How do you traverse int[][]?

Question

I tried doing:

for (int i=0; i<matrix.length; i++) {
  for (int j=0; j<matrix[].length; j++) {

but that doesn't work :(

well actually if it's a square matrix, then it's easy – Devoted Nov 08 '10 at 22:25 — Devoted, Nov 08 '10 at 22:25

score 8 · Accepted Answer · edited Nov 08 '10 at 22:28

8

Just index into the matrix using your current row.

for (int i=0; i<matrix.length; i++) {
  for (int j=0; j<matrix[i].length; j++) {
    // code
  }
}

This even factors in the possibility of a jagged array (i.e., a matrix with an inconsistent number of rows in the column).

edited Nov 08 '10 at 22:28

Jeffrey Hantin

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answered Nov 08 '10 at 22:27

Platinum Azure

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score 5 · Answer 2 · answered Nov 08 '10 at 22:26

5

for (int i=0; i<matrix.length; i++) {
    for (int j=0; j<matrix[i].length; j++) {

answered Nov 08 '10 at 22:26

aromero

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score 2 · Answer 3 · answered Nov 08 '10 at 22:27

2

for (int i=0; i<matrix.length; i++) {
  for (int j=0; j<matrix[i].length; j++) {

Notice the i I added.

However, it's best to cache both lengths while looping to avoid re-evaluation on each loop, which saves time at the only cost of one more int in memory (the fix still applies):

for (int i=0, il=matrix.length; i<il; i++) {
  for (int j=0, jl=matrix[i].length; j<jl; j++) {

answered Nov 08 '10 at 22:27

Delan Azabani

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Note that caching the array lengths will only work if the array won't resize in the loop. – Bitsplitter Nov 08 '10 at 22:49
That's true. It's good for if you're not changing the size, but only reading or writing. – Delan Azabani Nov 08 '10 at 22:51

How do you traverse int[][]?

3 Answers3