Introduction to Computer Science

• Iteration–the while loop–the for loop

Unit 8Unit 8

8- 2

The while loop

• Just like in the robot world, Java needs mechanisms for repeating an action or group of actions while some condition is true (combination of looping and conditional checking)

while ( condition )

statement

8- 3

The while loop

• Repeatedly execute statement while condition is true

• statement is called the body of the loop; can be a simple statement (as always, terminated by a ;) or a compound statement surrounded by { and }

• Can get us into an infinite loop

while ( condition )

statement

class Temperature {// Print a table of corresponding C/F

temperatures

public static void main (String[ ] args) {

final double LOW_TEMP = -10.0,HIGH_TEMP = 10.0;

double cent, // The Centigrade temperature.

fahr; // The Fahrenheit temperature.

System.out.println(“DEGREES C\tDEGREES F“);

cent = LOW_TEMP;

while (cent <= HIGH_TEMP) {fahr = ( (9.0/5.0) * cent ) + 32.0;

//Convert C to F System.out.println(“\t” + cent + “\t\t” + fahr);

cent = cent + 1.0; //Increment the C value.

}}

}

8- 5

Resulting Output

• The \t is called an escape sequence; it represents the tab character, which it causes to be output (lining things up in columns)

• The entire { ... } is executed each time, before the condition is checked again

DEGREES C DEGREES F-10.0 14.0-9.0 15.8-8.0 17.6...

8- 6

One More Simple Example

• Print out the following song:10 in a bed and the little one said,

“Roll over, roll over.”They all rolled over and one fell out,9 in a bed and the little one said,

“Roll over, roll over.”They all rolled over and one fell out,8 in a bed and the little one said, ...1 in a bed and the little one said,

“Alone at last.”

8- 7

How Do WeDivide It Up for Iteration?

• One possible structure:10 in a bed and the little one said,

“Roll over, roll over.”They all rolled over and one fell out,9 in a bed and the little one said,

“Roll over, roll over.”They all rolled over and one fell out,8 in a bed and the little one said, ...1 in a bed and the little one said,

“Alone at last.”

Before Loop

}}}

Body ofLoop

Body ofLoop

Body ofLoop

After Loop

8- 8

Program is Organized as Follows

print first line of versewhile ( more verses ) {

print rest of verseprint first line of next verse

}print rest of last verse

10 in a bed and the little one said,

“Roll over, roll over.”They all rolled over and one fell out,

??? in a bed and the little one said,

“Alone at last.”

class TenInABed {// Print the nursery rhyme “Ten In a Bed.”

static final int MAX_NUMBER_IN_BED = 10;

public static void main (String[ ] args) {

int numberInBed;

System.out.println(MAX_NUMBER_IN_BED+ “ in a bed and the little one said,”);

numberInBed = MAX_NUMBER_IN_BED - 1;

while (numberInBed > 0) {System.out.println(“\t\”Roll over, roll over.\””);System.out.println(

“They all rolled over and one fell out,”);System.out.println(

numberInBed + “ in a bed and the little one said,”);

numberInBed = numberInBed - 1; }System.out.println(“\t\”Alone at last.\””);}

}

8- 10

The for loop

• This is a very common kind of loop, where the variable is incremented each time through the loop

cent = LOW_TEMP;while (cent <= HIGH_TEMP) {

fahr = ( (9.0/5.0) * cent ) + 32.0;//Convert C to F System.out.println(“\t” + cent + “\t\t” + fahr);

cent = cent + 1.0; //Increment the C value.}

So Java provides a special loop form that makes it easy to do: the for statement

8- 11

The for loop

• This is a very common kind of loop, where the variable is incremented each time through the loop

cent = LOW_TEMP;while (cent <= HIGH_TEMP) {

fahr = ( (9.0/5.0) * cent ) + 32.0;//Convert C to F System.out.println(“\t” + cent + “\t\t” + fahr);

cent = cent + 1.0; //Increment the C value.}

So Java provides a special loop form that makes it easy to do: the for statement

1. initialize2. condition

3. increment

8- 12

The for loop

• This is exactly equivalent to writing the following while statement:

for ( statement1; condition; statement2; )statement3

statement1;while ( condition ) {

statement3;statement2;

