JAVA / CHAPTER 06

Iteration

Intermediate

Use loops with clear bounds, progress, and accumulators.

By the end of this chapter

Trace a for loop
Maintain an accumulator
Identify termination conditions

The idea

Loops repeat a controlled piece of work. A for loop exposes initialization, continuation testing, and update together. A while loop checks its condition before each iteration and may execute zero times. Trace the condition before the body rather than assuming the body always runs. For a loop over n positions beginning at zero, i < n usually visits exactly the valid indices.

An accumulator represents a result built so far. State what it means before the loop starts and after each update: total can mean the sum of all accepted values already examined. Separately identify how the loop makes progress. Without a change that eventually falsifies the condition, execution can continue indefinitely.

Java example

public class Main {
  public static void main(String[] args) {
    int total = 0;
    for (int n = 1; n <= 5; n++) {
      if (n % 2 == 1) total += n;
    }
    System.out.println(total);
  }
}

Result

The loop examines one through five and adds only one, three, and five. The total changes only when the condition succeeds.

Try it yourself

Count the positive values in a fixed integer sequence, then test a sequence with no positive values.

Use your own editor for this exercise. Runnable challenges on CodeQuest are available for JavaScript, Python, and SQL.

CHECK YOUR UNDERSTANDING

Iteration

By the end of this chapter

The idea

Try it yourself

Chapter checkpoint