CS 117 Spring 2002

Repetition

Hanly Chapter 4

Friedman-Koffman Chapter 5

Flow Control

• Three types of program flow– sequential (what we did first)– selection (what we just looked at)

• if - else• switch

– repetition (Chapter 4)• while• do - while• for

Repetition

• There are many situations where you need to repeat the same actions over and over.– averaging a group of numbers– reading a file one line at a time– searching through a group of objects– repeating a calculation for different input

values

Why iterate?

Use the computer's speed to do the same task faster than if done by hand.

Avoid writing the same statements over and over again.

Repetitive control structures

– Because many algorithms require many iterations over the same statements.• To average 100 numbers, we would need

300 plus statements.• Or we could use a statement that has the

ability to repeat a collection of statements:

Sum 100 values the hard way

int sum = 0;

cout << "Enter number: "; // <-Repeat these three

cin >> number; // <- statements for each

sum = sum + number; // <- number in the set

cout << "Enter number: ";

cin >> number;

sum = sum + number;

Sum 100 values the hard way

/*

...291 statements deleted ...

*/

cout << "Enter number: ";

cin >> number;

sum = sum + number;

average = sum / 100;

2/15/02

• Questions about program 2

• Holiday Monday - building may be closed

• seminar Wed Feb 20

• using namespace

Loop Behaviorinitialize loop control variableexecute body of loopupdate loop control variablecontinue after loop

check loop control

var

FalseTrue

while loop

• Basic syntaxwhile (cond)

doSomething;• As in the if statement, the first statement after the

while is considered to be the body of the loop. Use { } to make it more than one statement.

• If the condition is false to begin with, the loop doesn’t execute at all.

Sentinel Loop

• Useful for finding the end of a list of numbers• if you know all valid data is positive, stop the loop

when a negative value is input

double data = 1.0while (data > 0.0) {

cin >> data;processData;}

readme

• your name, assignment

• instructions for using the program

• anything you did to try to get extra credit

• what you liked, disliked, had particular trouble with

• what you would like to do to make the program better

while loop

• Basic syntaxwhile (cond)

doSomething;• As in the if statement, the first statement after the

while is considered to be the body of the loop. Use { } to make it more than one statement.

• If the condition is false to begin with, the loop doesn’t execute at all.

5.4 Conditional Loops

In many programming situations, you will not be able to determine the exact number of loop repetitions

Conditional Loop– Initialize the loop control variable– While a condition involving the loop control

variable is true–   Continue processing

–   Update the loop control variable

Input loop

• reading data until the input endswhile (!cin.eof())

{

cin >> data;

processData;

}

Flag-Controlled Loops

char getDigit(){ char nextChar; bool digitRead; digitRead = false; while (!digitRead)

{ cin >> nextChar; digitRead = (('0' <= nextChar) &&

(nextChar <= '9')); } return nextChar;}

Counting Loop

• a loop that repeats a pre-defined number of times

int count = 0, maxCount=10;

while (count<maxCount)

{

doWhatever;

count = count +1;

}

for loop

• Counting loop so common there is a special loop statement to handle it.for (init; cond; update)

forBody;– init initializes the loop control variable– cond is the loop repetition condition– update updates the loop control variable

counting loop with for

• The counting loop from above becomes

for (count=0; count<10; count=count+1)

doWhatever;

A different loop orderinitialize loop control variableexecute body of loopupdate loop controlvariablecontinue after loop

check loop control

var

FalseTrue

do-while loop

• like while except loop control variable is checked after the body has executed char ch;do {

mainAction;cout << "Enter a character, q to quit ";cin >> ch;

} while (ch!='q');• the loop body always executes at least once.

loop cautions

while (cond);– has an empty body.

– Infinite loops: control variable doesn’t get changed so loop repeats forever

2/20/02

• Friday - last drop date

Program 3

• Program 3– Menu loop– Selection

• to select requested action

• converting between bearing and heading

– Alternate version slightly more difficult

Program 3

• Heading 0-360– N=0 = 360– S = 180– E = 90– W = 270

• Bearing – face north or south– degrees (0-90) to turn from

that direction– direction to turn - either

east or west

Loop Review :while

• Most commonly used when repetition is not counter controlled

• condition test precedes each loop repetition• loop body may not be executed at all

while (condition) dothis;

Loop Review: do-while

• Convenient when at least one repetition of loop body must be ensured

• test condition after execution of body

dothat;

while (condition);

Loop Overview

• Counting loop - number of repetitions – known ahead of time – can be controlled by a counter

• also convenient for loops involving non counting loop control with simple initialization and updates

• condition test precedes the execution

for (initialization; condition; modification)doSomething;

5.8 Nested Loops

Possible to nest loops – Loops inside other loops– Start with outer jump to inner loop– Inner loop continues until complete– Back to outer loop and start again

NestLoop.cpp and Triangle.cpp examples

Nested Logic

outer = 1;

while(outer <= 3)

{

inner = 1; // Resets for each iteration of the outer loop

while(inner <= outer)

{

cout << outer << " " << inner << endl;

inner = inner + 1;

} // End of the nested loop

outer = outer + 1; // Increment the outer loop counter

} // The end of the outer loop

increment and decrement operators

• For looping operations, you will frequently need to increment or decrement by 1.

• C++ has special operators, ++ and -- to do this.

– i++; is equivalent to i = i + 1;

– j--; is equivalent to j = j - 1;

Compound Assignment Operators

Lets look at the idea of adding together a group of numbers

Short hand notationtotalPay += pay;– same as

totalPay = totalPay + pay; See Table 5.2