COMP 14Introduction to Programming

Mr. Joshua StoughFebruary 16, 2005

Monday/Wednesday 11:00-12:15

Peabody Hall 218

Announcements

• Mid-Term Exam - March 9– covers Chapters 1-5– look at previous mid-term, homework

exercises, in-class exercises, words in bold print in textbook

• Monday– sit with your recitation group– each group should have laptop, textbook,

and lab manual

Loops• Allow us to repeat statements some number of

times

• Must use a loop control variable– controls how many times to loop

• 4 Parts to Every Loop– initialization - set loop control variable before

condition– condition - when to stop– update - change to the loop control variable– body - actions to repeat

Typical Uses of Loops

• Repeat a section of code a specified number of times - counter-controlled

• Repeat a section of code (reading input) until the a specific value is read - sentinel-controlled

• Repeat a section of code (reading input) until a valid value is entered - input validation

• Repeat a section of code (reading from a file) until the end of the file is reached - EOF-controlled

• Repeat a section of code until a boolean variable becomes false - flag-controlled

Questions

What type of while loop should be used?

1. Read every line in a file until the number -99 is read.

2. Read every line in a file where the length of the file is unknown.

3. Make sure that the user enters a lowercase letter.

4. Print the even numbers between 0 and 50.

sentinel-controlled

EOF-controlled

input validation

counter-controlled

The while Loop

while (expression)statement

The while Loop

final int LIMIT = 3;

int count = 0;

while (count < LIMIT) {

System.out.println (count);

count++;

}

System.out.println (“All done!”);

booleancondition

loopbodyupdate

initialization

Output:012All done!

In-Class Exercise

Write a while loop that prints2468

int i = 2;while (i <= 8) {

System.out.println (i);i += 2;

}

int i = 1;while (i <= 4) {

System.out.println (i*2);i++;

}

In-Class Exercise

Write a while loop that prints the odd numbers between 0-100.

int i = 0;while (i < 100) {

if ((i % 2) == 1) {System.out.println (i);

}i++;

} int i = 1;while (i <= 99) {

System.out.println (i);i+=2;

}

Question

What is the loop control variable in the following while loop?

int count = 0, num;num = Integer.parseInt(keyboard.readLine());while (num < 0) {System.out.println (num);num = Integer.parseInt(keyboard.readLine());count++;

}

num

Today in COMP 14

• The for loop• The do...while loop• break and continue• Nested loops

• Textbook Reference: Ch 5 (pgs. 229-247)

The for Loop

• Specialized form of while loop• Simplifies the writing of count-controlled loops

Basic Form:

for (initialization; condition; update) {statement(s);

}

The for LoopExecution

1. initial statement executes

2. loop condition evaluated

3. If loop condition evaluates to true, execute for loop statement and execute update statement

4. Go back to step 2 and repeat until loop condition is false

The for LoopSyntax

for ( initialization; condition; update ) { loop body;}

Reservedword

The initializationis executed once

before the loop begins

The loop body isexecuted until the

condition becomes false

The update portion is executed at the end of each iterationThe condition-loop body-update cycle is executed repeatedly

for vs. while

A for loop is functionally equivalent to the following while loop structure:

initialization;while ( condition ){ loop body; update;}

The for LoopExample

final int LIMIT = 3;

int count;

for (count=0; count<LIMIT; count++) {

System.out.println (count);

}

System.out.println (“All done!”);

booleancondition

loop bodyupdate

initialization

Output:012All done!

Comparing while and for

While Loop

final int LIMIT=3;

int i = 0;

while (i < LIMIT) {

System.out.println (i);

i++;

}

System.out.println

(“All done!”);

For Loop

final int LIMIT=3;int i;

for (i=0; i<LIMIT; i++) {System.out.println (i);

}

System.out.println("All done!");

The for Loop

• Like a while loop, the condition of a for statement is tested prior to executing the loop body

• The body of a for loop will execute zero or more times

• It is well suited for executing a loop a specific number of times that can be determined in advance

• Anything you can do in a for loop, you can do in a while loop

The for Loop

• Does not execute if initial condition is false

• Update expression changes value of loop control variable, eventually making it false

• If loop condition is always true, result is an infinite loop

• for statement ending in semicolon is empty; does not affect program

for (count=0; count<LIMIT; count++);

Examples

for (i=1; i<=5; i++) {System.out.println ("Hello");System.out.println ("*");

}

for (i=1; i<=5; i++)System.out.println ("Hello");System.out.println ("*");

for (i=1; i<=5; i++);System.out.println ("*");

Hello*Hello*Hello*Hello*Hello*

HelloHelloHelloHelloHello*

*

Multiples.java Example

• Ask the user for a positive number• Ask the user for a limit• Print all of the multiples of the

positive number up to limit

• What happens if the user enters a negative number?

In-Class Exercisefor vs. while Loopsfinal int MAX = 20;

int i;

for (i = 0; i<MAX; i+=3) {

System.out.println (i);

}• Predict the output• Translate this to a while loop.

Output:0369121518

final int MAX = 20;int i = 0;while (i < MAX) {

System.out.println (i);i += 3;

}

The do…while Loop

• Statements executed first, then expression evaluated

• Statement(s) executed at least once then continued if expression is true

• Basic Form:

do { statement(s);} while (expression);

The do...while LoopSyntax

do{ loop body;}while ( condition );

do andwhile arereserved

words

The loop body is executed once initially,and then the condition is evaluated

The loop body is executed repeatedlyuntil the condition becomes false

do…while Loop(Post-test Loop)

do { statement(s);} while (expression);

The do...while Loop

• Like a while loop, but its condition is at the end of the loop

• Loop body always executes at least once

• Must also be checked for termination (not an infinite loop)

• Anything you can do with a do...while loop, you can do with a while loop

The do...while LoopExamplefinal int LIMIT = 3;

int count = 0;

do {

System.out.println (count);

count++;

} while (count < LIMIT);

System.out.println (“All done!”);

booleanconditio

n

loopbodyupdat

e

initialization

Output:012All done!

