📓2.8: for Loops

Table of Contents

• For Loops
  • Three Parts of a For Loop
  • Decrementing Loops

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📖 This page is a condensed version of CSAwesome Topic 2.8

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For Loops

Another type of loop in Java is a for loop. This is usually used when you know how many times you want the loop to execute. It is often a simple counter-controlled loop to do the loop body a set number of times.

Three Parts of a For Loop

A for-loop combines all 3 parts of writing a loop in one line to initialize, test condition, and change the loop control variable. The 3 parts are separated by semicolons (;):

// LOOP HEADER
for (initialize; test condition; change) {
     // LOOP BODY
}

The for-loop is like a shortcut way to write a while loop, with all three steps that you need in one line.

Watch the following which compares a while loop and for loop line by line.

Here is a control flow diagram for a for loop:

• The code in the initialization area is executed only one time before the loop begins
• The test condition is checked each time through the loop and the loop continues as long as the condition is true
• The loop control variable change is done at the end of each execution of the body of the loop, just like a while loop.
• When the loop condition becomes false, execution will continue at the next statement after the body of the loop.

Two common patterns in for-loops are to count from 0 up to an number (using <) or count from 1 to a number including the number (using <=). Remember that if you start at 0 use <, and if you start at 1, use <=.

The two loops below using these two patterns both run 10 times:

// These loops both run 10 times
// If you start at 0, use <
for(int i = 0; i < 10; i++) {
    System.out.println(i);
}
// If you start at 1, use <=
for(int i = 1; i <= 10; i++) {
    System.out.println(i);
}

The variable i (stands for index) is often used as a counter in for-loops.

Decrementing Loops

You can also count backwards in a loop starting from the last number and decrementing the loop counter down to 0 or 1. All 3 parts of the loop must change to count backwards including the test of when to stop.

For example, for (int i=5; i > 0; i--) counts from 5 down to 1.

💬 DISCUSS: What do you think will happen when you run the code below? How would it change if you changed line 11 to initialize i’s value to 3?

String line1 = " bottles of pop on the wall";
String line2 = " bottles of pop";
String line3 = "Take one down and pass it around";

// loop 5 times (5, 4, 3, 2, 1)
for (int i = 5; i > 0; i--) {
    System.out.println(i + line1);
    System.out.println(i + line2);
    System.out.println(line3);
    System.out.println((i - 1) + line1);
    System.out.println();
}

💻 In-Class Activity: Turtle Loops

1. Go to
2. Make sure you SIGN IN!
3. Complete the Programming Challenge: Turtles Drawing Shapes activity in pairs.

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Acknowledgement

Content on this page is adapted from Runestone Academy - Barb Ericson, Beryl Hoffman, Peter Seibel.