Question

I am getting a weird problem while using scanf() to store data into a union.

Here’s my code

#include <stdio.h>

union Student
{
    float score;
    char grade;
};

int main(void)
{
    union Student jack;

    printf("Enter student score : ");
    scanf("%f", &jack.score);
    printf("Score : %f", jack.score);

    jack.score=0;

    printf("Enter student grade : ");
    scanf("%c", &jack.grade);
    printf("Grade : %c", jack.grade);

}

I get the following output

searock@searock-desktop:~/Desktop$ ./union
Enter student score : 12
Score : 12.000000Enter student grade : Grade :

but if I change my code to:

#include <stdio.h>

union Student
{
    float score;
    char grade;
};

int main(void)
{
    union Student jack;

    printf("Enter student grade : ");
    scanf("%c", &jack.grade);
    printf("Grade : %c\n", jack.grade);

    printf("Enter student score : ");
    scanf("%f", &jack.score);
    printf("Score : %f\n", jack.score);

}

It gives me the exact output [correct output]. I know this is not a good example, but can someone explain me what’s going wrong?

Modified Code : Add \n before format string. [scanf(“\n%c”, &ch);]

#include <stdio.h>

    union Student
    {
        float score;
        char grade;
    };

    int main(void)
    {
        union Student jack;

        printf("Enter student score : ");
        scanf("%f", &jack.score);
        printf("Score : %f", jack.score);

        jack.score=0;

        printf("Enter student grade : ");
        scanf("\n%c", &jack.grade);
        printf("Grade : %c", jack.grade);

    }

Answer 1

In the first example, the first scanf() reads a number up to but not including the newline (assuming you don’t type just blanks and newlines; if you do, it will continue waiting for input until you provide a number or a non-number – where blanks etc do not count as either a number or a non-number). Then the second scanf() with the ‘%c‘ format specifier does not skip white space (unlike most other format specifiers) and reads the newline (assuming you typed a newline immediately after the end of the number; if you typed something else – white space or a letter, perhaps, it reads that character), completely ignoring anything you typed on the second line. (Indeed, when I run the code, it does not wait for me to enter anything for the second lot of input. Try typing “3.14+C” or “3.14C” and a newline.)

In the second example, the first scanf() reads the first character. The second scanf() skips over white space, including newlines, until it finds either a number or something that is definitively not a number (like a letter).

This is a beautiful demonstration of why most people avoid scanf(); it is very difficult to provide satisfactory control over it. You would be better off reading lines (perhaps using fgets(); definitely not using gets()) and then parsing them with sscanf(). You would then get sane behaviour for both examples.

Note that your problem is completely unrelated to the use of a union; your code is fine in the way that is working. Your problem is all tied up with the use of scanf().

Minor recommendation: when printing lines of output – other than prompts – include a newline at the end of the format string.