Linear Search in AP CSA: Complete Guide (2025-2026)

Linear search is the go-to algorithm for finding a value in an unsorted array or ArrayList. It is Unit 4 content (Data Collections, 30–40% of the exam) and appears in both MCQ trace questions and FRQ method-writing. Master first-match, last-match, and count variants—and never confuse returning the value vs. the index.

How Linear Search Works

Iterate from index 0 to arr.length - 1. For each element, check if it matches the target. Return the index (or value, or boolean) on a match. Return a sentinel (-1 or false) after the loop if no match was found.

Search Variants

• First-match: return i immediately inside loop; return -1 after
• Last-match: update a variable on every match; return it after loop
• Count: increment counter on every match; return after loop
• Boolean exists: return true on first match; return false after loop

Code Examples

Example 1: First-Match Linear Search

public class Search {
    public static int firstMatch(int[] arr, int target) {
        for (int i = 0; i < arr.length; i++) {
            if (arr[i] == target) {
                return i;
            }
        }
        return -1;
    }
    public static void main(String[] args) {
        int[] arr = {4, 7, 2, 7};
        System.out.println(firstMatch(arr, 7));   // first 7
        System.out.println(firstMatch(arr, 99));  // not found
    }
}

Example 2: Count and Last-Match

public class MultiSearch {
    public static int countOf(int[] arr, int t) {
        int count = 0;
        for (int v : arr) if (v == t) count++;
        return count;
    }
    public static int lastMatch(int[] arr, int t) {
        int lastIdx = -1;
        for (int i = 0; i < arr.length; i++)
            if (arr[i] == t) {
                lastIdx = i;
            }
        return lastIdx;
    }
    public static void main(String[] args) {
        int[] data = {5, 3, 5, 9, 5};
        System.out.println("Count: " + countOf(data, 5));
        System.out.println("Last idx: " + lastMatch(data, 5));
    }
}

Example 3: String Array Search

public class WordSearch {
    public static boolean contains(String[] words, String target) {
        for (String w : words) {
            if (w.equals(target)) {
                return true;
            }
        }
        return false;
    }
    public static void main(String[] args) {
        String[] vocab = {"alpha", "beta", "gamma"};
        System.out.println(contains(vocab, "beta"));
        System.out.println(contains(vocab, "delta"));
    }
}

Common Pitfalls

Check for Understanding

public static int search(int[] arr, int target) {{
    for (int i = 0; i < arr.length; i++) {{
        if (arr[i] == target) {
            return arr[i];
        }
    }}
    return -1;
}}

public static int search(int[] a, int t) {{
    for (int i = 0; i < a.length; i++)
        if (a[i] == t) {
            return i;
        }
    return -1;
}}

public static int firstNegative(int[] arr) {{
    for (int i = arr.length - 1; i >= 0; i--) {{
        if (arr[i] < 0) {
            return i;
        }
    }}
    return -1;
}}

1: public static int findWord(ArrayList words, String w) {{
2:   for (int i = 0; i <= words.size(); i++) {{
3:     if (words.get(i).equals(w)) return i;
4:   }}
5:   return -1;
6: }}

Frequently Asked Questions

Related Topics