C programming scanf() not working correctly

Question

I want to know the reason why this code not run properly

#include<stdio.h>
main()
{
    char a;
    int n;
    do
    {
        printf("enter the number");
        scanf("%d",&n);
        printf("the squre is %d",n*n);
        printf("want any more so Y for yes N for no");
        scanf("%c%[^\n]",&a);
    }while(a=='Y');
}

you probably meant to ask on [so], not here. Also, please look at the formatting help or use the formatting toolbar. Also, please describe the error you're observing (input, expected output and why, actual output or error message (copy-pasted with the line number identified)). "This code doesn't run properly" is as non-descriptive as it can get. — John Dvorak, Jan 01 '14 at 08:21
This question has been answered before, for example here: [C: Mutliple scanf's, when I enter in a value for one scanf it skips the second scanf.](http://stackoverflow.com/questions/9562218/c-mutliple-scanfs-when-i-enter-in-a-value-for-one-scanf-it-skips-the-second-s) — Thomas Padron-McCarthy, Jan 01 '14 at 10:05

score 3 · Answer 1 · edited Jan 01 '14 at 10:28

3

Reasons are
1. scanf("%c%[^\n]",&a); needs two parameters. Remove %[^\n].
2. \n character left behind by the previous scanf on pressing Enter key. Next scanf will read this \n character in the buffer. You need to consume this \n. Use a space before %c specifier to eat up this \n.

Try this:

scanf(" %c",&a);  
       ↑ A space before %c specifier

A space before %c specifier is able to eat ant number of newline characters.
Your code after modification:

#include<stdio.h>

int main(void)
{
    char a;
    int n;
    do
    {
         printf("enter the number\n");
         scanf("%d",&n);
         printf("the squre is %d\n",n*n);
         printf("want any more so Y for yes N for no\n");
         scanf(" %c",&a);
    }while(a=='Y');

    return 0;
}

edited Jan 01 '14 at 10:28

Maroun

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answered Jan 01 '14 at 09:51

haccks

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Hey @Maroun, how can I print this arrow? I tried with unicode but getting some weird character. :) – haccks Jan 01 '14 at 10:30
Just copy-paste it, no need to unicode it. 10$ please. – Maroun Jan 01 '14 at 10:32
@MarounMaroun; Do not worry. I already transferred it to your account :P – haccks Jan 01 '14 at 10:33

score 2 · Answer 2 · edited Jan 03 '14 at 18:09

2

Here is a solution to your problem.

#include<stdio.h>
main()
{
    char a;
    int n;
    do
    {
        printf("enter the number");
        scanf("%d",&n);
        a = getchar(); // <-- Here is a change.
        printf("the squre is %d",n*n);
        printf("want any more so Y for yes N for no");
        while(a == '\n') a = getchar();
    }while(a=='Y');
}

What actually is making you problem is this line.

scanf("%d",&n);

Suppose, you entered 10 then pressed 'Enter', now scanf() takes 10 and leaves a newline behind in the buffer. It is making problem. By getchar() you are eating up that new line each time after taking a input with scanf. Yes there are other solutions too with scanf() tricks but it seems simpler to me. So I shared it.

edited Jan 03 '14 at 18:09

haccks

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answered Jan 01 '14 at 10:29

taufique

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What if a user press Enter key multiple times after inputting the value of `n`? – haccks Jan 01 '14 at 10:35
If multiple enter keys are pressed after first input (integer) then it's not gonna work, as enter is taken as a character other that 'Y'. But if you press multiple enter after Y or N, it won't do any problem. – taufique Jan 01 '14 at 10:39
Then you should fix that. – haccks Jan 01 '14 at 10:44
1

I did something like that. Look again at my answer. – taufique Jan 01 '14 at 11:03
My previous edit was wrong. I rolled back the edit what you had done. My apologies. – haccks Jan 03 '14 at 18:11

Legong.Sun · Answer 3 · 2014-01-01T10:35:54.353

0

Wrong code: scanf("%c%[^\n]",&a);
Right code: scanf(" %c%*[^\n]",&a);

edited Jan 01 '14 at 10:35

answered Jan 01 '14 at 10:00

Legong.Sun

1
1

@haacks: The star means suppress assignment, so no parameter is needed for that. – Thomas Padron-McCarthy Jan 01 '14 at 10:10
@ThomasPadron-McCarthy; True. But still a space is needed before `%c`. – haccks Jan 01 '14 at 10:23
1

@haccks: yes, you are right. 'scanf(" %c%*[^\n]",&a);' – Legong.Sun Jan 01 '14 at 10:31

score 0 · Answer 4 · edited Jan 01 '14 at 10:22

0

just use scanf(" %c",&a); and could be done.

edited Jan 01 '14 at 10:22

haccks

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answered Jan 01 '14 at 10:03

vinayawsm

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@haccks yeah!! but why is that space given? – vinayawsm Jan 01 '14 at 10:14

C programming scanf() not working correctly

4 Answers4