Segfault while allotting 2D array using double dimensional pointer - c

Question

I'm currently working on a codechef practice problem http://www.codechef.com/problems/STEPUP#
I'm trying to set up a 2D array using pointers to accept the data and enter in into the 2D array as i receive it using scanf.
#include<stdio.h>
#include<stdlib.h>
int main(int argc, char **argv)
{
int m,n,i,k,j;
int ex;
scanf("%d",&ex);
for(i=0;i<ex;i++)
{
int **edgegraph=NULL,temp1,temp2;
scanf("%d %d",&n,&m);
edgegraph=malloc(m*sizeof(int));
for(k=0;k<m;k++)
{
*(edgegraph+k)=malloc(m*sizeof(int));
if(!*(edgegraph+k))
exit(0);
}
for(k=0;k<m;k++)
{
scanf("%d %d",&temp1,&temp2);
*(*(edgegraph+m*temp1)+temp2)=1;
}
for(i=0;i<m;i++)
{
for(j=0;j<m;j++)
printf("%d ",*(*(edgegraph+m*i)+j));
printf("\n");
}
}
}
The error i get is
(gdb) run
Starting program: /home/vishwa/codechef/valid
2
2 2
1 2
Program received signal SIGSEGV, Segmentation fault.
0x000000000040079d in main (argc=1, argv=0x7fffffffded8) at validedge.c:24
24 *(*(edgegraph+m*temp1)+temp2)=1;
(gdb) quit
What I intend to do is create an m*m matrix, set all valid edges to 1 and then sort in ascending order of number of edges. I'm unsure if this will solve the problem, but would like to know where I'm messing up.

Answer 1

You malloc the wrong number of bytes: edgegraph=malloc(m*sizeof(int)); should have malloc(m * sizeof(int *)); . To avoid this sort of error you can use the following pattern:
ptr = malloc( N * sizeof *ptr );
which always allocates N of whatever ptr is a pointer to.
Next, the syntax x[y] is much simpler to read than *(x+y) especially when the expressions get complicated. Using that syntax would have avoided the mistake dconman points outs. You seem to have put an extra m * into your calculation where it is not required.
Also you mix up m and n later in your code. To avoid this sort of error, use more descriptive variable names.
So a fixed version of your allocation code could look like:
if ( 2 != scanf("%d %d",&num_edges, &num_vertices) )
exit(EXIT_FAILURE);
edgegraph = malloc( num_vertices * sizeof *edgegraph );
for (int vertex = 0; vertex < num_vertices; ++vertex)
{
edgegraph[vertex] = malloc( num_vertices * sizeof **edgegraph );
if ( edgegraph[vertex] == NULL )
exit(EXIT_FAILURE);
}
Note that it is possible to replace that malloc series with a single allocation:
int (*edgegraph)[num_vertices] = malloc( num_vertices * sizeof *edgegraph );
Moving onto your code to read edges. You wrote for(k=0;k<m;k++) however I think you meant n there. Using more descriptive variable names and the x[y] syntax:
for(int edge = 0; edge < num_edges; ++edge)
{
if ( 2 != scanf("%d %d",&temp1,&temp2) )
exit(EXIT_FAILURE);
if ( temp1 < 0 || temp1 >= num_vertices || temp2 < 0 || temp2 >= num_vertices )
exit(EXIT_FAILURE); // maybe display an error message
edgegraph[temp1][temp2] = 1;
edgegraph[temp2][temp1] = 1; // add this if undirected graph!
}
Now the final loop, for(i=0;i<m;i++). You have used the same variable i as control variable for this loop and for your outer loop. To avoid this sort of error, use scoped control variables:
for (int i = 0; i < num_edges; ++i)
Finally you will need to free the memory you malloc'd at the end of each time around the outer loop.

Answer 2

You are so close: lose the m* in your expressions to access an array element. Remember, you set up your 2d array as an array of rows, each with its own pointer (you allocated each independently).

Answer 3

*(*(edgegraph+m*temp1)+temp2)=1;
should be
*(*(edgegraph+temp1)+temp2)=1;
And the same change where you do that later in your code.
Is there a reason you are not using array indices?
EDIT
here is my input
2
10
10
4 3
4 9
7 3
3 7
4 3
4 5
7 4
3 5
9 0
5 2
And I got this output
0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 1 0 1 0 0
0 0 0 1 0 1 0 0 0 1
0 0 1 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0
0 0 0 1 1 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0
1 0 0 0 0 0 0 0 0 0