Copyright © Texas Education Agency, 20131 Computer Programming For Loops.

transcript

Computer Programming

For Loops

List some tasks that you have to do over and over again

Mow the lawn Brush your teeth Take out the trash

Wouldn’t it be great if you only had to do the task one time, then it repeated itself automatically?

Looping is a central idea in programming

‘Looping’ is the programming term for repeating a task more than once.

The ‘for’ loop is a basic structure for repetition

for ( int j = 0; j < 7; j++)

System.out.println(“Counting ” + j);

// The loop above goes from 0 to 6

for loops are used when…

The same task must be executed several times.

You know how many iterations* are needed

* An iteration is a single execution of a loop, which may execute many times.

The ‘for’ loop starts with the reserved word…

for ( int j = 0; j < 7; j++)

Then comes a header with 3 parts…

for ( int j = 0; j < 7; j++)

The first header item is the initialization expression

for ( int j = 0; j < 7; j++)

This determines where the loop starts counting. In this case, the loop starts at 0.

The second header item is the condition or control expression

for ( int j = 0; j < 7; j++)

This determines where the loop stops counting. In this case, the loop stops when j is not less than 7. The condition must be true in order for the loop to keep running. If the condition is not true to start with, the loop will never run.

The third header item is the step expression

for ( int j = 0; j < 7; j++)

This determines how much the loop changes by. In this case, ‘j’ changes by 1 each time.

Let’s see a for loop in action

for ( int j = 0; j < 5; j++) System.out.println(“Counting ” + j);

Counting 0

Counting 1

Counting 2

Counting 3

Counting 4

The initialization does not have to start with 0

for ( int j = 0; j < 7; j++)

for ( int j = 4; j < 7; j++)

for ( int j = 1; j < 32; j++)

The control must evaluate to true/false

for ( int j = 0; j < =7; j++)

for ( int j = 4; j*2 < 100; j++)

for ( int j = 1; j < x; j++)

The step must move the condition towards false

for ( int j = 0; j < =7; j++)

for ( int j = 400; j > 0; j--)

for ( int j = 1; j < 100; j=j+4)

When the condition is false, the loop is finished running

for ( int j = 0; j < 7; j--)

The loop above will never stop running, because j is becoming more negative, it will always be less than 0. This is known as an infinite loop.

When the loop condition begins as false, the loop will never run

for ( int j = 0; j > 7; j++)

The loop above will never start running, because j is not more than 7.

There is special step syntax

for (int j = 0; j < 100; j=j+1)

for (int j = 0; j < 100; j++) // means the same thing

for (int j = 0; j < 100; j=j+8)

for (int j = 0; j < 100; j+=8) // means the same thing

for (int j = 0; j < 100; j=j*2)

for (int j = 0; j < 100; j*=2) // means the same thing

Do you know the special syntax for the expressions below?

j = j – 1

j = j – 2

j = j / 2

Did you guess right?

j = j – 1 same as j - = 1 or j --

j = j – 2 same as j -= 2

j = j / 2 same as j /=2

j = j +1 same as j += 1 or j++

How do you make more than one line execute?

for ( int j = 0; j < 7; j++)

System.out.println(“Printing ” + j);

// Only the “Counting” line is in the for loop.

// The “Printing” line runs after the loop ends

Use curly brackets to make a block statement

for ( int j = 0; j < 7; j++) {

System.out.println(“Printing ” + j);

// Now both “Counting” and “Printing” execute inside the loop. The line or lines that execute inside a loop are known as the ‘body’ of the loop.

Initialization Variables

for ( int j = 0; j < 7; j++)

The initialization variable tells the loop where to start.

The variable ‘j’ is only valid inside the for loop, where it was declared, but not outside the loop

for ( int j = 0; j < 7; j++) {

System.out.println( j);

// System.out.println(j); // this would not work

The variable ‘k’ is valid outside the for loop, where it was declared, as well as inside the loop

int k;

for ( k = 0; j < 7; k++) {

System.out.println( k);

System.out.println(k); // this works

The loop count variable ‘k’ is incremented until the condition fails -- when it fails the loop is over

int k;

for ( k = 0; j < 3; k++) {

System.out.println( k); // k=0, 1, 2

System.out.println(k); // k is 3

Review

What is this part called and what doesit do?

for ( int j = 4; j < 9; j++)

What is this part called and what does it do?

for ( int j = 2; j < 9; j++)

What is this part called and what does it do?

for ( int j = 14; j > 7; j -=2)

Can you write a for loop?

Copyright © Texas Education Agency, 20131 Computer Programming For Loops.

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