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CS 177 Week 5 Recitation Slides

Loops

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Announcements Project 2 due next Thursday at 9PM. Exam 1 this evening (switch and loop not

covered) Old exams posted

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QUESTIONS???

While Loops

Uses repetition to perform a lot of work very fast They look a lot like if statements:

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int x = 0;if(x < 5){ /*do work using statements*/}

int x = 0;while(x < 5){ /*do work using statements*/ }

When here, finishedWhen here, loop back upAnd test again…

x += 1;

Need to assurea statement isin place to cause the expressionto eventuallyevaluate to false

While

Braces are optional for a single statementwhile (i < 10)

sum += i++;

As with ifs, only when the expression is true will the statements within the while block be executed.

The big difference is that when the last statement of a while is executed (if the expression was first true), the expression will be re-evaluated.

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MUST BE CAREFUL that the condition will depend on a value that changes as the loop progresses or a “break” will be needed (to be discussed).

Small contrived exampleboolean value = true;

//program statements…

while (value)

{

System.out.println(“If here, value is true!”);

value = false;

System.out.println(“value can change…”)

System.out.println(“But ONLY matters at the end”);

value = true;

}

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With value set to true in the endit will loop forever

boolean value = true;

//program statements…

int count = 0;

while (value)

{

System.out.println(“If here, value is true!”);

value = false;

//… lots of work

value = true;

count++;

}

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if (count > 10)

value = false;

This causes the loop to stop…

After how many iterations???i.e. how many times willIf here, value is true!print?

Keep multiplying input numbers until a sentinel reached

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int i = 1;int product = 1;

while( i != 0 ){product *= i;System.out.print(“Enter number: “);i = StdIn.readInt();

}System.out.println(“Product: “ + product);

What is the sentinel here?

Why is i initialized to 1?

So that the while expression will evaluate to true to first enter the loop but also once inside the loop, multiplying by 1 does not affect the final value (as opposed to i = 2).

From lecture:

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int i = 0;double sum = 0;int count = 0;

while( i >= 0 ){sum += i;count++;System.out.print(“Enter number: “);i = StdIn.readInt();

}count--; //fixes extra count for sentinelSystem.out.println(“Average: “ + (sum/count));

Getting the sentinel valuecounted as1 input, so, just calculate with (count-1) to calculate with just the actual values

+(sum/(count-1)));

Infinite Loops

int i = 0;

int sum = 0;

while(i <= 10)

{

sum += i;

}

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This loop will run

forever!

WHY???

How do we fix this?

i++;

This run-time error is a little harderto make when using for loops…

for loops

for(init; condition; inc)

{

//statements

}

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init; //statement

while(condition)

{

//statements

inc; //statement

}

Without increment statements, loops can run forever (unless there is a break… to-be-discussed)but with for loops, you add it right at the beginning.

Side by Side

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int i = 0;

int sum = 0;

while(i <= 10)

{

sum += i;

i++;

}

int sum = 0;

for(int i = 0; i <= 10; i++)

{

sum += i;

}

for loops cont.

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int sum = 0;

for(int i = 0; i <= 10; i++)

{

sum += i;

}

init performed once onlyat beginning of loop.

condition is checked at the beginning and beforeeach next loop iteration

inc is performed at the endof every loop iteration just before condition is rechecked

Another Painful Error for Both while and for Loops

int i = 0;

int sum = 0;

while(i <= 10);

{

sum += i;

i++;

}

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int sum = 0;

for(int i = 0; i <= 10; i++);

{

sum += i;

}

Semicolons placed here will give you a not-so-expected result!!What will happen in the two cases and why???Will be different for each. For loop will have a compilation error since i is declared inside the loop and i is used “outside” of loop…

What about

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int sum = 0;

int i;

for(i = 0; i <= 10; i++);

{

sum += i;

}

What will sum and i be here (no error this time…)?

When do we use whiles and when do we use fors?

The answer is not EXACTLY clear cut, BUT there is a general use for each that seems natural:

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for loops are great for “count controlled” loops; i.e. loops that have a count with them.

Loops from 1 to 100 for instance

while loops are great for “event controlled” loops; i.e. loops that keep iterating until some event has occurred

Loop until program gets correct input for instance

From lecture…

The following code will tell you if a number isn’t prime

What’s the problem?

