String comparison C - strcmp()

Question

I\'m trying to compare two strings, but I fail achieving that. Why?

#include 
#include 

int main(){
    float a = 1231.23123;
            


        

Answer 1

You are comparing these 2 strings here:

1231.23123
1231.231201

which are different indeed, thus strcmp returns non-zero value.

The actual problem here is that when you do float a = 1231.23123;, the number you want to store in a can't be represented as a float, the nearest number that can be represented as a float is 1231.231201171875 in this case. Have a look at OMG Ponies!!! (Aka Humanity: Epic Fail) ;)

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To solve your problem I would start with using double instead of float to get more precise accuracy. Then you could specify the precision (%.5lf) while printing this number into the string to make sure that the number is rounded just like you need it:

double d = 1231.23123;
char str[32];
sprintf(str, "%.5lf", d);
// strcmp(str, "1231.23123") would return 0 here

Answer 2

If you want a set number of digits to compare against in a string, use the precision specifier in sprintf - %.5f, and as others have pointed out, the number you've picked cannot be represented by a float, but can be represented by a double. i.e.

double a = 1231.23123;
char b[32];
sprintf(b, "%.5f",a);

Answer 3

It's because the precision of float cannot support so many digits. So b is not "1231.23123". In my test, it's "1231.231201".

Answer 4

The two strings are clearly different, so strcmp() is working as it should.

The issue is that 1231.23123 cannot be represented as a float. In fact, the nearest number that can be represented as a float is 1231.231201171875, which is what you're seeing (rounded by sprintf() to six decimal places).