Iteration with while and for Loops

Iteration: while and for Loops

By the end of this lesson, you will be able to write and trace while and for loops, avoid infinite loops, and select the appropriate loop for a given task.

The while Loop

while (condition) {
    // body — executes while condition is true
    // MUST update the condition variable to avoid infinite loop!
}

Trace: sum 1 to 5

int sum = 0, i = 1;
while (i <= 5) {
    sum += i;
    i++;
}
// sum = 15

do-while: the body always executes at least once (condition checked after).

do {
    // body (runs at least once)
} while (condition);

The for Loop

for (initialization; condition; update) {
    // body
}

All three parts are optional (but the semicolons are required).

Count from 1 to 5:

for (int i = 1; i <= 5; i++) {
    System.out.print(i + " ");
}
// Output: 1 2 3 4 5

Count down:

for (int i = 10; i > 0; i -= 2) {
    System.out.print(i + " ");
}
// Output: 10 8 6 4 2

Common Loop Patterns

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

Find a sum:

Find a maximum:

int max = arr[0];
for (int i = 1; i < arr.length; i++) {
    if (arr[i] > max) max = arr[i];
}

Count elements meeting a condition:

int count = 0;
for (int i = 0; i < arr.length; i++) {
    if (arr[i] % 2 == 0) count++;
}

Nested Loops

for (int row = 1; row <= 3; row++) {
    for (int col = 1; col <= 4; col++) {
        System.out.print("* ");
    }
    System.out.println();
}

The inner loop completes fully for each iteration of the outer loop.

Output:

* * * *
* * * *
* * * *

Total iterations of inner loop body: 3 × 4 = 12.