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CS 177Week 5: Loops

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Repetition

Repetition is the ability to do something over and over again

With repetition in the mix, we can solve practically any problem that can be solved with a computer

Repetition leverages the most famous ability of the computer: speed

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Mechanics

The main way that repetition works in Java is through loops

A loop is a block of code that will be executed repeatedly some number of times (perhaps even zero times!)

As the statements are executed, variables may change

It isn’t just repeating the same thing over and over

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while loop

The simplest loop in Java is the while loop

It looks similar to an if statement The difference is that, when you get

to the end of the while loop, it jumps back to the top

If the condition in the while loop is still true, it executes the body of the loop again

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Anatomy of a while loop

while( condition ){

statement1;statement2;…statementn;

}

A whole bunch of

statements

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A while loop with only one statement

A while loop will usually have multiple statements in its body

However, it is possible to make a while loop with only a single statement

Then, like an if-statement, the braces are optional

while( condition ) statement;

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Printing numbers

Let’s print the numbers from 1 to 100

int i = 1;

while( i <= 100 ){System.out.println(i);i++;

}

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Rules for while

The while loop checks if the condition is true, and if so, executes each statement one by one

When execution gets to the bottom, it jumps back up to the top

If the condition is still true (i.e., i <= 100), it repeats the loop

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Workings of a while loop

//line Awhile( condition ){

//line Bstatement1;statement2;//line C…statementn;

}//line D

Line Condition

A n

B

C

D

Line Condition

A Unknown

B

C

D

Line Condition

A Unknown

B True

C

D

Line Condition

A Unknown

B True

C Unknown

D

Line Condition

A Unknown

B True

C Unknown

D False

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Summing numbers

We can also use while loops to help us deal with input

What if we wanted to sum all of the numbers that a person entered?

How would we know when to stop? One solution is to keep adding

numbers until the person enters a negative number

This is called using a sentinel value

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Summing numbers

Solution:

int i = 0;int sum = 0;

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

}System.out.println(“Sum: “ + sum);

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Finding the average

We could also find the average: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));

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Tracking the biggest input

What if we wanted to find the biggest input?

Somehow we would have to check each input and see if it were larger than the current largest

Solution: use an if-statement inside of a while loop

if (i > iBig) iBig = i;

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Guessing game

Let’s say that you wanted to write a program to guess a number that a person had come up

The number is between 1 and 100 Every time the computer guesses a

number, the person enters: H if the number is too high L if the number is too low F if the number was found

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Guessing game algorithm

1. Start with the minimum and maximum of the range

2. Find the midpoint3. Ask the user if the midpoint is correct4. If the answer is too high, go to Step 1

using the minimum and the midpoint as the new range

5. If the answer is too low, go to Step 1 using the midpoint and the maximum as the new range

6. If the midpoint is correct, you’re done!

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Nested loops

Just as with if-statements, it’s possible to nest loops

A repetitive task can be done inside of another repetitive task

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C0mmon Pitfalls with while Loops

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Infinite loops

Loops can go on forever if you aren’t careful

int n = 40;int i = 1;

while( i <= n ){System.out.println(i);//supposed to print all the numbers//less than 40, but i never increases

}

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Badly-written loops

int n = 40;int i = 1;

while( i <= n ){System.out.println(i);i--; //woops, should have been i++

}

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Fencepost errors

Being off by one is a very common loop error

int n = 40;int i = 1;

while( i < n ) //won’t //reach n

{System.out.println(i);i++;

}

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Skipping loops entirely

If the condition isn’t true to begin with, the loop will just be skipped

int n = 40;int i = 1;

while( i >= n ) //oops, should be <={System.out.println(i);i++;

}

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Misplaced semicolon

A misplaced semicolon can cause an empty loop body to be executed (often infinitely)int n = 40;

int i = 1;

while( i <= n ); //semicolon is wrong{System.out.println(i);i++;

}

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3 loops in Java

1. while loops Used when you don’t know how many

times you are going to need to repeat2. for loops

Used when you do know how many times you are going to repeat

3. do-while loops Used almost never Oh, okay, they are used whenever you need

to be guaranteed the loop runs at least once

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Loops are interchangeable

Any problem that uses loops can use any kind of loop

The choice is supposed to make things easier on the programmer

Some loops are more convenient for certain kinds of problems

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for loops

for loops are great when you know how many times a loop will run

They are the most commonly used of all loops

They are perfect for any task that needs to run, say, 100 times

A for loop has 3 parts in its header:1. Initialization2. Condition3. Increment

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Way to Progress

Ending Point

Starting Point

Anatomy of a for loop

for( init; condition; inc ){

statement1;statement2;…statementn;

}

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A for loop will usually have multiple statements in its body

However, it is possible to make a for loop with only a single statement

Then, like if-statements and while-loops, the braces are optional

for( init; condition; inc )statement;

A for loop with only one statement

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for loop example

Let’s print the numbers from 1 to 100 (again)

Remember how this was done with while:

i = 1;

while( i <= 100 ){System.out.println(i);i++;

}

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for loop example

A for loop is specifically designed for this sort of thing:

The initialization and the increment are built-in

for( i = 1; i <= 100; i++ ){System.out.println(i);

}

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Finding primes

To see if a number is prime, we can divide by all the numbers less than the number to see if it can be evenly divided

Sounds like a perfect opportunity for a for loop

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Testing for a prime

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

What’s the problem?

int number = StdIn.readInt();

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

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

}}

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Better testing for a prime

By adding a boolean, we can keep track of whether or not the number is prime

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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do-while loops

They work just like while loops The only difference is that they are

guaranteed to execute at least once Unlike a while loop, the condition

isn’t checked the first time you go into the loop

Sometimes this is especially useful for getting input from the user

The syntax is a little bit different

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Anatomy of a do-while loop

do{

statement1;statement2;…statementn;

} while( condition );

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Common pitfalls

Almost all of these are issues with while, for and do-while loops as well

Many of these errors have to do with incorrect starting or ending conditions for the loop

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Infinite loops

Loops can go on forever if you aren’t careful

int n = 40;int i = 1;

do{System.out.println(i);//supposed to print all the numbers//less than 40, but i never increases

} while( i <= n );

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Infinite for loops

for( int i = 0; i < 10; i++ ){System.out.println(i);//lots of other codei--; //woops

}

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Bad increment

int i;

for( i = 1; i <= 40; i-- )//woops, should have been i++System.out.println(i);

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Fencepost errors

Being off by one is a very common loop error

int i;

for( i = 1; i < 40; i++ )//runs 39 timesSystem.out.println(i);

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Skipping loops entirely

If the condition isn’t true to begin with, the loop will just be skipped

for( int i = 1; i >= 40; i++ ) //oops, should be <=System.out.println(i);

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Misplaced semicolon

A misplaced semicolon can cause an empty loop body to be executed

Everything looks good, loop even terminates

But, only one number will be printed: 41

for( int i = 1; i <= 40; i++ ); //semicolon is wrong

{System.out.println(i);

}