for Loops

In the last lesson, you met the while loop, which is great for repeating code as long as some condition is true. Now, let's meet its more structured cousin: the for loop. A for loop is perfect for count-controlled iteration, where you know you need to do something exactly 5, 10, or 100 times.

The Anatomy of a for Loop

The power of a for loop is that it bundles all the logic for counting into one clean line. The header of a for loop has three distinct parts, separated by semicolons.

for (initialization; boolean expression; update)

Let's use an analogy of a road trip from Boston to Chicago.

1. 1
   Initialization

   int i = 0; This is where you set up your counter, or loop control variable. It runs only once at the very beginning. Think of this as setting your car's trip odometer to 0 before you pull out of the driveway.
2. 2
   Boolean Expression (Condition)

   i < 10; This is the gatekeeper. Before each loop cycle begins, Java checks this condition. If it's true, the loop body runs. If it's false, the loop stops completely, and the program moves on. This is like asking, "Have we driven 10 miles yet?" before you start the next leg of your journey.
3. 3
   Update

   i++ This part runs after the code inside the loop body has finished, but before the condition is checked again for the next cycle. This is you driving one more mile. After you've done your "driving" (the loop body), your odometer clicks up.

Here’s a simple example that prints the numbers 0 through 4:

for (int i = 0; i < 5; i++) {
  System.out.println(i);
}

Let's trace the execution step-by-step:

1. 1
   Initialize

   int i is created and set to 0. This only happens once.
2. 2
   Check

   Is i < 5? Yes, 0 < 5 is true.
3. 3
   Execute Body

   The code inside the {} runs. System.out.println(0); prints "0".
4. 4
   Update

   i++ runs. i is now 1.
5. 5
   Check

   Is i < 5? Yes, 1 < 5 is true.
6. 6
   Execute Body

   System.out.println(1); prints "1".
7. 7
   Update

   i++ runs. i is now 2. ...and so on. This continues until:

• Check
  Is i < 5? Let's say i is now 4. Yes, 4 < 5 is true.
• Execute Body
  System.out.println(4); prints "4".
• Update
  i++ runs. i is now 5.
• Check
  Is i < 5? No, 5 < 5 is false.
• Stop
  The loop terminates. The program continues with whatever code comes after the loop's closing brace }.

for Loops vs. while Loops

The AP Exam expects you to know that any for loop can be rewritten as a while loop, and vice versa. This is a great way to truly understand the three parts of the for loop header.

Let's convert our for loop into a while loop.

The for loop:

for (int i = 0; i < 5; i++) {
  System.out.println(i);
}

The equivalent while loop:

// 1. Initialization happens BEFORE the loop
int i = 0;

// 2. The boolean expression is the while condition
while (i < 5) {
  // The loop body is the same
  System.out.println(i);

  // 3. The update is the LAST thing in the loop body
  i++;
}

See how the three parts just get rearranged?

• The for loop's initialization becomes a statement right before the while loop.
• The for loop's boolean expression becomes the while loop's condition.
• The for loop's update becomes the very last line inside the while loop's body.

This equivalence shows why for loops are so useful. They don't add new functionality, but they organize the code for count-based loops much more cleanly, reducing the chance you'll forget to initialize or update your counter variable. You're much less likely to accidentally write an infinite loop with a for loop than with a while loop.

Let's walk through a couple of common scenarios you'll see on the AP exam.

public int sumUpTo(int n) {
  int sum = 0; // Initialize our total

  // Loop from 1 up to n (inclusive)
  for (int i = 1; i <= n; i++) {
    // Add the current number to the sum
    sum = sum + i; // or more concisely: sum += i;
  }

  return sum; // Return the final calculated sum
}

String message = "hello";

// Initialize at the last index, count down to 0
for (int i = message.length() - 1; i >= 0; i--) {
  System.out.println(message.charAt(i));
}

Ready to try a few on your own? Don't worry about getting it perfect on the first try. The goal is to start thinking in loops.

Problem 1: The Even Number Printer Write a for loop that prints all the even numbers between 1 and 20 (inclusive). The output should be 2, 4, 6, ... 20.

• Hint 1
  You can start your loop at 2. What would your update statement need to be to only hit the even numbers?
• Hint 2
  Alternatively, you could loop from 1 to 20 and use an if statement inside the loop with the modulus operator (%) to check if a number is even before printing it. Both approaches are valid!

Problem 2: The Countdown Your school basketball team, the Wildcats, is playing their rival. Write a for loop that simulates a countdown for the final seconds of the game. It should print: 10... 9... ... 1... WILDCATS WIN!

• Hint: You'll want to count down, not up. What should your initialization, condition, and update be? The final print statement should happen after the loop is finished.