Read adjacency matrix from file - Competitive programming - c

Question

I have to do an assignment where I have to read a file that contains an adjacency matrix and later do some stuff.
I have all working but my code is very slow, at least for the benchmarking system.
I'm reading all the file rows in this code snippet below:
while (fgets(buf, sizeof(buf), stdin) != NULL) {
parse(buf);
i++;
}
and then I initialize my 2d array with all the values using strtok and atoi:
void parse(char *str, int count, char *sep) {
//char *aux = malloc(count * sizeof(char*));
char *aux;
aux = strtok(str, sep);
int j = 0;
while (aux) {
array[(i*DIM) + j] = atoi(aux);
j++;
aux = strtok(NULL, sep);
}
//free(aux);
}
Arrays are DIM*DIM size and each INT is separated by a comma.
Sample input for a 3*3 matrix:
1,20,1
0,111,3
4,7,10
How can I improve this for better performances?
EDIT:
array definition:
array = malloc(DIM*DIM*sizeof(int));

Answer 1

The malloc part makes no sense since you just need one single character pointer for strtok. Similarly, functions like strtol or atoi already parse the data, so you don't even need strtok - it just takes up extra time in this case. Furthermore, atoi doesn't have any error handling so it should never be used.
So you can just call strtol in a loop and it will do what you want. By checking the endptr argument you can see if each read was successful or not (man strtol). And then next lap in the loop, start over from endptr + 1.
If combining this with your 2D int array requirement, the function might look like this:
#include <stdio.h>
#include <stdlib.h>
void csv_to_int (size_t col, size_t row, int dst[col][row], const char* str)
{
const char* ptr = str;
char* end;
for(size_t c=0; c<col; c++)
{
for(size_t r=0; r<row; r++)
{
int val=strtol(ptr,&end,10);
if(ptr==end)
{
return ;
}
dst[c][r]=val;
ptr = end+1;
}
}
}
int main (void)
{
const char* input = "1,20,1\n0,111,3\n4,7,10\n";
int arr[3][3];
csv_to_int(3, 3, arr, input);
for(size_t i=0; i<3; i++)
{
for(size_t j=0; j<3; j++)
{
printf("%3d ", arr[i][j]);
}
puts("");
}
}
Output:
1 20 1
0 111 3
4 7 10
This is of course assuming that the input suits the 3x3 format - this code has almost no error handling.

Answer 2

How can I improve this for better performances?
So do not use these functions, if you think they are slow. Limit your requirements - do not handle locale specific digits. So read and convert it yourself. Blatantly disregard error checking. Something along:
#define _GNU_SOURCE 1
#include <stdio.h>
int main() {
char data[] ="1,20,1\n0,111,3\n4,7,10\n";
FILE *f = fmemopen(data, sizeof(data), "r");
int i = 0, j = 0;
#define DIM 3
int array[20];
// this reading part
int buf = 0;
for (int c; (c = fgetc(f)) != EOF; ) {
if (c == '\n') {
array[i * DIM + j] = buf;
buf = 0;
++i;
j = 0;
} else if (c == ',') {
array[i * DIM + j] = buf;
buf = 0;
++j;
} else {
buf *= 10;
buf += (c - '0');
}
}
array[i * DIM + j] = buf;
++j;
// checking
for (int i = 0; i < 3; ++i) {
for (int j = 0; j < 3; ++j) {
printf("%d %d = %d\n", i, j, array[i * DIM + j]);
}
}
}
Even further, ignore portability, and use system calls (on unix - read(STDIN_FILENO)) instead of C API.