Topic 4.14 — Enhanced `for` Loop for 2D Arrays

Goal: use the enhanced for loop (for-each) to traverse 2D arrays, understand what each loop variable represents, and know when you need indexed loops instead.

The big idea

For a 2D array, the enhanced for loop is usually two enhanced loops: one for each row (which is itself an array), and one for each element in the row.

It’s perfect for reading all values (sum/count/max), but not great when you need row/col indexes.

Syntax (2D)

for (Type[] row : grid) {
  for (Type value : row) {
    // use value
  }
}

The first loop gives you each row array. The second loop gives you each element.

Example: print every value

for (int[] row : grid) {
  for (int x : row) {
    System.out.print(x + " ");
  }
  System.out.println();
}

Great use: sum / count / max

// sum
int sum = 0;
for (int[] row : grid) {
  for (int x : row) {
    sum += x;
  }
}

// count negatives
int neg = 0;
for (int[] row : grid) {
  for (int x : row) {
    if (x < 0) neg++;
  }
}

// max (assume grid has at least 1 element)
int max = grid[0][0];
for (int[] row : grid) {
  for (int x : row) {
    if (x > max) max = x;
  }
}

Limitation: you don’t get indexes

In a for-each loop, you don’t directly know r and c. If you need coordinates (like “row 2 col 3”), use indexed loops.

// Need row/col? Use indexes:
for (int r = 0; r < grid.length; r++) {
  for (int c = 0; c < grid[r].length; c++) {
    // grid[r][c] with (r,c)
  }
}

Updating is tricky

Assigning to the loop variable won’t update the array element.

for (int[] row : grid) {
  for (int x : row) {
    x = 0; // ❌ does not change grid
  }
}

If you need to change values, use indexed loops and assign to grid[r][c].

Safe with jagged arrays

Enhanced loops naturally handle jagged arrays because each row has its own length.

for (int[] row : jagged) {
  for (int x : row) {
    // works even if rows have different lengths
  }
}

Quick self-check

In for (int[] row : grid), what is row?
In the inner loop, what does x represent?
Why is for-each good for summing/counting?
Why is for-each not ideal when you need row/col positions?
How do you correctly update every element in a 2D array?

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Topic 4.14 — Enhanced for Loop for 2D Arrays