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int number = StdIn.readInt();

for( int i = 2; i < number; i++){if( number % i == 0 ){

System.out.println(number + “ is not prime.”);

}}

cont. By adding a boolean, we can keep track of whether or

not the number is prime

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int number = StdIn.readInt();boolean prime = true; for( int i = 2; i < number && prime; i++){if( number % i == 0 )

prime = false;}

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break;

Some might think this would work? (and in a way it does…)

BUT What about when the loop is finished and we want to know LATER if the number is actually prime withoutdoing the loop again??

; i++)

do-while

Work just like while loops, only execute statements once before ever checking the condition…

do

{

//statements…

} while(condition);

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Note here the semicolon, because it is at the endNOT at the beginning, it denotes the end of the statement.

Get a password/combination

String combinationForDruidea; //anyone name the movie?

do

{

System.out.println(“Please enter password:”);

combinationForDruidea = StdIn.readString();

} while( !(combinationForDruidea.equals(“1234”) );

//”Amazing, now I’m going to have to change the

//combination on my luggage!”

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If you know the loop will have to execute the statements AT LEASTonce, then you can use a do-while loop.

Side by Side for product

int product = 1;

System.out.print(prompt);

int x = StdIn.getInt()

while(x != 0)

{

product *= x;

System.out.print(prompt);

x = StdIn.getInt();

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int product = 1;

int x;

do

{

System.out.print(prompt);

x = StdIn.getInt();

product *= x;

} while(x != 0)

String prompt = “Please enter a number to multiply to the productEnter 0 to quit\n”);

import lejos.nxt.*;

public class BasicRobotTest

{

public static void main(String[] args)

{

TouchSensor tSensor = new TouchSensor(SensorPort.S1);

int x = 3;

LCD.drawInt(x, 0, 0);

Sound.beep();

pause(1000);

LCD.drawString("Touch me to", 0, 0);

LCD.drawString("shut me up!", 0, 1);

while (!tSensor.isPressed())

{

Sound.buzz();

}

Motor.A.setSpeed(720);

Motor.C.setSpeed(720);

Motor.A.forward();

Motor.C.forward();

pause(1000);

Motor.A.stop();

Motor.C.stop();

}

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int x = 3; LCD.drawInt(x, 0, 0); Sound.beep(); pause(1000); LCD.drawString("Touch me to", 0, 0); LCD.drawString("shut me up!", 0, 1);

while (!tSensor.isPressed()) { Sound.buzz(); } Motor.A.setSpeed(720); Motor.C.setSpeed(720); Motor.A.forward(); Motor.C.forward(); pause(1000); Motor.A.stop(); Motor.C.stop();

Displays a 3 and beeps for a second

Displays “Touch me to shutme up!”

Loop to buzz forever UNTILthe sensor is pressed

Sets the speed of the motorsTells the motors to move forwardat that speed for a secondthen to stop.

Next week’s Lego Lab Starter

Last Bit on Loop Errors

BE CAREFUL!! When using loops, you have to follow the syntax and the logic

you want EXACTLY or a lot can go wrong.

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If you have a loop and the program just seems to sit there, you may have an infinite loop. Infinite Loops can be caused for various reasons…

In cases, your value can be off by 1 if you’re not careful.

Sometimes, a condition can look right but is actually wrong, a loop may never execute…

And the misplaced semicolon can make life miserable!

Infinite Loops

int x = 0;

do

{

System.out.println(x);

}while(x < 10)

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for(int x = 0; x < 10; x++)

{

System.out.println(x);

/*code that make you feel you’ve accomplished something */

x--;

//even more code

}How would we fix either?

What about?

x--

Off by 1

int ranThrough = 0;

for(int i = 1; i < 40; i++)

{

System.out.println(i);

ranThrough++;

}

System.out.println(“The loop ran through “ + ranThrough

+ “ times”);

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What will print here?

What will be the printed value of i on theLAST print statement?

ranThrough is 39!

Problems?

for(int i = 1; i >= 40; i++)

System.out.println(i);

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This will never print anything…. WHY?

Looks right but a logic problem with expression…

int i;

for(i = 1; i <= 40; i++);

System.out.println(i);

What is the problem? What will actually print and how many times?

for(int i = 1; i <= 40; i++);

System.out.println(i);

//gives compilation error!

//i used “outside” loop

//where it was declared.

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Final QUESTIONS???