Loops and Iteration

The three loops

// 1. while -- test first; may run zero times
int i = 0;
while (i < 5) { System.out.print(i); i++; }

// 2. do...while -- test last; always runs at least once; note the semicolon
int j = 0;
do { System.out.print(j); j++; } while (j < 5);

// 3. for -- initialisation; condition; update, all in one line
for (int k = 0; k < 5; k++) { System.out.print(k); }

The for header is executed in this order: the initialisation once; then the condition; then the body; then the update; then the condition again. A variable declared in the header exists only inside the loop — using k after the loop is a compile-time error.

All three are interchangeable in principle; choosing the natural one is a readability decision that is regularly marked.

Counters, accumulators and the value after the loop

int sum = 0;              // accumulator: initialise BEFORE the loop
int i = 0;                // counter
while (i < 5) { sum = sum + i; i++; }
System.out.print(i + " " + sum);      // 5 10

The counter ends at the first value that fails the test (here 5, not 4), and the sum is 0+1+2+3+4 = 10. Questions on this pattern are almost guaranteed in a quiz.

break and continue

break leaves the loop immediately; continue skips the rest of the body and goes to the next test (in a for, the update still happens). In a while loop, continue placed before the update can produce an infinite loop:

int balance = 10;
while (balance >= 1) {
    if (balance < 9) continue;    // balance is never changed again -> infinite loop
    balance = balance - 9;
}

In nested loops, break leaves only the innermost loop.

Nested loops

for (int row = 1; row <= 3; row++) {
    for (int col = 1; col <= row; col++) System.out.print("*");
    System.out.println();
}                       // prints *, **, *** on three lines

An outer loop of m passes containing an inner loop of n passes runs the inner body m × n times. When the inner limit depends on the outer counter, count the passes one row at a time.

Input validation

Scanner in = new Scanner(System.in);
int age;
do {
    System.out.print("Age (0-120): ");
    while (!in.hasNextInt()) { in.next(); System.out.print("Numbers only: "); }
    age = in.nextInt();
} while (age < 0 || age > 120);

The pattern is: ask, read, test, repeat while the value is unacceptable. A do loop is the natural choice because at least one question must be asked.

The three ways a loop never ends

1. The control variable is never updated inside the body.
2. The update moves the variable away from the terminating condition.
3. A continue in the wrong place, or a stray semicolon (while (i < 5);) detaches the update from the loop.