The value of NULL is 0 (defined in stdio.h), which is an integer value, and the data of double type is floating point type. Strictly speaking, these two types are different, but as internal types, the system will force conversion by itself, that is, plastic and floating-point data can be assigned to each other, and of course the compiler will give a warning.

But the problem is that ""contains a string (even if the actual length is 0), and it cannot be converted to a floating-point type because the compiler will give an error message. I want to know, why should I assign ""to floating-point data?