Declare matrix at file scope, dimensions from user input? C

Question

I have a program based on a matrix and need various functions to be able to access this matrix and it's dimensions, which I get from the user. I've managed to do it by passing them as arguments to each separate function, but that doesn't seem efficient.

When I try to declare this:

int lin, col;
char matrix[lin][col];

I get an error: variable length array declaration not allowed at file scope. I'm guessing it's because at that point, I haven't asked the user for 'lin' and 'col'? My question is: is there a way to make my matrix of variable dimensions have global scope? Or a way to access this matrix and dimensions without passing them as arguments to various functions?

If you cannot use dynamic allocation, just put greater values than needed. — Boiethios, Apr 20 '16 at 15:34
I hadn't yet learned about dynamic memory allocation. :) Since it was mentioned here, I googled and figured out what it meant. — Aryella Lacerda, Apr 22 '16 at 11:35

sjsam · Accepted Answer · 2016-04-20T16:00:28.463

Do

char **matrix;

Read rows r and columns c as input from the user.

Then do :

*matrix[c]=malloc(sizeof(matrix[r][c]); //You get contiguous memory here

Then you access the matrix like matrix[x][y] , the same way you access a 2D array.

At the end of the day, clear the memory allocated :

clear(matrix);

Or a way to access this matrix and dimensions without passing them as arguments to various functions?

Well, yes . You usually you pass the arguments like below :

void print_matrix(char** matrix_copy,int rows,int cols)
{
//Access the matrix using matrix_copy[x][y] format
}

You can just pass the matrix from main() like below :

print_matrix(matrix,r,c); // As simple as that.

Since you don't need arguments you need a workaround which is simple.

Declare

char **matrix;
int r,c; // rows & columns

outside all the functions, so that they are accessible across the program. In other words declare them in file scope. But in this case make sure that you access the matrix only after it is allocated some space using malloc.

This is probably the worst solution. It has, like, _zero_ memory locality for the data. — Lightness Races in Orbit, Apr 20 '16 at 16:13
@LightnessRacesinOrbit : Spatial locality may be guaranteed if the op has a small size matrix spanning only a few bytes/characters — sjsam, Apr 20 '16 at 16:24
Quite the opposite because you have an additional layer of indirection. That's the problem. Speaking generally, double-pointers = bad. — Lightness Races in Orbit, Apr 20 '16 at 16:45

score 0 · Answer 2 · answered Apr 20 '16 at 16:04

0

Dynamic Memory Allocation is a viable solution to your problem, as sjsam said. The only thing I would like to point out is that the code can look like this:

int i,lin,col;
char **matrix;
matrix=malloc(lin * sizeof(char*));
//check for NULL
for(i=0;i<lin;i++){
    matrix[i]=malloc(col * sizeof(char));
    //again,check for NULL
}

Then again, to free your allocated memory, you do:

for(i=0;i<lin;i++)
    free(matrix[i]);
free(matrix);

Hope that answers your question!

answered Apr 20 '16 at 16:04

D.Christodoulou

181
8

1

In this case memory allocated is not contiguous, ie it is fragmented. You get slow crappy code that burns.and crashes . – sjsam Apr 20 '16 at 16:11
Yes you are right but as the OP mentioned she is a beginner C programmer, I thought a simpler code might help her understand about pointers and the "malloc" function. Thank you for pointing that out though! – D.Christodoulou Apr 20 '16 at 16:14
This is not simpler either. – sjsam Apr 20 '16 at 16:14

Declare matrix at file scope, dimensions from user input? C

2 Answers2