Initialized variable skips 0 during for loop

Question

This is a simple version of my code that still doesn't work when I run it. When I initialize i to be the counter for my for loop, it skips 0 for some reason, this has never happened to me before and I don't know why.

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

int main() {
    int x, i;
    scanf("%d", &x);
    char array[x][10];
    for (i = 0; i < x; i++) {
        printf("%d\n", i);
        gets(array[i]);
    }
    return 0;
}

edit: I input 5 for x, therefore I would have a character array size 5 rows, 10 columns. For i, it is supposed to start at 0, so I input array[0] first but skips to 1, so I start my input with array[1], that means my first row has no input.

[___DO NOT___ use `gets()`, it is dangerous. use `fgets()` instead.](https://stackoverflow.com/a/41383540/2173917) — Sourav Ghosh, Mar 15 '18 at 06:37
How would you say that 0 is skipped? What is the expected behavior, what is the observed behavior? — harper, Mar 15 '18 at 06:39
If you use the C tag, it is assumed that you program in standard C. But this code cannot compile as standard C because it is too old. Therefore you need to add an extra tag corresponding to the dinosaur version of C that you are for some reason using, such as C90 or C99. — Lundin, Mar 15 '18 at 09:16
Also make sure to burn the book or teacher that told you to use gets, then get a new one. — Lundin, Mar 15 '18 at 09:17

Stephen Docy · Accepted Answer · 2018-03-15T06:54:26.250

1

scanf() leaves some characters in the input stream, which are picked up by the first call to gets(), causing the first gets() to finish immediately and the program to continue and prompt for the input for array[1], hence "skipping" over 0.

edited Mar 15 '18 at 06:54

answered Mar 15 '18 at 06:45

Stephen Docy

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How do I prevent that? I used fgets() and the result is the same. – Hele Mar 15 '18 at 06:51
You could add an extra `fgets()` prior to the loop to drain the input stream, or possibly just continue to use `scanf()` instead, if all you want to do is get one integer at a time. – Stephen Docy Mar 15 '18 at 06:53
@Hele try fflush(stdin); – Emre Acar Mar 15 '18 at 06:54
@EmreAcar `fflush(stdin)` is UB. – Jabberwocky Mar 15 '18 at 07:57

chqrlie · Answer 2 · 2018-03-15T06:59:24.067

scanf() only consumes the characters that make up the converted number. Any remaining input, especially the newline that you typed after 5 is left in the input stream.

The for loop does start at 0, and 0 must be output by your program, but gets() reads the pending input upto and including the newline, so the next iteration comes immediately: 1 is output and the program only then waits for input.

Note that you must not use obsolete function gets() because it cannot determine the maximum number of characters to store to the destination array, so undefined behavior on invalid input cannot be avoided. Use fgets() or getchar() instead.

Here is how to fix your program:

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

int main() {
    int x, i, j, c;
    if (scanf("%d", &x) != 1)
        return 1;
    while ((c = getchar()) != EOF && c != '\n')
        continue;
    char array[x][10];
    for (i = 0; i < x; i++) {
        printf("%d\n", i);
        for (j = 0; (c = getchar()) != EOF && c != '\n';) {
            if (j < 9)
                array[i][j++] = c;
        }
        array[i][j] = '\0';
    }
    return 0;
}

Initialized variable skips 0 during for loop

2 Answers2