The elements of an array can be of any data type, including arrays! An array of arrays is called a multidimensional array.
1 |
int array[3][5]; // a 3-element array of 5-element arrays |
Since we have 2 subscripts, this is a two-dimensional array.
In a two-dimensional array, it is convenient to think of the first (left) subscript as being the row, and the second (right) subscript as being the column. This is called row-major order. Conceptually, the above two-dimensional array is laid out as follows:
[0][0] [0][1] [0][2] [0][3] [0][4] // row 0 [1][0] [1][1] [1][2] [1][3] [1][4] // row 1 [2][0] [2][1] [2][2] [2][3] [2][4] // row 2
To access the elements of a two-dimensional array, simply use two subscripts:
1 |
array[2][3] = 7; |
Initializing two-dimensional arrays
To initialize a two-dimensional array, it is easiest to use nested braces, with each set of numbers representing a row:
1 2 3 4 5 6 |
int array[3][5] = { { 1, 2, 3, 4, 5 }, // row 0 { 6, 7, 8, 9, 10 }, // row 1 { 11, 12, 13, 14, 15 } // row 2 }; |
Although some compilers will let you omit the inner braces, we highly recommend you include them anyway, both for readability purposes and because of the way that C++ will replace missing initializers with 0.
1 2 3 4 5 6 |
int array[3][5] = { { 1, 2 }, // row 0 = 1, 2, 0, 0, 0 { 6, 7, 8 }, // row 1 = 6, 7, 8, 0, 0 { 11, 12, 13, 14 } // row 2 = 11, 12, 13, 14, 0 }; |
Two-dimensional arrays with initializer lists can omit (only) the leftmost length specification:
1 2 3 4 5 6 |
int array[][5] = { { 1, 2, 3, 4, 5 }, { 6, 7, 8, 9, 10 }, { 11, 12, 13, 14, 15 } }; |
The compiler can do the math to figure out what the array length is. However, the following is not allowed:
1 2 3 4 5 |
int array[][] = { { 1, 2, 3, 4 }, { 5, 6, 7, 8 } }; |
Just like normal arrays, multidimensional arrays can still be initialized to 0 as follows:
1 |
int array[3][5] = { 0 }; |
Note that this only works if you explicitly declare the length of the array! Otherwise, you will get a two-dimensional array with 1 row.
Accessing elements in a two-dimensional array
Accessing all of the elements of a two-dimensional array requires two loops: one for the row, and one for the column. Since two-dimensional arrays are typically accessed row by row, the row index is typically used as the outer loop.
1 2 3 |
for (int row = 0; row < numRows; ++row) // step through the rows in the array for (int col = 0; col < numCols; ++col) // step through each element in the row std::cout << array[row][col]; |
In C++11, for-each loops can also be used with multidimensional arrays. We’ll cover for-each loops in detail later.
Multidimensional arrays larger than two dimensions
Multidimensional arrays may be larger than two dimensions. Here is a declaration of a three-dimensional array:
1 |
int array[5][4][3]; |
Three-dimensional arrays are hard to initialize in any kind of intuitive way using initializer lists, so it’s typically better to initialize the array to 0 and explicitly assign values using nested loops.
Accessing the element of a three-dimensional array is analogous to the two-dimensional case:
1 |
std::cout << array[3][1][2]; |
A two-dimensional array example
Let’s take a look at a practical example of a two-dimensional array:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 |
#include <iostream> int main() { // Declare a 10x10 array const int numRows = 10; const int numCols = 10; int product[numRows][numCols] = { 0 }; // Calculate a multiplication table for (int row = 0; row < numRows; ++row) for (int col = 0; col < numCols; ++col) product[row][col] = row * col; // Print the table for (int row = 1; row < numRows; ++row) { for (int col = 1; col < numCols; ++col) std::cout << product[row][col] << "\t"; std::cout << '\n'; } return 0; } |
This program calculates and prints a multiplication table for all values between 1 and 9 (inclusive). Note that when printing the table, the for loops start from 1 instead of 0. This is to omit printing the 0 column and 0 row, which would just be a bunch of 0s! Here is the output:
1 2 3 4 5 6 7 8 9 2 4 6 8 10 12 14 16 18 3 6 9 12 15 18 21 24 27 4 8 12 16 20 24 28 32 36 5 10 15 20 25 30 35 40 45 6 12 18 24 30 36 42 48 54 7 14 21 28 35 42 49 56 63 8 16 24 32 40 48 56 64 72 9 18 27 36 45 54 63 72 81
Two dimensional arrays are commonly used in tile-based games, where each array element represents one tile. They’re also used in 3d computer graphics (as matrices) in order to rotate, scale, and reflect shapes.
6.6 -- C-style strings ^{[1]} |
Index ^{[2]} |
6.4 -- Sorting an array using selection sort ^{[3]} |