Lesson: For Loops

Previous Lesson: If Statements

Why Loops?

Programs often need to repeat actions. For example, printing a sequence, calculating totals, checking multiple inputs — all involve doing the same thing multiple times.

Copy-pasting code works, but it’s inefficient, error-prone, and inflexible:

System.out.println("Hello!");
System.out.println("Hello!");
System.out.println("Hello!");

If you wanted to print “Hello!” 100 times, this wouldn’t be realistic. That’s where loops come in.

What is a For Loop?

A for loop repeats a block of code a specific number of times:

for (int counter = 0; counter < 3; counter++) {
    System.out.println("Hello!");
}

Output:

Hello!
Hello!
Hello!

This loop runs 3 times, because:

1. counter variable is initialized and starts at 0.
2. The loop continues while counter < 3.
3. After each repetition, counter++ increases counter by 1.

Visualizing the Flow

For Loop Structure

Part	Description	Example
Initialization	Create and start counter	int i = 0
Condition	Continue while this is true	i < 5
Update	Change the counter after each run	i++

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

Output:

i = 0
i = 1
i = 2
i = 3
i = 4

Examples

There are many uses for for loops — from counting, to pattern generation, to graphics and user interaction. Here are a few examples.

Example 1: Counting Up

for (int num = 1; num <= 10; num++) {
    System.out.println(num);
}

Output:

1
2
3
4
5
6
7
8
9
10

This prints numbers 1 to 10, inclusive.
What happens if you wrote num < 10 for the condition?

Example 2: Counting Down

for (int num = 10; num >= 1; num--) {
    System.out.println(num);
}
System.out.println("Blastoff!");

Output:

10
9
8
7
6
5
4
3
2
1
Blastoff!

The loop counts down before printing Blastoff!.
Notice that the final statement runs only after the loop is finished.

Example 3: Using Variables in Loops

You can use the counter inside the loop to calculate or build text dynamically.

for (int i = 1; i <= 5; i++) {
    System.out.println("Square of " + i + " is " + (i * i));
}

Output:

Square of 1 is 1
Square of 2 is 4
Square of 3 is 9
Square of 4 is 16
Square of 5 is 25

Example 4: Drawing with Loops (Processing)

Repetition isn’t just for math — it can be used to create visual patterns.
The example below draws five circles across the screen, evenly spaced:

for (int x = 50; x <= 250; x += 50) {
    ellipse(x, 100, 40, 40);  // Draw a circle
}

Output:

Each circle is spaced 50 pixels apart horizontally.

Example 5: Summing User Input

Loops can also process repeated user input:

public class SumInputs extends ConsoleProgram {
    public void run() {
        int count = readInt("How many numbers? ");
        int total = 0;

        for (int i = 0; i < count; i++) {
            int value = readInt("Enter a number: ");
            total = total + value;
        }

        System.out.println("Total = " + total);
    }
}

Sample Output:

How many numbers? 3
Enter a number: 4
Enter a number: 7
Enter a number: 9
Total = 20

What would you need to do to get this program to calculate the average (mean)?

Example 6: Multiplication Table

Loops are perfect for generating repetitive structures like tables:

int num = readInt("Enter a number: ");
for (int i = 1; i <= 10; i++) {
    System.out.println(num + " × " + i + " = " + (num * i));
}

Sample Output:

Enter a number: 7
7 × 1 = 7
7 × 2 = 14
7 × 3 = 21
...
7 × 10 = 70

Example 7: Loop with Logic

You can combine loops and conditionals to apply logic to each iteration:

for (int i = 1; i <= 10; i++) {
    if (i % 2 == 0) {
        System.out.println(i + " is even");
    } else {
        System.out.println(i + " is odd");
    }
}

Output:

1 is odd
2 is even
3 is odd
4 is even
5 is odd
6 is even
7 is odd
8 is even
9 is odd
10 is even

