Why my inner loop only run once?

Question

a.c

#include <stdio.h>

int main(int argc, char *argv[])
{

    int i, counter=0;
    char c;

    FILE *file=fopen("a.txt","r");

    for (i = 0x41 ; i < 0x45; i++)
    {
        printf("%c(%x) ",i ,i);
        while ((c = fgetc(file)) != EOF)
        {
            if (i == (char) c)
                counter++;
        }
        printf("%d\n", counter);
        counter=0;
    }
    fclose(file);
    return 0;
}

a.txt

AAABBBAAA

I don't understand why the for loop runs perfectly but the while loop only runs once.

The output looks like

Shiekh

maybe you need to rewind? – Inequitable

How did you determine that the while loop ran only once? – Davedaveda

@DavidSchwartz probably by seeing the counter value. (I posted and deleted the same comment a while ago.) :) – Inequitable

Because after you read the file once you're now at the end of the file? – Shellback

rewind(file) after while loop will do the job – Quinquennium

ideone.com/A5e5zk – Xanthe

Answer 1

Read the man page for fgetc(), (emphasis mine)

fgetc() reads the next character from stream and returns it as an unsigned char cast to an int, or EOF on end of file or error.

So, once the while loop has run, it will exit once fgetc() returns EOF and it will keep returning EOF for all successive calls.

You need to reset the file pointer to the beginning of the stream (using rewind() or fseek()) to start over from the beginning.

That said,

• fgetc() returns an int and a char is too short to store all the possible return value (for example, EOF itself). Change the type of c to int.
• For a hosted environment, int main() should at least be int main(void) to conform to the standard.

Inequitable

You're welcome. You can also consider accepting an answer that helped you. – Inequitable

Answer 2

In the first run of the for loop, the whole file gets used up, and you never reset it. So it will only successfully search for character 0x41.

You need to do one of these two operations after the while loop:

fseek(file, 0, SEEK_SET);

rewind(file);

The first is preferred because of better error handling.

Coracorabel

Answer 3

This should do the job for you:

#include <stdio.h>

int main(void)
{

    int i, counter=0;
    int c;

    FILE *file=fopen("a.txt","r");

    for (i = 0x41 ; i < 0x45; i++)
    {
        printf("%c(%x) ",i ,i);
        while ((c = fgetc(file)) != EOF)
        {
            if (i == c) counter++;
        }
        rewind(file);
        printf("%d\n", counter);
        counter=0;
    }
    fclose(file);
    return 0;
}

Orlan