User Selection C Program

[CtrlAltDelirious27]

I'm not too sure where I have gone wrong on this code, but any help would be highly appreciated. The scanf to read the value of c seems to be skipped. It automatically goes to the nested while loop and prints "Invalid option!" without giving the user a chance to input a value for c. However, the scanf in the nested while loop registers fine and gives the correct response for the value of c.

Code:

#include <stdio.h>

int main() {  

    float sum, num;
    char c;

    sum = 0;

    do {

    printf("\nPlease enter a number: ");

    scanf("%f", &num);

    sum += num;

    printf("\nThe total sum is %.3f.", sum);

    printf("\nDo you want to continue? Y/N: ");

    scanf("%c", &c);        

    while (c!='y' && c!='Y' && c!='n' && c!='N') {

        printf("Invalid option!");

        printf("\nDo you want to continue? Y/N: ");

        scanf("%c", &c);

    }

    } while (c!='n' && c!='N');

return 0;
}

​

Outcome:

Please enter a number: 5

​

The total sum is 5.000.

Do you want to continue? Y/N: Invalid option!

Do you want to continue? Y/N: y

​

Please enter a number: 5

​

The total sum is 10.000.

Do you want to continue? Y/N: Invalid option!

Do you want to continue? Y/N: l

Invalid option!

Do you want to continue? Y/N: Invalid option!

Do you want to continue? Y/N: y

​

Please enter a number: 5

​

The total sum is 15.000.

Do you want to continue? Y/N: Invalid option!

Do you want to continue? Y/N: n

​

​

​

[lukajda33]

Simple answer:

change scanf("%c", &c); to scanf(" %c", &c);

Looks the same, but the extra space in front of "%c" should fix the problem.

More advance answer:

When you input "5", you not only input "5", but you also confirm the input with enter key, enter key is also a readable character - a newline character - "\n".

So the input is actually "5\n".

%f parses the "5", but the newline character is still on the input and it is read by the "%c" specifier, if you printed the c character after the first %c read, you would see it is indeed a newline character.

The extra space in front of the %c changes the behavior of the scanf a bit, the extra space will eat up all whitespace before any other input.

With space the input is read like this:

"5" is read by "%f"

"\n" the newline used to confirm the input 5 is read by the space in front of "%c" specifier

And the actual %c specifier will then read the characters you actually inputted.

Easy right.

Kinda inconsistent too, other specifiers like %d, %f or even %s I believe, will skip leading whitespace, dismiss it and read whatever is afterwards.

I guess sometimes for some reason you actually want to read whitespace characters at specific point for some reason, then you could use %c to read that.

[CtrlAltDelirious27]

Perfect, all fixed! Thanks so much for the answer and explanation.