Common Mistakes

Error	Code Example	Explanation
Missing braces	for (int i = 0; i < 3; i++) System.out.println(i); System.out.println("Done!");	Only the first line repeats — "Done!" prints once, not in the loop. Always use {} for clarity.
Uninitialized counter	for (i = 0; i < 5; i++)	Causes a syntax error — the loop variable must be declared (int i = 0;).
Off-by-one error	for (int i = 1; i < 10; i++) (when you wanted 1–10)	Common logic mistake — loop ends one short or one extra. Check your condition carefully.
Wrong condition	for (int i = 0; i > 10; i++)	Never runs — the condition is false at the start.
Infinite loop	for (int i = 0; i >= 0; i++)	Runs forever — the condition never becomes false.
Wrong update direction	for (int i = 10; i < 20; i--)	Logic reversed — the counter moves away from the stop condition.
Shadowed variable	int i = 0; for (int i = 0; i < 5; i++)	Declares two is — causes a compile-time error. The outer i is hidden.

Practice Problems

1. Printing a Pattern of Dashes
   Ask how long a line to draw and print that many - characters.
   Hint: There's a difference between print() and println() for output.

   Example Output:

   How long a line? 12
   ------------
   

2. Counting Coins
   Ask how many coins you have and print clink! that many times.

   Example Output:

   How many coins do you have? 5
   clink!
   clink!
   clink!
   clink!
   clink!
   

3. Running Lap Tracker
   Ask the user for the number of laps to run for a workout. Print Lap # complete for each lap, replacing # with the lap number. After the loop, print Workout complete!.

   Example Output:

   How many laps do you want to run? 3
   Lap 1 complete
   Lap 2 complete
   Lap 3 complete
   Workout complete!
   

4. Cumulative Distance
   Assume you run 2 km per day for 14 days. Each day, print the total distance so far.
   Hint: Try to use an accumulator variable.

   Example Output:

   Day 1 - Total Distance: 2 km
   Day 2 - Total Distance: 4 km
   Day 3 - Total Distance: 6 km
   ...
   Day 14 - Total Distance: 28 km
   

5. Power of Two Table
   Print powers of two from 2^1 up to 2^10.

   Example Output:

   Power | Result
   ------+-------
   2^1   | 2
   2^2   | 4
   2^3   | 8
   ...
   2^10  | 1024
   

6. Investment Interest Simulation
   You invest $1000 for 5 years earning 5% interest compounded annually.
   Print the balance after each year.

   Example Output:

   Year 1: $1050.00
   Year 2: $1102.50
   Year 3: $1157.63
   Year 4: $1215.51
   Year 5: $1276.28
   

7. Temperature Warning System
   Simulate hourly temperature readings for 6 hours using random values between 15–35°C.
   If the temperature is above 30, print Warning: too hot! after the reading.

   Example Output:

   Hour 1: 27°C
   Hour 2: 32°C
   Warning: too hot!
   Hour 3: 25°C
   Hour 4: 34°C
   Warning: too hot!
   Hour 5: 29°C
   Hour 6: 31°C
   Warning: too hot!
   

8. Divisible by Three Counter
   Ask the user for a number n. Print all numbers from 1 to n that are divisible by 3.
   At the end, print how many there were.
   Hint: Use % to test divisibility and a variable to count total valid numbers.

   Example Output:

   Divisible by Three
   ------------------
   Enter a number: 15
   3
   6
   9
   12
   15
   There are 5 numbers divisible by 3.
   

9. Guessing Game (Challenge)
   Generate a random number 1–10. Let the user guess up to 3 times.
   If the guess is correct, print You got it! and stop.
   Otherwise, give a hint (too high or too low).

   Example Output:

   I'm thinking of a number between 1 and 10.
   Enter your guess: 4
   Too low!
   Enter your guess: 8
   Too high!
   Enter your guess: 6
   You got it!
   

10. Pattern Builder (Challenge)
    Ask the user for a number of rows, r. Print r rows forming a right-aligned triangle of stars.

    Example Output:

    How many rows? 5
    
        *
       **
      ***
     ****
    *****
    

Summary

• Loops automate repetition and reduce duplication.
• for loops use a counter variable to control execution.
• Initialization → Condition → Update → Repeat.
• You can nest logic (if statements) inside loops.
• Common use cases: counting, summing, accumulating, table generation.