Practice›Java Fundamentals›Module 5. Arrays & Lists›5.3 Lists

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5.3 Lists — removing while looping

Lesson

Ловушка, которая молчит

Removing an element while walking forward through a list skips elements — and the program does not crash, so the bug hides. Every removal shifts the rest left by one, while the loop counter still moves right.

Смотрите сами

// list is [1, 2, 2, 3], remove every 2
for (int i = 0; i < list.size(); i++) {
    if (list.get(i) == 2) { list.remove(i); }
}
// result: [1, 2, 3]  — one 2 survived!

After removing the first 2, the second 2 slid into its place — but i had already moved past it.

The fix: walk backwards. Removals then shift only the part you have already visited:

for (int i = list.size() - 1; i >= 0; i--) {
    if (list.get(i) == 2) { list.remove(i); }
}
// result: [1, 3]

▶ Try it: run this, then change something and run it again.

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<Integer> bad = new ArrayList<Integer>();
        bad.add(1); bad.add(2); bad.add(2); bad.add(3);
        for (int i = 0; i < bad.size(); i++) {
            if (bad.get(i) == 2) {
                bad.remove(i);
            }
        }
        System.out.println(bad);

        ArrayList<Integer> good = new ArrayList<Integer>();
        good.add(1); good.add(2); good.add(2); good.add(3);
        for (int i = good.size() - 1; i >= 0; i--) {
            if (good.get(i) == 2) {
                good.remove(i);
            }
        }
        System.out.println(good);
    }
}

Removing while walking forward...

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Why does walking backwards fix it?

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Type exactly what this program prints.

import java.util.ArrayList;

public class Main { public static void main(String[] a){
    ArrayList<Integer> l = new ArrayList<Integer>();
    l.add(5); l.add(5); l.add(7);
    for (int i = l.size() - 1; i >= 0; i--) {
        if (l.get(i) == 5) {
            l.remove(i);
        }
    }
    System.out.println(l);
}}

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What makes this bug dangerous?

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True or false?

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