Loops

After this lesson, you will be able to:

Write for loops for counted repetition and while loops for conditional repetition
Trace through loop execution step by step (the master debugging skill)
Choose the right loop type for a given problem
Avoid infinite loops, off-by-one errors, and fence-post problems

Why Loops?

Without loops, printing numbers 1 through 1000 would take 1000 println statements. With a loop, it takes three lines:

for (int i = 1; i <= 1000; i++) {
    System.out.println(i);
}

Loops let you execute a block of code repeatedly — either a fixed number of times or until some condition changes.

The for Loop

Use for when you know how many times to repeat:

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

The three parts inside the parentheses:

Part	Purpose	Example
`int i = 0`	Initialize — runs once before the loop	Start counting at 0
`i < 5`	Test — checked before each iteration	Keep going while true
`i++`	Update — runs after each iteration	Move to next count

From CSCD 110: Python’s for i in range(5) does the same thing. Java’s for loop makes the initialization, test, and update explicit rather than hiding them inside range().

How many times does this loop print? for (int i = 1; i <= 10; i++) { System.out.println(i); }

A 9 times

B 10 times

C 11 times

D It depends on the value of i

Answer: B. The loop starts at i = 1 and runs while i <= 10. So it executes for i = 1, 2, 3, ..., 10 — that's 10 iterations. When i becomes 11, the test 11 <= 10 is false and the loop stops.

The while Loop

Use while when you don’t know how many times to repeat — you repeat until a condition becomes false:

Scanner console = new Scanner(System.in);
System.out.print("Enter a positive number: ");
int num = console.nextInt();

while (num <= 0) {
    System.out.print("That's not positive. Try again: ");
    num = console.nextInt();
}
System.out.println("You entered: " + num);

This is input validation — one of the most common uses of while. The loop runs zero or more times depending on what the user enters.

The do-while Loop

A do-while loop runs the body at least once, then checks the condition:

int choice;
do {
    System.out.println("1. Play  2. Quit");
    System.out.print("Choice: ");
    choice = console.nextInt();
} while (choice != 1 && choice != 2);

Use do-while when you need the body to execute before you can check the condition (like showing a menu before reading a choice).

Loop Tracing: The Master Skill

The single most important debugging technique for loops is tracing — stepping through each iteration and tracking variable values:

int sum = 0;
for (int i = 1; i <= 4; i++) {
    sum = sum + i;
}

Iteration	`i`	`sum` before	`sum` after
1	1	0	1
2	2	1	3
3	3	3	6
4	4	6	10

After the loop, sum is 10 (which is 1+2+3+4). If your loop produces unexpected output, trace it on paper.

What is the value of result after this loop? int result = 1; for (int i = 0; i < 3; i++) { result = result * 2; }

A 6

B 8

C 3

D 16

Answer: B. Trace it: result starts at 1. After i=0: result = 1*2 = 2. After i=1: result = 2*2 = 4. After i=2: result = 4*2 = 8. The loop runs 3 times (i = 0, 1, 2), doubling each time: 1 → 2 → 4 → 8.

Choosing the Right Loop

Situation	Best Loop	Why
“Print 1 to N”	`for`	Known count
“Read until sentinel”	`while`	Unknown count
“Show menu, then validate”	`do-while`	Must run once first
“Process each element”	`for` or enhanced `for`	Index-based or collection

You need to keep asking the user for a password until they enter the correct one. Which loop type is the best choice?

Common Mistakes

Infinite loop — forgetting to update the loop variable:

int i = 0;
while (i < 10) {
    System.out.println(i);
    // Forgot i++! This runs forever.
}

Off-by-one — < vs <=:

for (int i = 0; i < 5; i++)   // 0,1,2,3,4 → 5 iterations
for (int i = 0; i <= 5; i++)  // 0,1,2,3,4,5 → 6 iterations
for (int i = 1; i <= 5; i++)  // 1,2,3,4,5 → 5 iterations

When in doubt, trace the first and last iterations.

Summary

for loops handle counted repetition. while loops handle conditional repetition. do-while guarantees at least one execution. Tracing — writing out variable values for each iteration — is the most reliable way to debug loop logic.