📓4.2: For Loops

For Loops
- Three Parts of a For Loop
- Decrementing Loops
⭐️ Summary

📖 This page is a condensed version of CSAwesome Topic 4.2

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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

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Complete the Programming Challenge: Turtles Drawing Shapes activity in pairs.

⭐️ Summary

There are three parts in a for loop header: the initialization, the test condition (a Boolean expression), and an increment or decrement statement to change the loop control variable.
```
// LOOP HEADER
for (initialize; test condition; change) {
   // LOOP BODY
}
```
In a for loop, the initialization statement is only executed once before the evaluation of the test Boolean expression. The variable being initialized is referred to as a loop control variable.
In each iteration of a for loop, the increment or decrement statement is executed after the entire loop body is executed and before the Boolean expression is evaluated again.
A for loop can be rewritten into an equivalent while loop and vice versa.

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Acknowledgement

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