4.2 Two-dimensional Arrays and Statistics

A two-dimensional array stores values in rows and columns.

cpp

int grid[2][3] = {
    {1, 2, 3},
    {4, 5, 6}
};

The first index chooses the row, and the second index chooses the column.

cpp

std::cout << grid[0][2] << std::endl;  // 3
grid[1][0] = 10;

Nested loops are the usual way to process a two-dimensional array.

cpp

#include <iostream>

int main() {
    int grid[2][3] = {
        {1, 2, 3},
        {4, 5, 6}
    };

    int total = 0;
    for (int row = 0; row < 2; row++) {
        for (int col = 0; col < 3; col++) {
            total += grid[row][col];
        }
    }

    std::cout << "Total: " << total << std::endl;
    return 0;
}

Matrix addition and subtraction work element by element. Values at the same row and column are combined to form the result matrix.

Element-wise Matrix Operations

131012+007521=138533

131012-007521=13-6-5-11

cpp

int A[3][2] = {{1, 2}, {3, 4}, {5, 6}};
int B[3][2] = {{6, 5}, {4, 3}, {2, 1}};
int C[3][2];

for (int row = 0; row < 3; row++) {
    for (int col = 0; col < 2; col++) {
        C[row][col] = A[row][col] + B[row][col];
    }
}

For statistics such as row totals, column totals, maximums, or averages, keep one accumulator for the value you are computing and update it inside the correct loop.

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