Traverse arrays and ArrayList

Track size and valid positions

An array has a fixed length. An ArrayList can change its size. Both use zero-based positions: an array's last valid index is length - 1 and an ArrayList's is size() - 1. A two-dimensional array is indexed by row and then column.

Removing an ArrayList element shifts later elements left. When removing through a forward index, the next element can move into the position you just visited. A backwards traversal avoids skipping it.

import java.util.ArrayList;
public class Demo {
    public static void main(String[] args) {
        ArrayList<Integer> scores = new ArrayList<Integer>();
        scores.add(40); scores.add(35); scores.add(70);
        for (int i = scores.size() - 1; i >= 0; i--) {
            if (scores.get(i) < 50) { scores.remove(i); }
        }
        System.out.println(scores);
    }
}

Test the traversal

The example prints [70]. Test an empty collection, all matching elements, and consecutive matching elements. For two-dimensional arrays, use each row's length rather than assuming all rows have the same number of columns.

When reading a dataset from a text file, separate parsing from analysis. Check missing or malformed values before adding them to a collection. A successful traversal cannot repair incorrect assumptions about the input data.

Check yourself