Question

Ok, I have a char that is a number. How could I convert it into a double?

char c;

I’ve tried (double)c, but it converts to zero.

Any ideas?

Answer 1

if you have a null terminated string that you wish to convert to double use atof:

const char *str = "3.14";
double x = atof(str);
printf("%f\n", x); //prints 3.140000

If you have a single character, casting should work:

char c = 'a'; //97 in ASCII
double x = (double)c; 
printf("%f\n", x); //prints 97.000000

If the character is zero then it print zeros of course:

char c = '\0';
double x = (double)c; 
printf("%f\n", x); //prints 0.000000

Note: atof and similar functions don’t detect overflows and return zero on error, so there’s no way to know if it failed (not sure if it sets errno), see also Keith’s comments about undefined behaviour for certain values, so the point is you should use strtol for converting from strings to int and strtod for converting to double those have much better error handling:

const char *str = "3.14";
double x = strtod(str, NULL);