How to avoid user inputs from going over, using two scanf?

Question

I'm new to c, and I'm not sure how to word this question. But for example using this code:

#include <stdio.h>

int main()
{
    int x,y,z;

    printf("Enter 2 numbers: ");
    scanf("%d %d", &x, &y);

    printf("Test: ");
    scanf("%d",&z);

    printf("x:%d y:%d z:%d\n",x,y,z);

    return 0;
}

If the user inputs "1 2 3", it would set x=1, y=2 and z=3. I was wondering if you could set x=1 and y=2 and ignore 3, then the user can input another value and not use the 3 given before.

You can flush the input after the first scanf. [See here](http://stackoverflow.com/questions/7898215/how-to-clear-input-buffer-in-c). — M.M, Oct 09 '16 at 02:56
You can use [`fgets()`](http://pubs.opengroup.org/onlinepubs/9699919799/functions/fgets.html) (or POSIX [`getline()`](http://pubs.opengroup.org/onlinepubs/9699919799/functions/getline.html)) to read a line and then scan it with [`sscanf()`](http://pubs.opengroup.org/onlinepubs/9699919799/functions/sscanf.html) instead of `scanf()`. This is often a better way to process data; it allows you to report errors better, and allows you to try alternative ways of scanning the data if the first format doesn't work. — Jonathan Leffler, Oct 09 '16 at 03:01
Totally agree with @JonathanLeffler: You should use `fgets()` to get one line, then `sscanf()` to process it carefully (checking the return values from those functions, of course). — John Zwinck, Oct 09 '16 at 03:05
Well you may want to use scanf("%d",&z); scanf("%d",&z); that is, scan the value two times. The first one will be overwritten This is not the obvious solution but it may help — Anish Sharma, Oct 10 '16 at 09:42

Masked Man · Answer 1 · 2016-10-10T08:30:44.017

0

This code will help you:

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

int main() {
    int x,y,z;
    char s[1024];

    printf("Enter 2 numbers: ");
    scanf("%d %d", &x, &y);
    fgets(s, 1024, stdin);

    printf("Test: ");
    scanf("%d",&z);

    printf("x:%d y:%d z:%d\n",x,y,z);

    return 0;
}

It initializes a string pointer with 1024 bytes and after reading the first two integers ignore the remaining content on that line, then prompts Test: and reads z variable.

EDIT Use code below as I tested it with line of 0 <= lentgh <= 8.1k characaters!

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

int main() {
    int x,y,z;
    char ch;

    printf("Enter 2 numbers: ");
    scanf("%d %d", &x, &y);
    while(scanf("%c", &ch)) if(ch == '\n') break;

    printf("Test: ");
    scanf("%d",&z);

    printf("x:%d y:%d z:%d\n",x,y,z);

    return 0;
}

edited Oct 10 '16 at 08:30

answered Oct 09 '16 at 10:09

Masked Man

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Never use `gets()`. Never, never, never! – Jonathan Leffler Oct 09 '16 at 14:24
@JonathanLeffler, give me an alternative `:)`. – Masked Man Oct 09 '16 at 14:51
See [Why `gets()` is so dangerous it should not be used — ever](http://stackoverflow.com/questions/1694036/why-is-the-gets-function-dangerous-why-should-it-not-be-used) for a list of alternatives. – Jonathan Leffler Oct 09 '16 at 14:57
@JonathanLeffler, updated the answer with `fgets()` `:)`. – Masked Man Oct 09 '16 at 15:03
1

So now we have a memory leak instead? Use `char s[1024];` instead. – Jonathan Leffler Oct 09 '16 at 15:08
What happens if the user enters a 2,000 character line? – Andrew Henle Oct 09 '16 at 15:09
@AndrewHenle, what if user enters a 10,000 characters line?! – Masked Man Oct 09 '16 at 16:01
@JonathanLeffler, I don't know why I didn't use that the first time! good reminder. – Masked Man Oct 09 '16 at 16:03
@MaskedMan *what if user enters a 10,000 characters line?!* Your code doesn't work. That's what happens. – Andrew Henle Oct 09 '16 at 22:23
10000 char line? use a `while()` loop, where the call to `fgets()` controls the `while()` loop I.E. `while( fgets( s, sizeof(s), stdin ); );` – user3629249 Oct 10 '16 at 02:12
@AndrewHenle it does not have line limitation anymore `:)`. – Masked Man Oct 10 '16 at 08:32

How to avoid user inputs from going over, using two scanf?

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