📓2.9: Implementing Selection & Iteration Algorithms

Table of Contents

📖 This page is a condensed version of CSAwesome Topic 2.9

Loops are not just for counting — they can perform many common algorithmic patterns, such as accumulating totals, finding maximums, or searching for a value.

Accumulator Pattern

An accumulator keeps a running total as the loop iterates.

Coding Exercise: Sum 1 to 5

int sum = 0;

for (int i = 1; i <= 5; i++) {
    sum = sum + i;
}

System.out.println("Sum is " + sum);

Counter Pattern

A counter counts how many times something happens.

Coding Exercise: Count Even Numbers

int count = 0;

for (int i = 1; i <= 10; i++) {
    if (i % 2 == 0) {
        count++;
    }
}

System.out.println("Number of even integers: " + count);

Finding the Maximum

Coding Exercise: Largest Number in Array

int[] nums = {5, 9, 2, 7, 3};
int max = nums[0];

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

System.out.println("Max is " + max);

Searching for a Value

Coding Exercise: Search Array for Target

int[] nums = {3, 6, 9, 12};
int target = 9;
boolean found = false;

for (int i = 0; i < nums.length; i++) {
    if (nums[i] == target) {
        found = true;
    }
}

System.out.println("Found? " + found);

Summary

  • Accumulator: running total.
  • Counter: counts matches.
  • Maximum finder: tracks largest value.
  • Search: checks for target presence.

AP Practice

Question 1</summary>

What will this print?

int sum = 0;
for (int i = 0; i < 4; i++) {
    sum += i;
}
System.out.println(sum);

Answer: 6 — because 0 + 1 + 2 + 3 = 6.

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Question 2</summary>

What does this code do?

int count = 0;
for (int i = 1; i <= 10; i++) {
    if (i % 2 != 0) {
        count++;
    }
}
System.out.println(count);

Answer: Counts the odd numbers from 1 to 10, so it prints 5.

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Acknowledgement

Content on this page is adapted from Runestone Academy - Barb Ericson, Beryl Hoffman, Peter Seibel.