2.7 while Loops

What you need to know

• while (condition) { body } — the condition is checked before each iteration. If it's false initially, the body runs zero times.
• Something in the body must eventually make the condition false, or the loop is infinite.
• Typical structure: initialize before the loop; test; body; update. Forgetting the update is the classic infinite loop.
• After the loop, variables keep their final values. If a loop runs while i < 5 incrementing i, then i is 5 afterward.
• while loops are the choice when the number of iterations isn't known in advance (reading input until a sentinel, searching until found).
• An if inside a while with a return is a common way to exit a search loop early.

Worked example

int n = 20;
int count = 0;
while (n > 1)
{
    n = n / 2;
    count++;
}
System.out.println(n + " " + count);

Output: 1 4.

Trace it yourself

Step through with the buttons, or use the ← → keys. Changed variables are highlighted.

Going deeper

The nuance, edge cases, and connections that turn a 3 into a 5.

• A while loop has four parts, even when they aren't adjacent: initialization (before), condition (header), body, update (in the body). Identify all four before tracing. A missing update is an infinite loop.
• The condition is checked before every iteration, including the first. Zero iterations is a real outcome. The exam includes loops whose condition is false at the start.
• Iteration count vs. final value: the body runs once for each check that passes. The variable's final value is the one that made the check fail. If the body runs for i = 3, 7, 11, then the loop stops with i = 15 — four values, three iterations.
• Sentinel loops read input until a special value: while (n != -1) { … n = input.nextInt(); }. The sentinel isn't processed. The first read happens before the loop.
• Search loops often have compound conditions: while (i < arr.length && arr[i] != target). The bounds check comes first (short-circuit). After the loop, i < arr.length tells you whether it was found.
• Modifying the condition's variable in unexpected ways — dividing, multiplying, non-uniform steps — is how the exam varies these. Trace with a table; don't assume a formula.
• A return inside a while loop (in a method) exits the loop and the method at once. Common in "find the first…" methods.

Mistakes that cost points

• Counting the failed check as an iteration. Body runs only when the check passes.
• Reporting the last-processed value as final. Final is one update past that.
• Missing the zero-iteration case. Check the condition against the initial values first.
• Assuming the update is +1. Read the body.

Practice questions

Written in the style of the real exam. Try each one before revealing the answer.

int k = 3;
while (k < 12)
{
    System.out.print("x");
    k += 4;
}

int i = 0;
while (i * i < 50)
{
    i++;
}