Thread: The variable is unexpectedly set zero while running while loop

Hello, I am a new learner of C.

I ran into a problem while using while loop.

The declared and initiated local int variable works well with its specified value while running through the 1st run of a while loop.

It is set zero while entering the 2nd run and the following unexpectedly.

However, the variable still exits.

I can't understand its mechanism please help me.

Thanks a lot

The following is the code with problem.

Code:

#include <stdio.h>


int main(void){
    
    const int x=6;
    char c='y';
     
     while(c=='y'){
             //const int x=6;
             printf("x = %d\n",x);
             printf("Enter:");
             scanf("%s",&c);   
             printf("\n");  
             }         
              
    system("pause");
    return 0;          
}

Thanks a lot!

Another problem raised is while loop breaks in the 2nd run as I use %c in replace of %s.

I can input the value of char variable using scanf with %c outside loop block without any error.

I don't realize that?

Thank you so much!

Originally Posted by HolmesChang

Thanks a lot!

Another problem raised is while loop breaks in the 2nd run as I use %c in replace of %s.

I can input the value of char variable using scanf with %c outside loop block without any error.

I don't realize that?

I think I might figure out why it still output unexpectedly.
After calling scanf, I input y and pressed enter which is recognized as a character.
scanf in the first loop read y and leave \n in the stdin, which is read by scanf in the second loop.
Then loop ended.

I use fflush(stdin) after scanf to clear stdin, then no character left to be mis-read.

I am not sure if my explanation is correct although it works.

Code:

#include <stdio.h>


int main(void){
    
    int x=6;
    char c[6];
    
    c[0] = 'y';
    c[1] = 'a';
    c[2] = 'l';
    c[3] = 'l';
    c[4] = 'o';
    c[5] = 0;    


     while(c[0]=='y'){
             //const int x=6;
             printf("x = %d\n",x);
             printf("Enter:");
             c[0] = scanf("%c",&c[0]);
             fflush(stdin);
             //getchar();   
             printf("\n");  
             }
           
     
     
     //scanf("%c",&c[5]);
     printf("%s\n",c);    
              
    system("pause");
    return 0;          
}

Originally Posted by stahta01

FAQ > Why fflush(stdin) is wrong - Cprogramming.com

Tim S.

Thank you.

I read the usage of fflush from a sample code of a textbook.
It talks about how to escape from reading the unwanted \n after getchar();
It also use a following getchar() to "absorb" the unwanted \n in the other way.
I think I can use the second way.

BTW, I still have another problem.
Is it supposed to stop and wait for some input and a following enter as I call scanf?
Then why scanf automatically read \n from input buffer in the 2nd run of the while loop?
Still looking up the textbook.

I don't quite understand your question but perhaps try:

Code:

scanf(" %c",&c); // note the leading space before the %c

This will consume any characters that isspace() would return true for (\n included)

Originally Posted by vart

It is not exactly right.

What you enter on the keyboard will be put into the stdin buffer only after you press enter.

scanf reads what is in the buffer, and wait only when buffer is empty.

So if you press 'y' <ENTER> 2 chars are posted in to the buffer 'y' and '\n'

So if first scanf reads 'y' and nothig has being done with '\n' the input buffer will not be empty when second scanf starts reading.

So it will not wait - it will try to read '\n'

I think it's the point to which I didn't know before.
Now I know it.
Thank you so much.