + C causes program to stop execution

8.1 General Repetition Concepts

Nested loop - loop embedded in another loop

Almost any statement can be placed in loop body - including other loops or conditional statements

Once finished, flow transfer's to the next statement following the loop

8.1 General Repetition Concepts

Three loop structures

while loop:

while (<condition>) <action>

do while loop:

do <action> while <condition>;

for loop:for (<init-expr>; <condition>; <post-loop-expr>) <action>

8.2 The while Loop

Pre-test loopSyntax:

while ( <condition> )<action>

• Action, or body, will continue to execute while the condition remains true

• If the body needs to include multiple statements, surround them (action) with curly braces

8.2 The while Loop

Sometimes see a loop written as: while ( 1 ) { ... }

The numeric literal 1 is treated as a true condition and never changes – causing an infinite loop

8.2 The while Loop

while ( 1 ) { ... }

• Requires additional statement within the body of the loop to terminate it

• Although there are reasons to use this type of loop, it should only be used by experienced programmers and then only with due consideration

8.2 The while Loop

char again = '\0';int operand1 = 0, operand2 = 0;

cout << "\nDo you wish to multiply two numbers (y/n)? ";cin >> again; // Priming read

while ( again == 'y' || again == 'Y' ) // Notice no semicolon{ // Start of the loop body (action) cout << "Enter first number: "; cin >> operand1;

cout << "Enter second number: "; cin >> operand2;

cout << "Result: " << operand1 << " * " << operand2 << " = " << operand1 * operand2 << endl; // Don’t forget to change the control variable cout << "\nDo you wish to multiply two more numbers (y/n)? "; cin >> again; } // End of the loop body (action)cout << "The End" << endl;

8.2 The while Loop

Priming read - a prompt prior to encountering a loop• Ensures the control variable has a user

provided value before evaluating the condition

• Must be a corresponding prompt as the last statement in the body of the loop

8.3 The do-while Loop

Post-test loop (the body of the loop always executes at least once)

Syntax:do<action>

while ( <condition> );

If the body needs to include multiple statements, surround them with curly braces

8.3 The do-while Loop

char menu_choice; float number;cout << "Please enter a number: ";cin >> number;do{ cout << "\n1) Square the Number\n" << "2) Cube the Number\n" << "3) Exit\n\n" << "Please enter menu choice: " << endl; cin >> menu_choice; switch ( menu_choice ) { case '1': cout << number << " Squared = " << number * number << endl; break; case '2': cout << number << " Cubed = " << number * number * number << endl; break; case '3': cout << "Goodbye" << endl; break; default: cout << "Invalid menu option" << endl; } } while ( menu_choice != '3' ); // Notice the semicolon

8.4 The for Loop

Generally used when the specific number of iterations are known

Both while loops and for loops are pre-test loops - could be used interchangeable

8.4 The for Loop

Syntax:

for ( <init-expr> ;

<condition> ;

<post-loop-expr> )

<action>

8.4 The for Loop

for ( <init-expr> ;

<condition> ;

<post-loop-expr> )

<action>

Four sections:• Requires the two semicolons (;) and the action• Can include multiple statements separated by

commas placed within both the <init-expr> and <post-loop-expr> sections

8.4 The for Loop

Order in which the parts are evaluated or executed:1.<init-expr>2.<condition>3.<action> (if true condition)4.<post-loop-expr>5.<condition>

The <init-expr> section is only executed once

8.4 The for Loop

Variable(s) declared in <init-expr> section only visible within the body of the loop

Not all compilers adhere to this standard - but you should

8.4 The for Loop

Common errors students make - put a semicolon after the closing parenthesis• Although legal, semicolon terminates the loop

• In effect - takes the place of the body of the loop

• Often a difficult error to find

8.4 The for Loop

for ( int i = 0; i < 5; i++ ) cout << i << ' ';// Output0 1 2 3 4

Start for loop

Initialize control variable(i = 0)

Check conditionIs i < 5?

Execute body of loop (Display contents of i)

Terminate for loop

False

True

Manipulatecontrol variable

(i++)

8.4 The for Loop

// Example 1for ( int i = 0; i < 5; i++ ) // No semicolon cout << i << endl;

// Example 2// Notice the multiple expressionsfor ( int i = 0, j = 5; i < 5; i++, j-- ) cout << i << ' ' << j << endl;// Example Output// Ex. 1 // Ex. 2 0 0 5 1 1 4 2 2 3 3 3 2 4 4 1

8.4 The for loop

int sum = 0, value;for ( int i = 0; i < 5; i++ ) // No semicolon{ cout << "Enter value " << i + 1 << ": "; cin >> value;

sum += value;} cout << "The sum of the five values is: " << sum << endl;// Example OutputEnter value 1: 4Enter value 2: 7Enter value 3: 3Enter value 4: 12Enter value 5: 99The sum of the five values is: 125

8.4.1 Nested for Loops

for loops can be nested

• Used in many algorithms

• Important when using multi-dimensional arrays

8.4.1 Nested for Loops

for ( int row = 0; row < 5; row++ ){ for ( int col = 0; col < 5; col++ ) cout << col << ' ';

cout << endl;}// Example Output0 1 2 3 40 1 2 3 40 1 2 3 40 1 2 3 40 1 2 3 4

Start for loop

Initialize control variable(row = 0)

Check condition

Is row < 5?

Initialize control variable(col = 0)

Check condition

Is col < 5?

Execute body of inner loop

Display contents ofcol

Print newline

End for loop

True

True

FalseChange

control variablecol++

ChangeControl variable

row++

False

8.4.1 Nested for Loops

for ( int row = 0; row < 5; row++ ){ for ( int col = row; col < 5; col++ ) cout << col << ' ';

cout << endl;}// Example Output0 1 2 3 41 2 3 42 3 43 44

8.5 Breakpoints and Loops

Debugging loops:• Time consuming and annoying if necessary to

step through the loop

• Breakpoints add functionality to break or stop execution, based upon a condition or after a line of code has been encountered a certain number of times

8.5 Breakpoints and Loops

Right clicking on a breakpoint shows popup menu that allows setting breakpoint properties

8.5 Breakpoints and Loops

The Breakpoint Condition allows setting a condition that when true, stops the loop

8.5 Breakpoints and Loops

Can also stop execution after a breakpoint has been encountered a specific number of times

8.7 C – The DifferencesIllegal in C to declare variables in the

initialization section of a for loop

All variables in C must be declared at the beginning of a block, right after an opening curly brace

C99 specifications allow the C++ conventions for variable declarations

8.7 C – The Differences

#include <stdio.h>int main(){ //Legal in both C++ and C: int count; for ( count = 0; count < 5; count++ ) printf( "%d\n", count );

// Legal in C++ (and C99) but not in older C: // for ( int i = 0; i < 5; i++ ) // cout << i << endl;

return 0;}