Comparing while and do...while

while Loop

final int LIMIT=3;int count = 0;

while (count < LIMIT) {System.out.println (count);count++;

}

System.out.println (“All done!”);

do...while Loop

final int LIMIT=3;int count = 0;

do {System.out.println (count);count++;

} while (count < LIMIT);

System.out.println("All done!");

while vs. do...while

i = 11;while (i <= 10) {

System.out.print (i + " ");i += 5;

}System.out.println();

i = 11;do {

System.out.print (i + " ");i += 5;

} while (i <= 10);System.out.println();

[blank line]

11

In-Class ExerciseThe do...while Loop

int x = 0, y = 0;

do {

System.out.println (x*y);

if (y < x) {

y += 2;

}

x++;

} while (x < 5);

• Predict the output of the loop

x

y0

0

Output:

1 2

004616

23

4

45

break Statements

• Used to exit early from a loop • Used to skip remainder of switch

structure• Can be placed within if statement

of a loop– If condition is met, loop exited

immediately

break Example

int i;for (i=0; i<5; i++) {

System.out.print ("i = ");if (i == 2) {

break;}System.out.println (i);

}

i = 0i = 1i =

continue Statements• Used in while, for, and do...while

structures• When executed in a loop, the remaining

statements in the loop are skipped; proceeds with the next iteration of the loop

• When executed in a while/do…while structure, expression evaluated immediately after continue statement

• In a for structure, the update statement is executed after the continue statement; then the loop condition executes

continue Exampleint i;for (i=0; i<5; i++) {

System.out.print ("i = ");if (i == 2) {

continue;}System.out.println (i);

}

i = 0i = 1i = i = 3i = 4

int i=0;while (i<5) {

System.out.print ("i = ");if (i == 2) {

continue;}System.out.println (i);i++;

}

i = 0i = 1i = i = i = ...(infinite loop)

Nested Control Structures• Provides new power, subtlety, and

complexity• if, if…else, and switch

structures can be placed within while loops

• for loops can be found within other for loops – each time through the outer loop, the

inner loop goes through its full set of iterations

Nested Control Structures Examplefor (int row = 1; row <= 5; row++) {for (int star = 1; star <= row; star++) {

System.out.print(“*”);}System.out.println();

}Output:***************

• Can't use the variable row outside the outer for loop.• Can't use the variable star outside the inner for loop.

Flips.java Example

• Flip a coin 5 times– choose a random number

• Print the total number of heads and tails

Loop Features

loop

initialization

(outside / header)

number of loop body

executions

update(inside / header)

conditional

(beginning / end)

while outside 0 or more

inside beginning

do outside 1 or more

inside end

for header 0 or more

header beginning

Choosing a Loop Structure• while

– you don't know how many times to execute the loop body

• do...while– you don't know how many times to execute

the loop body, but it's at least once

• for– you know exactly how many times to

execute the loop body

Loops

4 parts of every loop

1. initialization2. condition3. loop body4. update

Orderwhile and for

1. initialization2. condition3. loop body4. update

do...while1. initialization2. loop body3. update4. condition

Program 3

• Rock-Paper-Scissors-Spock-Lizard– variation on Rock-Paper-Scissors

• Have the computer choose one of the 5 shapes.– Hint: Math.random

• Ask the user to choose one of the 5 shapes or 'q' to quit.

• Decide who wins.• After the user quits, print out a tally of

the user wins, losses, and ties.

Program 3

• You must print the title of the game.• You must make sure the user knows

what to enter to choose a particular shape.

• Allow the user to enter invalid input and prompt them for valid input.

• You do not have to use letters for selecting the shapes.

• Start early!

Program 3Break Program into Smaller Steps1. Print out welcome message, options, and

directions to user2. Write a loop that allows the user to enter

a character and end the loop if the user enters 'q' (or 'Q')

3. Write code that will print out the shape (rock, paper, ...) based on what the user entered

4. Write code that will make the computer choose a shape

5. Write code that will print out what shape the computer chose

Program 3Break Program into Smaller Steps6. Write code that decides if the

computer or the user won7. Write code that will keep up with the

number of wins, losses, and ties for the user

8. Write code that will print out the number of wins, losses, and ties after the user chooses to quit

Math.random Example

double randomNum; int sample012;

// Math.random() will return [0.0, 1.0) randomNum = Math.random();

// sample012 will be {0, 1, 2} sample012 = (int) (randomNum * 3);

// print out {-1, 0, 1} System.out.println(sample012 - 1);

Print a random number in {-1, 0, 1}.

System.out.println ((int) (Math.random() * 3) - 1);

In-Class ExerciseRandom

Write a code fragment to choose a random number in {0, 1, 2, 3}.

Print 'A' if the number chosen is 0, 'B' if the number is 1, 'C' if the number is 2, and 'D' if the number is 3.– Hint: a switch statement would be

appropriate here

sample = (int) (Math.random() * 4);

int sample;

// sample will be {0, 1, 2, 3}sample = (int) (Math.random() * 4);

switch (sample) {case 0:

System.out.println("A");break;

case 1:System.out.println("B");break;

case 2:System.out.println("C");break;

case 3:System.out.println("D");break;

}

Next Time in COMP 14

• Programming Session

• Sit with your recitation group

• Each group should have a laptop, textbook, and lab manual