For loop iterating twice the expected iterations

Question

I'm writing a C program which prompts user for a file name and when entered it shows first 20 lines and then wait for the input from user. So, everything worked fine just that it showed 40 lines instead of 20, so I reduced the number of lines to 1 and it showed 2 lines instead. I even tried with while loop instead of for loop but still same result.

#include <stdio.h>

int main (void)
{
    FILE *in;
    char file[81], buffer[81];
    int flag = 1, c, i;

    printf("File: ");
    scanf("%s", file);

    if ((in = fopen (file, "r")) == NULL) {
        printf("Unable to open the file: %s\n", file);
        return 1;
    }

    while (flag) {
        for (i = 0; i < 1; i++) {
            if (fgets (buffer, 80, in) != NULL) {
                printf("%s", buffer);
            } else {
                flag = 0;
            }
        }

        c = getchar();
        if (c == (char) 'q')
            flag = 0;
    }

    fclose (in);

    return 0;
}

Problem in C by Stephen G Kochan, Chapter 16, Problem 6

Note: I'm not expecting an alternative method to solve the problem, I just want to know what is causing this problem.

I don't see anything that counts the number of lines. And what's the purpose of a `for` loop that counts from 1 to 1? — Barmar, Aug 17 '17 at 16:32
it loops because `flag` is set only when `fgets` fails. why this flag? — Jean-François Fabre, Aug 17 '17 at 16:32
@Barmar Read the question more closely, that `1` was `20` before ;-) — dhke, Aug 17 '17 at 16:33
@Barmar I just fixed it, it was supposed to be for just one iteration, — Ankit R Gadiya, Aug 17 '17 at 16:33
The book has an error. `char file[81]` and you can `fgets(...81...)`. — Antti Haapala -- Слава Україні, Aug 17 '17 at 16:34
`fgets()` already subtracts 1 to allow for the null byte, you don't have to do it yourself. — Barmar, Aug 17 '17 at 16:35
To clarify: It also shows the 20 (or 2) lines the first time round? Otherwise, I'd say `getchar()` picks up the newline you need to enter to submit the line. — dhke, Aug 17 '17 at 16:35
getchar reads standard input, fgets reads a file, not stdin. — Jean-François Fabre, Aug 17 '17 at 16:35
@dhke it showes 2 lines of text the first time with this exact code and when i replaced 1 with 20 it shows 40 lines of text. — Ankit R Gadiya, Aug 17 '17 at 16:41
@AnkitRGadiya No, I meant `getchar()`. See Jean-François Fabre's below. Your `getchar()` consumes the newline left there after the initial `scanf()`. If you hit enter on the prompt, you should only be getting 10 lines, however if you hit e.g. ` `, you'll be getting 20 lines. — dhke, Aug 17 '17 at 16:46
Aside: in `if (c == (char) 'q')` the `c` and the `'q'` are both of type `int` so no need for any cast. — Weather Vane, Aug 17 '17 at 16:51
@dhke `fgets` does not leave a newline in the input buffer - it reads it and includes it in the string that was read. — Weather Vane, Aug 17 '17 at 16:56
@WeatherVane I never mentioned `fgets()`. The code does a `scanf()` from stdin. After this, there's still a `'\n'`in the input buffer for `stdin`. That gets consumed by the subsequent `getchar()`, skipping the intended pause. There is no interaction with the `fgets()` from the file in this. I missed that part the first time round and though only of `q` `\n` being entered, causing two iterations to run. — dhke, Aug 17 '17 at 17:18

score 2 · Accepted Answer · answered Aug 17 '17 at 16:36

2

the issue is that there's a remaining linefeed from your first scanf("%s", file); statement that is picked up by your getchar() the first time, so the loop runs twice without any user input.

To prevent that, just getchar() before your loop.

Aside (as comments noted), fgets (buffer, 80, in) needs buffer[80] only.

And scanf("%s", file); could be protected like scanf("%80s", file); (here you need to keep 81 bytes, unlike in fgets)

answered Aug 17 '17 at 16:36

Jean-François Fabre

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If scanf call is the problem then why is it happening everything the loop iterates. Each consecutive time the for loop runs it iterates twice as many times not just the first time. Also, about the getchar() call, i cannot place it before the loop as the problem wants me to first print 20 lines and then prompt user for input, if the user enter 'q', then the program should stop, any other input should result in printing the next 20 lines. – Ankit R Gadiya Aug 17 '17 at 16:48
1

Because you have `getchar` in the loop. You enter a character that is not `'q'` not just a plain `Enter`? Then there will a newline in the input buffer. – Weather Vane Aug 17 '17 at 16:52
Okay i got it, I tried hardcoding the file name and thus removing the scanf call and it worked. Obviously not the correct idea though. Thanks – Ankit R Gadiya Aug 17 '17 at 17:00

For loop iterating twice the expected iterations

1 Answers1