2-D arrays: working with a grid

A grid of boxes

• A 2-D array is a grid: rows and columns of the same type.
• int g[2][3]; makes 2 rows and 3 columns — 6 ints in all.
• You reach one cell with two indexes: g[row][col].

Rows then columns (row-major)

• C stores a grid row by row: all of row 0, then all of row 1.
• To visit every cell, use a for loop for the rows, and a for loop inside it for the columns.
• g[r][c] is the cell in row r, column c. Both start at 0.

Passing a 2-D array

• When you pass a grid to a function, you must give the column count: int g[][3].
• C needs the number of columns to find each row in memory; the row count can stay blank.
• So a function takes int g[][3], int rows — the columns are fixed at 3, the rows are passed in.

#include <stdio.h>

int main(void) {
    int g[2][3] = {{1, 2, 3}, {4, 5, 6}};
    int total = 0;
    for (int r = 0; r < 2; r++) {
        for (int c = 0; c < 3; c++) {
            total += g[r][c];
        }
    }
    printf("%d\n", total);   // 21
    return 0;
}

Working one row at a time

• You can also work on a single row: loop only the columns for a fixed row r.
• This is how you sum one row, or find the biggest value in a row.
• Mixing the two — an outer row loop with an inner column loop — handles the whole grid.

Common mistakes

• Index a grid grid[row][col].
• The outer loop walks the rows, the inner loop the columns.

Now you try

• For the function tasks, the grid has 3 columns, so the parameter is int g[][3].
• Do not write a main — the checker provides one.