CS303E Loops 1

CS 303E Lecture 8:

LoopsSelf-referentially, short for GNU's not UNIX, a Unix-compatible software system developed by the Free Software Foundation (FSF). Which could be expanded to GNU’s not UNIX’s not UNIX,which could be expanded to GNU's not UNIX’s not UNIX’s not UNIX, whichcould be expanded to…….

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Iteration – a Basic OperationExample of iteration – building a fence:

Move tools and materials to starting point.

Install first fence post.

Repeat until finished:

Move to position of next fence post.

Install next fence post.

Install fencing between new post

and previous post.

LoopBody ofloop isrepeated.

Initialization

Terminationcondition

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while Loop — Syntax

while ( boolean_expression )statement;

or

while ( boolean_expression ){ statement;

. . .}

Continuationcondition

Loop body

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Syntax and Semantics ofwhile Statements

while (<Boolean expression>) <statement>

while (<Boolean expression>){ <statement 1> . <statement n>}

?

statement

true

false

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while Loop for Building Fence

Move tools and materials to starting point.

Install first fence post.

while (not finished)

{ Move to position of next fence post;

Install next fence post;

Install fencing between new post

and previous post;

}

Continuationcondition

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Loops — two basic patternsCount-controlled loop: Number of iterations is determined before

the loop starts. Counts each iteration in a counter variable. Stops when the desired number of iterations

has been performed.

Event-controlled loop: Before each iteration, checks to see whether

some event has occurred. Continues until that event occurs. Number of iterations not known beforehand.

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Count-controlled loop pattern

counter = initialValue;other initialization;while ( counter <= finalValue ){ do computation;

increment counter;}

Number of iterations is:finalValue – initialValue + 1

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Designing Correct Loops Initialize all variables properly

• Plan how many iterations, then set the counter and the limit accordingly

Check the logic of the termination condition

Update the loop control variable properly

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Count controlled loop exampleSpecification: Sum the squares of the first 10 positive

integers.

Algorithm:int sum = 0; // initialize sum so far

int count = 1; // initialize counter

while ( count <= 10 ) // 10 = final value

{ sum = sum + (count * count); // do computation

count = count + 1; // increment counter

}

sumField.setNumber(sum);

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Off-by-One Errorint counter = 1;while (counter <= 10){ //Executes 10 passes <do something> counter++;}

int counter = 1;while (counter < 10){ //Executes 9 passes <do something> counter++;}

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Example – Sum powers of 2

Specification: Sum the first max powers of 2: 2 + 4 + 8 + . . .

Algorithm in pseudo-code:

1. Get value for max;

2. Set count to 1, sum to 0, power to 1;

3. While ( count <= max )

{ a. Multiply power by 2; // 2, 4, 8, . . .

b. Increment sum by power; // 2 + 4 + 8 + . . .

c. Increment count by 1; // 1, 2, . . ., max

}

4. Display sum;

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Code – Sum powers of 2

Specification: Sum the first max powers of 2: 2 + 4 + 8 + . . .

Code:

max = maxField.getNumber();

int count = 1, sum = 0, power = 1;

while ( count <= max )

{ power = power * 2; // 2, 4, 8, . . .

sum = sum + power; // 2 + 4 + 8 + . . .

count = count + 1; // 1, 2, . . ., max

}

sumField.setNumber (sum);

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Tracing the loop

Suppose max = 5.

We want to calculate sum = 2 + 4 + 8 + 16 + 32.

max count power sum

Before loop starts: 5 1 1 0

After iteration 1: 5 2 2 2

After iteration 2: 5 3 4 6

. . .

After iteration 5: 5 6 32 62

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Exercises

Sum the odd numbers from 1 to 10. Summing from 1 to N is easy. (Thanks to

Gauss)

2)1(1

nnin

i

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Infinite Loopint counter = 1;while (counter <= 10){// Executes 5 passes <do something> counter = counter + 2;}

int counter = 1;while (counter != 10){// Runs forever <do something> counter = counter + 2;}

In general, avoid using != in loop termination conditions.

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Event-controlled loop patterninitialization;while ( event has not occurred ){ do work; }Work must eventually cause event to occur.

HelloWorld in an Event Driven Programming World

while(true){System.out.print(“Hello World!! “);

}

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Example – Sum powers of 2

Specification: Sum the powers of 2 up to a power <= limit.

Algorithm in pseudo-code:

1. Get value for limit;

2. Set sum to 0, power to 2;

3. While ( power <= limit )

{ a. Increment sum by power; // 2 + 4 + 8 + . . .

b. Multiply power by 2; // 4, 8, 16, . . .

}

4. Display sum;

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Code – Sum powers of 2

limit = inputField.getNumber();

sum = 0;

power = 2;

while ( power <= limit )

{ sum = sum + power; // 2 + 4 + 8 + . . .

power = power * 2; // 4, 8, 16, . . .

}

outputField.setNumber (sum);

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Mixing the two types of loops

Specification: Sum the first powers of 2, so long as power <= limit but not more than 10 powers.

max = maxField.getNumber();

limit = limitField.getNumber();

sum = 0; power = 2; count = 1;

while ( power <= limit && count <= max )

{ sum = sum + power; // 2 + 4 + 8 + . . .

power = power * 2; // 4, 8, 16, . . .

count = count + 1; // 2, 3, 4, . . .

}

outputField.setNumber (sum);

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CS 303E Lecture 9:Loops II

"An apprentice carpenter may want only a hammer and saw, but a master craftsman employs many precision tools. Computer programming likewise requires sophisticated tools to cope with the complexity of real applications,and only practice with these tools will build skill in their use."- Robert L. Kruse, Data Structures and Program Design

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Prime Numbers

Write a program to determine if a given number is prime.

