How to convert string to float?

Question

#include<stdio.h> #include<string.h>  int main()  {     char s[100] ="4.0800" ;       printf("float value : %4.8f\n" ,(float) atoll(s));      return 0;  } 

I expect the output should be 4.08000000 whereas I got only 4.00000000.

Is there any way to get the numbers after the dot?

Answer 1

Use atof() or strtof()* instead:

printf("float value : %4.8f\n" ,atof(s));  printf("float value : %4.8f\n" ,strtof(s, NULL));  

http://www.cplusplus.com/reference/clibrary/cstdlib/atof/
http://www.cplusplus.com/reference/cstdlib/strtof/

• atoll() is meant for integers.
• atof()/strtof() is for floats.

The reason why you only get 4.00 with atoll() is because it stops parsing when it finds the first non-digit.

*Note that strtof() requires C99 or C++11.

Answer 2

Unfortunately, there is no way to do this easily. Every solution has its drawbacks.

1. Use atof() or strtof() directly: this is what most people will tell you to do and it will work most of the time. However, if the program sets a locale or it uses a library that sets the locale (for instance, a graphics library that displays localised menus) and the user has their locale set to a language where the decimal separator is not . (such as fr_FR where the separator is ,) these functions will stop parsing at the . and you will stil get 4.0.

2. Use atof() or strtof() but change the locale; it's a matter of calling setlocale(LC_ALL|~LC_NUMERIC, ""); before any call to atof() or the likes. The problem with setlocale is that it will be global to the process and you might interfer with the rest of the program. Note that you might query the current locale with setlocale() and restore it after you're done.

3. Write your own float parsing routine. This might be quite quick if you do not need advanced features such as exponent parsing or hexadecimal floats.

Also, note that the value 4.08 cannot be represented exactly as a float; the actual value you will get is 4.0799999237060546875.