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? t Use the computer's speed to do the same task faster than if done by hand. t 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 /* 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 Behavior

while loop Basic syntax while (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 = 1.0 while (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 syntax while (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 t In many programming situations, you will not be able to determine the exact number of loop repetitions t 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 ends while (!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 order

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 –Menu loop –Selection to select requested action converting between bearing and heading –Alternate version slightly more difficult

Program 3 Heading –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 do that; 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 t 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 t 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 t Lets look at the idea of adding together a group of numbers t Short hand notation totalPay += pay; –same as totalPay = totalPay + pay; t See Table 5.2