2d array where dimensions are not known until runtime

Question

I am writing a function where it reads in the files and store characters in a 2 dimensional array. I want this to be a global variable as I want multiple functions outside of the main method to be able to access it, but the dimensions cannot be set until after I've read the file, so what would the be approach in doing this. I've read a fair few posts in stack overflow before asking this question, but I'm still confused/ the questions don't help me with this specific problem.

(I am restricted to using the libraries/functions available with ANSI c)

One option would be to hide the implementation behind an interface, e.g. provide inline functions for checking the array size and accessing particular elements — M.M, Aug 11 '14 at 22:26
Can you clarify whether you mean C89 specifically? (C99 and C11 are ratified by ANSI, but sometimes people say "ANSI C" to mean just C89) — M.M, Aug 11 '14 at 22:28

score 2 · Accepted Answer · answered Aug 11 '14 at 22:22

You want to use malloc:

int i;
t_data * rows;
t_data ** array;

// Allocate the 2d array as a dynamic 1d array of pointers to rows
array = malloc(sizeof(t_data *) * num_rows);

// Allocate each row as a dynamic 1d array of t_data
for ( i = 0; i < num_rows; i++ )
    array[i] = malloc(sizeof(t_data) * num_cols);

Here, t_data is the data type of the data you want to store (for each array element), num_rows is the read number of rows, and num_cols is the read number of columns.

You can access elements via array[i][j].

When you're done with the array, you free it "in reverse":

for ( i = 0; i < num_rows; i++ )
    free(array[i]);

free(array);

score 0 · Answer 2 · answered Aug 11 '14 at 22:20

0

Anything that needs 'dynamic' sizing in C, will require the use of pointers, malloc(), free(), and related tools. Do a search online (or in a book, if you have one) for dynamic memory allocation.

Note though that this is a fairly broad topic (with associated learning curves and pitfalls), so if you come back with some code that you've implemented, we can help better with any directly related issues.

answered Aug 11 '14 at 22:20

roelofs

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To be accurate, C99 allows for VLA's (variable length arrays), so the statement *"Anything that needs 'dynamic' sizing in C, will require the use of pointers"* is not strictly true. – Ed S. Aug 11 '14 at 22:23
VLAs are only usable at block scope but OP requested global variable – M.M Aug 11 '14 at 22:24
@Matt McNabb: Yeah I was just about to amend my comment with that :P – Ed S. Aug 11 '14 at 22:24
And also, the user mentions ANSI C? – roelofs Aug 11 '14 at 22:25
All versions of ISO C are also ratified by ANSI – M.M Aug 11 '14 at 22:27
I'm used to C89 being referred to as ANSI C by a lot of people, even though all are ratified by ANSI. Must be a local thing ;) – roelofs Aug 11 '14 at 22:28
Actualy, [this](http://stackoverflow.com/questions/17206568/what-is-the-difference-between-c-c99-ansi-c-and-gnu-c-a-general-confusion-reg) post has some interesting comments in the accepted answer about ANSI and ISO. – roelofs Aug 11 '14 at 22:30

score 0 · Answer 3 · answered Aug 11 '14 at 22:24

You would need a pointer type, for example uchar*. Then at runtime, when you know the dimensions m x n, you would need to use malloc, like:

uchar *matrix = (uchar*)malloc(m * n * sizeof(uchar))

And then to navigate it you would use a function that takes into account strides:

int stride = m;
// for element (i,j)
matrix[i * stride + j];

2d array where dimensions are not known until runtime

3 Answers3