对于scanf()，%f用于float.您需要将%lf用于double:

For scanf(), %f is for float. You need to use %lf for double:

printf("Enter a # for square root.\n");
scanf("%lf",&a);

这与printf()相反，在printf()中，类型促销允许%f用于float和double.

This is in contrast to printf() where type-promotion allows %f to be used for both float and double.