Analysis / Design (algorithm)

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Prime Number Implementation

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Other Loops

Java provides syntax for two other types of loops

do - while loop for loop Any of these could be rewritten as a while loop.

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do-while Loop Syntax

do

{ //body of loop

First_Statement;

...

Last_Statement;

} while(Boolean_Expression); Initialization code may precede loop body

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do-while Loop

Loop test is after loop body so the body must execute at least once (minimum of at least one iteration)

May be either count controlled or event controlled loop• Good choice for event controlled loop

Something in body of loop should eventually cause Boolean_Expression to be false

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do-while Exampleint count = 1;int number = 10;do //Display integers 1 - 10 on one line

{ System.out.print(count + “, “ ); count++;}while(count <= number);

Note System.out.print() is used and not System.out.println() so the numbers will all be on one line

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for loop

for — A simpler count-controlled loop.The following two loops are equivalent:

i = 1;while ( i <= n ) for ( i = 1; i <= n; i = i+1 )

{ work { work i = i + 1; }}

Still need while for event-controlled loops and mixed loops (count- and event-controlled).

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for Loop Syntax and Semanticsfor (initialization statement; termination condition; update statement) loop body statement(s)

terminationcondition

loop bodystatement(s)

true

falseinitializationstatement

updatestatement

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for Example

Compute and output first 10 integers and their squares:

for (int i = 1; i <= 10; i++) System.out.println(i + " " + i*i + "\n");

initialization statement int i = 1; termination condition i <= 10update statement i++ (or i = i + 1)

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for loop Initialization, loop test, and loop counter change are part of the

syntax. Contrast with while loop.

Execution sequence:1. Initialization- executes only the first iteration of the loop

2. Boolean_Expression - the loop test

3. loop body - execute only if loop test is true

4. After_Loop_Body - typically changes the loop counter

5. Boolean_Expression - Repeat the loop test (step 2), etc.

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Another for Example

Count down from 9 to 0for(int count = 9; count >= 0; count--)

{

System.out.print("T = " + count);

System.out.println(" and counting");

}

System.out.println("Blast off!");

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Testing Loops

Check with small values or boundary values (1, 0, -1, limit).

Find a formula for the n-th iteration. Display a trace if things aren’t working.

• System.out.println(“x = “ + x + “ y = “ + y);where x and y are the variables of concern.

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A Tester Programimport BreezyGUI.*;

public class Tester{

public static void main(String[] args){ int counter = 1; while (counter <= 10){ System.out.println("Counter = " + counter); counter = counter + 2; } GBFrame.pause(); }

}

A non-GUI program for trying things out quickly.

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Text Areas Another window component.

TextArea areaName = addTextArea (

“initial value”, row, col, width, height); Like a text field, except has scroll bars and can

display any width and number of rows of text. May want width and/or height > 1. A part of the Java awt, as is Label, TextField,

Button. BreezyGUI just makes them easier to use

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TextArea methods

setText (aString) -- void (no value returned).

getText () -- returns String.

append (aString) -- void (no value returned).

Appends aString to the end of the TextArea.

Remember, to go to next line you need \n

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Output 10 Numbers to a Text Area

TextArea outputArea = addTextArea("", 1,1,2,5);

for (int i = 1; i <= 10; i++) outputArea.append(i + "\n");

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Output 10 Numbers to a Text Area

TextArea outputArea = addTextArea("", 1,1,2,5);

for (int i = 1; i <= 10; i++) outputArea.append(i + "\n");

// or

String outputStr = "";for (int i = 1; i <= 10; i++) outputStr = outputStr + i + "\n";outputArea.setText(outputStr);

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Method Format.justifyFormat is part of the BreezyGUI package

Justify data (left, right, or centered) in a field of width characters, with precision decimal places.

String justify (char alignment, // ‘l’ | ‘c’ | ‘r’type data, // data to be justifiedint width, // field widthint precision) // decimal places (double only)

type = String or char or long or double

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Format.justify

Format.justify(<a letter>, <a string>, <columns>)

Format.justify(<a letter>, <an int>, <columns>)

Format.justify(<a letter>, <a double>, <columns>, <precision>)

These methods return String objects.

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Format.justifyFormat.justify(<a letter>, <a string>, <columns>)

Format.justify(<a letter>, <an int>, <columns>)

Format.justify(<a letter>, <a double>, <columns>, <precision>)

Format.justify('l', 34, 4) "34 "

Format.justify('r', 34, 4) " 34"

Format.justify('c', 34, 4) " 34 "

Format.justify('r', 34.346, 7, 2) " 34.35"

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The Two Versions

while (counter <= 20){ sum = sum + counter; output.append(Format.justify('r', counter, 5) + Format.justify('r', sum, 10) + "\n"); counter++;}

while (counter <= 20){ sum = sum + counter; output.append(counter + " " + sum + "\n"); counter++;}

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Justification of Text Output

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Some practical considerationswhen using loops

The most common loop errors are unintended infinite loops and off-by-one errors in counting loops

Sooner or later everyone writes an unintentional infinite loop• To get out of an unintended infinite loop enter ^C (control-C)

Loops should tested thoroughly, especially at the boundaries of the loop test, to check for off-by-one and other possible errors• "Tracing" a variable (outputting its value each time through the

loop) is a common technique to test loop counters and troubleshoot off-by-one and other loop errors