I need to do this to persist operations on the matrix as well. Does that mean that it needs to be passed by reference?

Will this suffice?

void operate_on_matrix(char matrix[][20]);

4 Answers

C does not really have multi-dimensional arrays, but there are several ways to simulate them. The way to pass such arrays to a function depends on the way used to simulate the multiple dimensions:

1) Use an array of arrays. This can only be used if your array bounds are fully determined at compile time, or if your compiler supports VLA's:

#define ROWS 4 #define COLS 5 void func(int array[ROWS][COLS]) { int i, j; for (i=0; i<ROWS; i++) { for (j=0; j<COLS; j++) { array[i][j] = i*j; } } } void func_vla(int rows, int cols, int array[rows][cols]) { int i, j; for (i=0; i<rows; i++) { for (j=0; j<cols; j++) { array[i][j] = i*j; } } } int main() { int x[ROWS][COLS]; func(x); func_vla(ROWS, COLS, x); } 

2) Use a (dynamically allocated) array of pointers to (dynamically allocated) arrays. This is used mostly when the array bounds are not known until runtime.

void func(int** array, int rows, int cols) { int i, j; for (i=0; i<rows; i++) { for (j=0; j<cols; j++) { array[i][j] = i*j; } } } int main() { int rows, cols, i; int **x; /* obtain values for rows & cols */ /* allocate the array */ x = malloc(rows * sizeof *x); for (i=0; i<rows; i++) { x[i] = malloc(cols * sizeof *x[i]); } /* use the array */ func(x, rows, cols); /* deallocate the array */ for (i=0; i<rows; i++) { free(x[i]); } free(x); } 

3) Use a 1-dimensional array and fixup the indices. This can be used with both statically allocated (fixed-size) and dynamically allocated arrays:

void func(int* array, int rows, int cols) { int i, j; for (i=0; i<rows; i++) { for (j=0; j<cols; j++) { array[i*cols+j]=i*j; } } } int main() { int rows, cols; int *x; /* obtain values for rows & cols */ /* allocate the array */ x = malloc(rows * cols * sizeof *x); /* use the array */ func(x, rows, cols); /* deallocate the array */ free(x); } 

4) Use a dynamically allocated VLA. One advantage of this over option 2 is that there is a single memory allocation; another is that less memory is needed because the array of pointers is not required.

#include <stdio.h> #include <stdlib.h> #include <time.h> extern void func_vla(int rows, int cols, int array[rows][cols]); extern void get_rows_cols(int *rows, int *cols); extern void dump_array(const char *tag, int rows, int cols, int array[rows][cols]); void func_vla(int rows, int cols, int array[rows][cols]) { for (int i = 0; i < rows; i++) { for (int j = 0; j < cols; j++) { array[i][j] = (i + 1) * (j + 1); } } } int main(void) { int rows, cols; get_rows_cols(&rows, &cols); int (*array)[cols] = malloc(rows * cols * sizeof(array[0][0])); /* error check omitted */ func_vla(rows, cols, array); dump_array("After initialization", rows, cols, array); free(array); return 0; } void dump_array(const char *tag, int rows, int cols, int array[rows][cols]) { printf("%s (%dx%d):\n", tag, rows, cols); for (int i = 0; i < rows; i++) { for (int j = 0; j < cols; j++) printf("%4d", array[i][j]); putchar('\n'); } } void get_rows_cols(int *rows, int *cols) { srand(time(0)); // Only acceptable because it is called once *rows = 5 + rand() % 10; *cols = 3 + rand() % 12; } 

(See srand() — why call it only once?.)

Easiest Way in Passing A Variable-Length 2D Array

Most clean technique for both C & C++ is: pass 2D array like a 1D array, then use as 2D inside the function.

#include <stdio.h> void func(int row, int col, int* matrix){ int i, j; for(i=0; i<row; i++){ for(j=0; j<col; j++){ printf("%d ", *(matrix + i*col + j)); // or better: printf("%d ", *matrix++); } printf("\n"); } } int main(){ int matrix[2][3] = { {0, 1, 2}, {3, 4, 5} }; func(2, 3, matrix[0]); return 0; } 

_{Internally, no matter how many dimensions an array has, C/C++ always maintains a 1D array. And so, we can pass any multi-dimensional array like this.}

I don't know what you mean by "data dont get lost". Here's how you pass a normal 2D array to a function:

void myfunc(int arr[M][N]) { // M is optional, but N is required .. } int main() { int somearr[M][N]; ... myfunc(somearr); ... } 

2D array:

int sum(int array[][COLS], int rows) { } 

3D array:

int sum(int array[][B][C], int A) { } 

4D array:

int sum(int array[][B][C][D], int A) { } 

and nD array:

int sum(int ar[][B][C][D][E][F].....[N], int A) { }