}

8- 13

The for loop

• This is exactly equivalent to writing the following while statement:

for ( statement1; condition; statement2; )statement3

statement1;while ( condition ) {

statement3;statement2;

}

initialize condition increment

8- 14

Rewriting Our Temperature Loop

for (cent = LOW_TEMP; cent <= HIGH_TEMP; cent =cent + 1.0;) {

fahr = ( (9.0/5.0) * cent ) + 32.0; //Convert C to F System.out.println(“\t” + cent + “\t\t” + fahr);}

8- 15

Comparison withOriginal While Loop

for (cent = LOW_TEMP; cent <= HIGH_TEMP; cent =cent + 1.0;) {

fahr = ( (9.0/5.0) * cent ) + 32.0; //Convert C to F System.out.println(“\t” + cent + “\t\t” + fahr);}

initialize condition increment

cent = LOW_TEMP;while (cent <= HIGH_TEMP) {

fahr = ( (9.0/5.0) * cent ) + 32.0;//Convert C to F System.out.println(“\t” + cent + “\t\t” + fahr);

cent = cent + 1.0; //Increment the C value.}

1. initialize2. condition

3. increment

8- 16

One Advantage

• It keeps all the loop controlling statements in one place, not spread throughout the loop

• For example, if we wanted to print the table in reverse order, all changes could be made at the top of the loop:

for (cent = HIGH_TEMP; cent >= LOW_TEMP; cent =cent - 1.0;) {

fahr = ( (9.0/5.0) * cent ) + 32.0; //Convert C to F System.out.println(“\t” + cent + “\t\t” + fahr);}

8- 17

do-while Loops

• Another form of loops in Java:

dostatement

while ( condition );

The body of the loop is always executed at least once; the condition is checked after the body of the loop. This is basically a convenience.

8- 18

In Other Words

• This is exactly equivalent to writing the following while statement:

dostatement

while ( condition );

statementwhile ( condition )

statement;

8- 19

Example: Count Number of Digits in an int

numberOfDigits = 0;rest = number;while (rest > 0) {

// The number of digits in number is numberOfDigits

// plus the number of digits remaining in rest

rest = rest / 10;numberOfDigits++;

}•What happens when number < 0?

•How do we fix it?

•What about when number = 0?

8- 20

One Solution:also works for number <= 0

numberOfDigits = 0;rest = number;do {

// The number of digits in number is numberOfDigits

// plus the number of digits remaining in rest

rest = rest / 10;numberOfDigits++;

} while (rest != 0) ;Loop is always executed once, even when the number = 0; notice the comment is unchanged.

8- 21

Example: Reading Input in a Loop

Enter score (eof ends the data): 85Enter score (eof ends the data): 62Enter score (eof ends the data): 93Enter score (eof ends the data): 87Enter score (eof ends the data): 51Enter score (eof ends the data): ^D or ^Z

5 scores were entered.The average score was 75.6The maximum score was 93The minimum score was 51

8- 22

The Structure of the Problem

Read a test scoreIf end of file, stop the loopProcess the test scoreRead a test scoreIf end of file, stop the loopProcess the test scoreRead a test scoreIf end of file, stop the loop…Process the test scoreRead a test scoreIf end of file, stop the loop

8- 23

Two Ways of Grouping the Iteration: First Way

Read a test scoreIf end of file, stop the loopProcess the test scoreRead a test scoreIf end of file, stop the loopProcess the test scoreRead a test scoreIf end of file, stop the loop…Process the test scoreRead a test scoreIf end of file, stop the loop

}}

while loop:test for end-of-file is at the beginning of each repeated section

8- 24

Second Way ofGrouping the Iteration

Read a test scoreIf end of file, stop the loopProcess the test scoreRead a test scoreIf end of file, stop the loopProcess the test scoreRead a test scoreIf end of file, stop the loop…Process the test scoreRead a test scoreIf end of file, stop the loop

}}

do-while loop:test for end-of-file is at the end of each repeated section

8- 25

Using the while Loop

read scorewhile ( not end of file ) {

process scoreread score

}

Short, clear, natural

8- 26

Using the do-while Loop

read scoreif ( not end of file )

do {process scoreread score

} while ( not end of file );

