Reading file character by character, line by line in C

Question

I have a file with multiple lines. Each line has two numbers separated by a white-space. I need to use this code, which prints the first line, to print the whole file, but doing it line by line

do
{
   fscanf(fp,"%c", &c);
   if(c == ' ')
     break;
   printf("%c", c);
}
while (c != ' ');

do
{
   fscanf(fp, "%c", &c);
   printf("%c", c);
}
while( c != '\n');

I tried to use fgets but got into an infinite loop.

while(fgets(buf, sizeof buf, fp) != NULL) // assuming buf can handle the line lenght
{
   //code above
}

Why i cant use fgets like that to print it line by line?

Sample

Input:

10 5003 20 320 4003 200

Output

10 5003
20 320
4003 200

Answer 1

Obviously I can't use your exact code, or you would not have asked the question, but this is a similar idea, done a bit differently, and also using the fgets which was causing you trouble. It works by searching each line for digits, and non-digits. Note that I have #define MAXLEN 2000 to be generous, because in the previous question, you say each number can have 500 digits, so the line might be at least 1000 characters.

#include <stdio.h>
#include <ctype.h>

#define MAXLEN  2000

int main(void)
{
    FILE *fp;
    char line[MAXLEN];
    char *ptr;
    if((fp = fopen("test.txt", "rt")) == NULL)
        return 0;                               // or other action

    while(fgets(line, MAXLEN, fp) != NULL) {
        ptr = line;
        // first number
        while(*ptr && !isdigit(*ptr)) {        // skip non-digits
            ptr++;
        }
        while(*ptr && isdigit(*ptr)) {
            printf("%c", *ptr++);              // print digits
        }
        printf(" ");

        // second number
        while(*ptr && !isdigit(*ptr)) {        // skip non-digits
            ptr++;
        }
        while(*ptr && isdigit(*ptr)) {
            printf("%c", *ptr++);              // print digits
        }
        printf("\n");

    }
    fclose(fp);
    return 0;
}

EDIT you could make it more concise like this, with a loop to read each set of digits:

char *terminate = " \n";                    // 1st number ends with space, 2nd with newline
int i;
while(fgets(line, MAXLEN, fp) != NULL) {
    ptr = line;
    for(i=0; i<2; i++) {
        while(*ptr && !isdigit(*ptr)) {     // skip non-digits
            ptr++;
        }
        while(*ptr && isdigit(*ptr)) {
            printf("%c", *ptr++);           // print digits
        }
        printf("%c", terminate[i]);         // space or newline
    }
}

Program output (from your input):

10 5003
20 320
4003 200

Answer 2

If we replace printf(fp, "%c", c) with printf("%c", c) (because we are not printing in a file, right?), the following sample should do the job (it creates test file with ten lines).

#include <stdio.h>
#include <stdlib.h>

int main() {
    char c;
    FILE* fp;

    fp = fopen("test.txt", "w+");

    for (int i = 0; i < 10; i++)
    {
        fprintf(fp, "Test line %i\n", i);
    }

    rewind(fp);

    do {
        do {
            fscanf(fp, "%c", &c);
            if (c == ' ')
                break;
            printf("%c", c);
        } while (c != ' ');
        do {
            fscanf(fp, "%c", &c);
            printf("%c", c);
        } while (c != '\n');
    }while ((c=fgetc(fp))!=EOF?printf("%c", c):0);

    fclose(fp);

}