C 2 dimensional array with variable size rows

Question

is there any convenient way to create a matrix without using malloc? This kind of works:

int *arr2d[3];
int arr0[] = { 0 };
int arr1[] = { 0, 1 };
int arr2[] = { 0, 1, 2 };

arr2d[0] = arr0;
arr2d[1] = arr1;
arr2d[2] = arr2;

printf(%d, arr2d[i][j]);

It doesn't allow you to loop through the values easily as you can't get the size of each subarray as you could using sizeof for arr2d[3][3].

sizeof arr2d[i] / sizeof arr2d[i][0]

Is there any better way to do this?

For reference, here is the same kind of question for C++:

C++ 2 dimensional array with variable size rows

Answer 1

You cannot do that: in this case, sizeof is evaluated statically, and it represents the size of the pointer. If you need to implement a jagged array with different sizes per row, you have two options:

• When the size can be computed from the row index, eg in a "triangular" array like yours (array length is row+1 ) you do not store anything at all
• When the size is arbitrary, create a separate array size_t len[rows] , and store each length individually.

Answer 2

Using a struct and compound literals can be done on the stack only.

typedef struct
{
    size_t size ;
    int* a ;

} jag_array ;

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

This has limitations. When you copy the struct the arrays themselves are not copied.

Separate functions and macros could help handling this, but it is not that pretty.