三个 scanf 导致两个并取垃圾值

Question

Here Is code of Basic Structure , But the output is not as expected. There is three scanf function but only two is being executed. Middle one contains garbage value.

#include <stdio.h>

int main()
{
    struct book
    {
        char name;
        float price;
        int pages;
    };

    struct book b1,b2,b3;

    printf("Enter names , prices & no of pages of 3 books\n");
    scanf("%c%f%d",&b1.name,&b1.price,&b1.pages);
    scanf("%c%f%d",&b2.name,&b2.price,&b2.pages);
    scanf("%c%f%d",&b3.name,&b3.price,&b3.pages);

    printf("And this is what you entered\n");
    printf("%c%f%d",b1.name,b1.price,b1.pages);
    printf("%c%f%d",b2.name,b2.price,b2.pages);
    printf("%c%f%d",b3.name,b3.price,b3.pages);

    return 0;
}

Answer 1

Simply change

scanf("%c%f%d", &bx.name, &bx.price, &bx.pages);

To

scanf(" %c%f%d", &b1.name, &b1.price, &b1.pages);

After you press Enter , a '\\n' is left in stdin , which will later be consumed by "%c". Having read a character('\\n'), scanf() expects a floating-point number, as "%f" in the format string indicates. However, instead of getting that desired floating-point number, it meets a character, and it returns sadly. As a result, &bx.price and &bx.pages are not updated, so they remain uninitialised, giving you the garbage values.

With a leading space in scanf() , all whitespace characters(if any) are discarded before reading starts. Since \\n is discarded, following reading process will(presumably) be successful.

Also, just a tip: Always check the return value of scanf() , because you'll never know that stuff users will input.

Example code:

#include <stdio.h>

struct book
{
    char name;
    float price;
    int pages;
};

int main()
{
    struct book b1, b2, ..., bx;

    printf("Enter names, prices & no of pages of x books:\n");
    while (scanf(" %c%f%d", &bx.name, &bx.price, &bx.pages) != 3)
    { 
        fputs("Error reading bx. Please try again:\n", stderr);
        scanf("%*[^\n] ");
    }
    ......

    printf("And this is what you have entered:\n");
    printf("%c %f %d", bx.name, bx.price, bx.pages);
    ......

    return 0;
}

Example input & output:

Enter names, prices & no of pages of x books:
asd wedewc efcew
Error reading bx. Please try again:
a 12.34 42
And this is what you have entered:
a 12.340000 42

Answer 2

scanf, usually, reads the characters from standard input stream, which includes your newlines as well as spaces. so try to flush all unwanted inputs left in the stdin stream before you use scanf("%c");

Also remember: do not let %c fall somewhere in the middle of the format string

Hope this could help. unfortunately you should use it before all scanf in "Your" program

printf("Enter names , prices & no of pages of 3 books\n");
fflush(stdin);
scanf("%c%f%d",&b1.name,&b1.price,&b1.pages);

You may also use a space before %c in scanf to read whitespaces.

scanf(" %c"....);