while loop condition doesn't terminate when reading file is finished

Question

this program will keep reading next 20 lines until user enters 'q' or file reading is finished.

now for file reading is finished part, I've condition on line 29 of this program flag != NULL

as per my understanding fgets() returns NULL after it reach EOF but this condition doesn't work. what can I do to stop the loop after file has been read?

I'm using this file for testing and here's my program:

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

int main()
{
    char name[66];
    printf("Enter name of  file: ");
    scanf("%65s", name);

    FILE *fp = fopen(name, "r");

    if (!fp) {
        printf("can't open file for reading\n");
        return 1;
    }
    int  i = 1;

    char buffer1[1000];
    char *flag;
    char c;

    do {
        do {
            i++;
            flag = fgets(buffer1, 999, fp);
            printf("%s", buffer1);
            //printf("%d***\n", i);  //debug - print line number
        } while (i % 20 != 0 && flag != NULL);
        scanf(" %c", &c);
    } while (c != 'q');

    return 0;
}

isn't scanf blocking... meaning it will actually pause each 20 until the user reacts — Jonathan
– Jonathan, Commented Jan 11, 2017 at 11:24
Yes, that's what I want. scanf will ask user input after each 20 lines. — Maulik
– Maulik, Commented Jan 11, 2017 at 11:32
Sidenote: flags are typically boolean values, not pointers. Using moire appropriate names makes code much more clear and help spotting logical errors. — too honest for this site
– too honest for this site, Commented Jan 11, 2017 at 12:29

Andrew Henle · Accepted Answer · 2017-01-11 11:48:58Z

3

Your outer do-while loop won't terminate until the user enters q even when EOF is reached.

do {
    do {
        i++;
        flag = fgets(buffer1, 999, fp);
        printf("%s", buffer1);
        //printf("%d***\n", i);  //debug - print line number
    } while (i % 20 != 0 && flag != NULL);
    scanf(" %c", &c);
} while (c != 'q'); // no EOF checking here

This will probably do what you want:

do {
    do {
        i++;
        flag = fgets(buffer1, 999, fp);
        // only print if fgets() returned new data
        if (flag != NULL) {
            printf("%s", buffer1);
        }
        //printf("%d***\n", i);  //debug - print line number
    } while (i % 20 != 0 && flag != NULL);
    scanf(" %c", &c);
} while (c != 'q' && flag != NULL);

edited Jan 11, 2017 at 11:48

answered Jan 11, 2017 at 11:19

Andrew Henle

35.1k3 gold badges29 silver badges62 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Maulik Over a year ago

gist.github.com/ganganimaulik/951ec5cbf0fb109b06de893d5742ee3a I've done this and it solves the issue but last line gets printed twice :o

Andrew Henle Over a year ago

@MaulikGangani last line gets printed twice That's because you call printf() even if your call to fgets() fails and returns NULL. See my edit

chux Over a year ago

Note: 1000-1 not needed in char buffer1[1000]; ... fgets(buffer1, 999, fp); Better to use 1000 via fgets(buffer1, sizeof buffer, fp);.

Andrew Henle Over a year ago

@chux Those would be improvements, but I decided to not make such changes and just make changes relevant to the question. I also considered the additional parenthesis used in the other answer, but again, I tried to stay close to the original posted code. (I became a big fan of parenthesizing everything when I spent close to a week trying to run down a bug that I finally traced to a huge conditional clause that omitted such parenthesis...)

Rishikesh Raje · Accepted Answer · 2017-01-11 11:19:02Z

2

You are not exiting the outer loop. In the outer while loop change

while (c != 'q');

to

while ((c != 'q') && (flag !=NULL));

answered Jan 11, 2017 at 11:19

Rishikesh Raje

8,6222 gold badges18 silver badges33 bronze badges

Collectives™ on Stack Overflow

while loop condition doesn't terminate when reading file is finished

2 Answers 2

4 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related