calculation in C program always results in 0

Question

This always results in 0 for me and I don't know why... Other examples on internet generally cause of division.

in Codeblocks same code results normally but in Atom I have this problem. please help.

#include <stdio.h>
#include <conio.h>

#define PI 3.14
int main() {
    float r,a,c;
    printf("enter radius: \n");
    scanf("%.2f",&r);
    c = r * PI * 2;
    a = r * r * PI;
    printf("area:%.2f circumference:%.2f \n",c,a );
    return 0;
}

Answer 1

C11 7.21.6.2p3 says the following of fscanf et al:

3 [...] Each conversion specification is introduced by the character %. After the %, the following appear in sequence:

• An optional assignment-suppressing character *.
• An optional decimal integer greater than zero that specifies the maximum field width (in characters).
• An optional length modifier that specifies the size of the receiving object.
• A conversion specifier character that specifies the type of conversion to be applied.

The length modifier means the extra letter(s) such as l in %lf meaning double. Notice that while %2f would be valid and would mean that only to 2 characters of input can be consumed, it is nowhere said that you can write %.2f, i.e. %.2f is an invalid conversion specification, and hence the behaviour is undefined (C11 7.2.6.2p13).

Answer 2

If there's no reason you want to use %.2f to scan, just using %f will work

#include <stdio.h>
#include <conio.h>

#define PI 3.14
int main() {
    float r,a,c;
    printf("enter radius: \n");
    scanf("%f",&r);
    c = r * PI * 2;
    a = r * r * PI;
    printf("area:%.2f circumference:%.2f \n",c,a );
    return 0;
}

Example:

daniel@daniel-FX503VM:~/Documents/test$ ./zero 
enter radius: 
1
area:6.28 circumference:3.14 

The .2 in the printf is a printing format specifier, and shouldn't have an impact on a scanf

Where you're putting the .2 in the scanf is actually the width field,which specifies the maximum number of characters to be read in the current reading operation (optionally). However, in this context, .2 characters as a width is not a meaningful sentiment, seeing as the width is required to be an integer.

If what you're actually trying to do is only read 2 characters, you could say

scanf("%2f", &r);

But be warned that the '.' would also count as a character.If you specifically just want the output to be two decimal places, then the proposed code above should suffice. If you want to round it internally, I'd suggest you read this post