Copyright © Texas Education Agency, 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 Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]2

Wouldn’t it be great if you only had to do the task one time, then it repeated itself automatically? Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]3

Looping is a central idea in programming ‘Looping’ is the programming term for repeating a task more than once. Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]4

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 Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]5

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. Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]6

The ‘for’ loop starts with the reserved word… for ( int j = 0; j < 7; j++) System.out.println(“Counting ” + j); Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]7

Then comes a header with 3 parts… for ( int j = 0; j < 7; j++) System.out.println(“Counting ” + j); Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]8

The first header item is the initialization expression for ( int j = 0; j < 7; j++) System.out.println(“Counting ” + j); This determines where the loop starts counting. In this case, the loop starts at 0. Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]9

The second header item is the condition or control expression for ( int j = 0; j < 7; j++) System.out.println(“Counting ” + 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. Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]10

The third header item is the step expression for ( int j = 0; j < 7; j++) System.out.println(“Counting ” + j); This determines how much the loop changes by. In this case, ‘j’ changes by 1 each time. Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]11

Let’s see a for loop in action Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]12

for ( int j = 0; j < 5; j++) System.out.println(“Counting ” + j); Counting 0 Counting 1 Counting 2 Counting 3 Counting 4 Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]13

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++) Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]14

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++) Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]15

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) Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]16

When the condition is false, the loop is finished running Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]17 for ( int j = 0; j < 7; j--) System.out.println(“Counting ” + 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 Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]18 for ( int j = 0; j > 7; j++) System.out.println(“Counting ” + 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 Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]19

Do you know the special syntax for the expressions below? j = j – 1 j = j – 2 j = j / 2 Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]20

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++ Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]21

How do you make more than one line execute? for ( int j = 0; j < 7; j++) System.out.println(“Counting ” + j); System.out.println(“Printing ” + j); // Only the “Counting” line is in the for loop. // The “Printing” line runs after the loop ends Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]22

Use curly brackets to make a block statement for ( int j = 0; j < 7; j++) { System.out.println(“Counting ” + 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. Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]23

Initialization Variables for ( int j = 0; j < 7; j++) The initialization variable tells the loop where to start. Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]24

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 Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]25

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 Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]26

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 Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]27

Review Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]28

What is this part called and what does it do? for ( int j = 4; j < 9; j++) System.out.println(“Counting ” + j); Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]29

What is this part called and what does it do? for ( int j = 2; j < 9; j++) System.out.println(“Counting ” + j); Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]30

What is this part called and what does it do? for ( int j = 14; j > 7; j -=2) System.out.println(“Counting ” + j); Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]31

Can you write a for loop? Copyright © Texas Education Agency, 2013IT: [Computer Programming] - [For Loops]32