I can't read data into an array - c

Question

I'm trying to read data from a text file and copy them into an array, but my code doesn't work. This is the code:
#include <stdio.h>
int main(int argc, char *argv[])
{
int i;
FILE *data;
data = fopen(argv[1], "r");
.......
fclose(data);
}
and when I replace
data = fopen(argv[1], "r");`
with
data = fopen("(the file name)", "r");
it works.
this is the full code
#include <stdio.h>
int main(int argc, char *argv[])
{
int i;
i=0;
FILE *data;
data = fopen(argv[1], "r");
while (!feof(data)) {
fscanf(data, "%i", &aa[i]);
i++;
}
fclose(data);
printf("%i\n", aa[0]);
}
and the text file is
3
2 1
2 2
2 3

Answer 1

You should always write basic error checking code for anything which might reasonably fail, e.g.:
#include <stdio.h>
#include <errno.h>
int main(int argc, char *argv[])
{
int i;
FILE *data;
if (argc < 2)
{
fprintf(stderr, "Missing arguments: %s\n", usage);
exit(1);
}
data = fopen(argv[1], "r");
if (data == NULL)
{
fprintf(stderr, "fopen('%s') failed, errno = %d, argv[1], errno);
exit(2);
}
.......
fclose(data);
return (0);
}
This way if fopen fails then (a) you won't crash and (b) you will get some useful info as to why the file was not opened.

Answer 2

You might be passing the file which is not available in the current directory.
Give the valid file directory with file name as argument
check for null after 'fopen'
compile with gcc -g -Wall main.c
and use gdb debugger to identify the error

Answer 3

You should pass filename while running the code.
./a.out file.txt.
If file.txt does not exist then it will throw error as segmentation fault.
Try this code, its working
#include <stdio.h>
int main(int argc, char *argv[])
{
int i;
int j = 0;
int aa[100]; //declare an int array to store the data, as you are retrieving using %i
i=0;
FILE *data;
data = fopen(argv[1], "r");
while (!feof(data)) {
fscanf(data, "%i", &aa[i]);
i++;
}
i--;
fclose(data);
for(j; j < i; j++)
printf("%i\n", aa[j]);
}
in file.txt give whatever data you are giving.
and while running pass ./a.out file.txt
See to it that file.txt is in same folder or give whole path