Less natural

8- 27

Checking end-of-file

• We use the class for this course, creating a SimpleInput object and sending it the eof( ) message, which returns true or false

import intro2cs.utils.*;class Readnumbers { public static void main (String[ ] args) {

int trial;SimpleInput sinp = new

SimpleInput(System.in);System.out.print(“Type an integer: ”);trial = sinp.readInt();if ( sinp.eof( ) )

System.out.print(“We’re at end of file.”);

else … }}

8- 28

So Our While Loop Looks Like This

read scorewhile ( !sinp.eof( ) ) {

process scoreread score

}

8- 29

What the Processing Looks Like

• We’ll have statements like:

numberOfScores++; // how many scoressumOfScores = sumOfScores + score; //update sum

• But for these variables numberOfScores and sumOfScores to be updated correctly within the loop, they need to be initialized correctly before the loop, to the value 0

8- 30

Loop Invariant

• A “loop invariant” is a statement that states the status of variables and their relationship during execution of the loop

• It is something that is supposed to be true every time we execute the loop (including the first time)

• We write it as a comment inside the loop

8- 31

So far, what do we have? (partial)

int numberOfScores = 0;int sumOfScores = 0;SimpleInput sinp = new

SimpleInput(System.in);

System.out.print(“Enter score (eof ends the data): “); score = sinp.readInt( );

while ( !sinp.eof( ) ) {numberOfScores++; //new scoresumOfScores = sumOfScores +

score; //update sum// numberOfScores is the number of scores

read// so far and sumOfScores is their sum System.out.print(“Enter score (eof

ends the data): “);score = sinp.readInt( );

}

8- 32

Afterwards

• Once the loop has ended, we can compute the average score simply as follows:

(double) sumOfScores / numberOfScores

• Why do we need to cast sumOfScores to a double? What do we need to check before we do this computation?

8- 33

What About Maximum and Minimum Scores?

• Consider how the invariant could be changed to reflect the fact that, in addition to numberOfScores and sumOfScores, we also have maxOfScores and minOfScores:

// The numberOfScores is the number// of scores read so far and sumOfScores is// their sum; maxOfScores is the largest// score and minOfScores is the smallest// score read so far

8- 34

In Java Code

• To maintain the truth of that invariant, we add the following to the loop:

if (maxOfScores < score) //new largest scoremaxOfScores = score;

if (minOfScores > score) //new smallest scoreminOfScores = score;

• We must also initialize these two new variables, so the invariant is always true. We initialize them to the first score read.

public static void main (String[ ] args) {int score;int sumOfScores = 0;int numberOfScores = 0;SimpleInput sinp = new

SimpleInput(System.in); System.out.print(“Enter score (eof ends the

data): “); score = sinp.readInt( );

int maxOfScores = score;int minOfScores = score;

while ( !sinp.eof( ) ) {numberOfScores++; //new scoresumOfScores = sumOfScores +

score; //update sumif (maxOfScores < score) //new largest

scoremaxOfScores = score;

if (minOfScores > score) //new smallest score

minOfScores = score; // numberOfScores is the number of scores read// so far and sumOfScores is their sum; maxOfScores

is// the largest score and minOfScores is the smallest// score read so far

System.out.print(“Enter score (eof ends the data): “);

score = sinp.readInt( );}

…

etc…

8- 36

The break Statement in Loops

• We saw the break statement before, in switch statements

• They can also be used to terminate the execution of while and for loops

• When a break is encountered during the execution of one of these loops, the loop ends immediately, and execution continues with the statement following the loop

8- 37

Example

while ( condition1 ) {statement1if ( condition2 )

break;statement2

}With this use of the break

statement, we can jump out of the loop in the middle. When is

this useful?

8- 38

The Loop-and-a-Half Problem

• Before, we wrote the loop:read scorewhile (! sinp.eof( ) ) {

process scoreread score

}

• But what we really wanted was to executeread scoreprocess score

as long as there are scores to process

8- 39

But we had to read score to find out that there were no more scores

• How about this?

while (true) {read scoreprocess score

}

Doing this an extra “half time”, quitting in the middle when read score fails (i.e., we hit end of file)

8- 40

So we can use break

• We might write it as follows:

while (true) {read scoreif ( sinp.eof( ) ) break;process score

}

That way, the “read” part of the “read-process” loop doesn’t get written down twice, before the loop and during the loop