Question

I’m currently working through Project Euler problems as a way of learning C and I’m having trouble with problem #8, the actual maths and main bit of program is fine but the method requires reading individual digits from a large (1000 digit) number, which I put in a file.

The code below is for a test case, where my file “test.txt” is the number “123456789”. What I am trying to do is print out each digit on a separate line.

#include <stdio.h>

int main()
{
    int c = 0;
    FILE *fp;
    fp = fopen("test.txt", "r");

    while(c != EOF)
    {
        c = fgetc(fp);
        printf("%i\n", c);      

    }

    fclose(fp);

}

When I run this code, I get the output

49
50
51
52
53
54
55
56
57
10
-1

I have a hunch I am missing something really simple from my code and I would expect it is in the fgetc bit. If this is the case, what would be a better function to use and I’m also curious of why I get such a weird output – when I try it with my 1000 digit number I find a similar pattern with numbers around 49 to high 50s being output almost cyclically. Please bear in mind I am only a beginner with C, cheers.

Answer 1

Those are ASCII for the digits:

48 = '0'
49 = '1'
50 = '2'

etc. They’re in that format because they’re characters stored in a text file which represent integer digits.

You need to print:

if((48 <= c) && (c <= 57))
  printf("%d\n", c - '0');

Note: You’ll want that if check because the 10 is a newline and the -1 is EOF, you’re printing more then you wanted.

Of course there are lots of ways to do this if check:

if(('0' <= c) && (c <= '9'))

or

if(isdigit(c))

The point is simply that you need to check if you only want digits.