QUESTION 39 · Matrices and Dimensions

Why must the row width match in a two-dimensional array parameter?

2 marks

Answer

The function needs that width to advance correctly from one row to the next.

For an actual int a[2][3], a suitable parameter is int a[][3].
The adjusted parameter is a pointer to an array of three int, not int **.

Explain how a two-dimensional array is passed to a function.

5 marks

Answer in brief

A two-dimensional array argument normally supplies a pointer to its first row. The function must use the correct row type and valid active dimensions.

Fixed-width interface

void show(int a[][3], int rows);
/* equivalent parameter type: int (*a)[3] */

Meaning

The actual array has rows containing three ints. In the parameter, the outer array form adjusts to a pointer to one such row. The first bound may be omitted because the function receives a pointer, but the row width 3 describes the row type and the step used by a[i]. It must match the caller’s actual object.

Example explanation

main holds a 2-by-3 matrix and calls show(a, 2). The function visits two rows and three columns. The example prints the same matrix without copying the whole object. Since its parameter is not const-qualified, the function could modify elements, although this show function only reads them.

Capacity and active dimensions

In the matrix calculation programs, actual arrays have MAX columns even when only some columns are active. Their parameters therefore also use MAX as the row width. Passing cols as a logical count does not change the actual row width. C also supports other suitable dimension-aware forms, but this unit uses fixed-width parameters for clarity.

Important points and complexity

Do not cast an incompatible matrix to int ** or use a wrong row width. Pass and validate the row count separately. A display function for r rows and a fixed c-column width takes O(rc) time and O(1) auxiliary space beyond the input array.

C program and output

Download .c Open compiler

/* CodeBhavya | PPS AR25 | Learning example */
#include <stdio.h>
void show(int a[][3], int rows)
{
    for (int i = 0; i < rows; ++i) {
        for (int j = 0; j < 3; ++j) {
            if (j > 0) { putchar(' '); }
            printf("%d", a[i][j]);
        }
        putchar('\n');
    }
}

int main(void)
{
    int a[2][3] = {{1, 2, 3}, {4, 5, 6}};
    show(a, 2);
    return 0;
}

Sample input and output

This example uses values already written in the program.

Output

1 2 3
4 5 6