arrayscrecursionmatrix

# Printing 2D array using recursion (C programming)

I'm currently practicing recursion so I can use it in the Tideman CS50 problem. Specifically, I'd like to learn to traverse a matrix (2D array) using recursion.

For practice purposes, I've created a simple 3x3 2D array and I want to print the contents of that array.

Unfortunately, my logic breaks after the first row is successfully printed: printing subsequent elements of the array is incorrect. More precisely, my recursive function "breaks" when trying to print an element positioned at `i = 1`, `j = 0`. I would like to know why is this happening.

The code:

``````#include <stdio.h>

void recursive_print(int lenght, int i, int j, int array[i][j]);

int main(void)
{
int lenght = 3;
int i = 0;
int j = 0;

int array[lenght][lenght];
array[0][0] = 0;
array[0][1] = 1;
array[0][2] = 2;
array[1][0] = 3;
array[1][1] = 4;
array[1][2] = 5;
array[2][0] = 6;
array[2][1] = 7;
array[2][2] = 8;

recursive_print(lenght, i, j, array);

return 0;
}

void recursive_print(int lenght, int i, int j, int array[i][j])
{
// Base case
if (i == lenght - 1 && j == lenght - 1)
{
printf("Base case\n");
printf("%i\n", array[i][j]);
}

// Recursive case(s)
else if (j == lenght - 1)
{
printf("Recursive case 1\n");
printf("%i\n", array[i][j]);
recursive_print(lenght, i + 1, j - j, array);
}
else
{
printf("Recursive case 2\n");
printf("%i\n", array[i][j]);
recursive_print(lenght, i, j + 1, array);
}
}
``````

Actual output:

``````Recursive case 2
0
Recursive case 2
1
Recursive case 1
2
Recursive case 2
0
Recursive case 2
2
Recursive case 1
4
Recursive case 2
0
Recursive case 2
3
Base case
6
``````

Expected output:

``````Recursive case 2
0
Recursive case 2
1
Recursive case 1
2
Recursive case 2
3
Recursive case 2
4
Recursive case 1
5
Recursive case 2
6
Recursive case 2
7
Base case
8
``````

Solution

• Your argument for `int array[i][j]` is wrong, it should be either `int array[3][3]`, or more elegantly `array[][3]`.

Assume that `array` is organized `1d` in the memory. The `array[][3]` lets the compiler know to skip `3` positions when going down a line (i.e. increasing the first dimension).

I think you are getting messy results because recursively calling the function with `array[1][0]` tells skip `0` when going to a new line, so actually printing `array[1][0]` would point to the integer at the address `array + 1 * 0 + 0 = array`, so it prints `0`.

``````array[1][1] -- skip 1
array[1][1] -> *(array + 1 * 1 + 1) = *(array + 2) = 2

array[1][2] -- skip 2
array[1][2] -> *(array + 1 * 2 + 2) = *(array + 4) = 4

array[2][0] -- skip 0
array[2][0] -> *(array + 2 * 0 + 0) = *(array + 0) = 0

array[2][1] -- skip 1
array[2][1] -> *(array + 2 * 1 + 1) = *(array + 3) = 3

array[2][2] -- skip 2
array[2][2] -> *(array + 2 * 2 + 2) = *(array + 6) = 6
``````