Traversing 2D Arrays

Introduction

Unlike arrays, 2D arrays require nested iterative statements to traverse and access all elements. The easiest way to accomplish this is in row-major order. There may be some problems that use different algorithms so it is also important to consider additional traversal patterns, such as back and forth or column-major.

Because 2D arrays are traversed using nested loops, the number of times a code segment executes is multiplied. Recall that in a nested loop, the inner loop must complete all iterations before the outer loop can continue.

Printing a 2D Array

Nested iteration statements are used to traverse and access all elements in a 2D array. Since 2D arrays are stored as arrays of arrays, the way 2D arrays are traversed using for loops is like array objects. “Row-major order” refers to an ordering of 2D array elements where traversal occurs across each row, while “column-major order” traversal occurs down each column.

To print a 2D array, use a nested for loop. Consider the box 2d array from the previous lesson. This example shows a “row-major order”.

int [ ] [ ] box = { {1, 1, 0}, {9, 1, 2, 1}, {5} };
for (int row = 0; row < box.length; row++)
{
for (int col = 0; col < box[row].length; col++)
{
System.out.print (box[row] [col] + " ");
}
System.out.print1n ( ) ;
}

Nested iteration statements can also be written to traverse the 2D array in “column-major order”. This example shows a 2D array being initialized in column-major order.

int arr [ ] [ ] = new int [row] [col];

for (int i = 0; i < col; i++)
for (int j = 0; j < row; j++)
arr [j] [i]++;

If an enhanced for loop is used to traverse a 2D array, the variable must be the type of each row, which is an array. The inner loop traverses a single row. Therefore, the inner for-each loop variable must be the same type as the elements stored in the array.

This example will print the elements of the 2D array using a for-each loop.

int [ ] [ ] arr2D = { {8, 4, 9}, {1, 2, 0}, {2, 0, 2} };

for (int [ ] arr : arr2D) {
for (int num : arr) {
System.out.print (num) ;
}
System.out.print1n ( ) ;
}