Data type is very important. For example, now that all your variables are 64-bit and one variable is 32-bit, the storage space of this 32-bit variable is smaller than that of the 64-bit variable. Suppose a value can be stored in 64 bits, but not in a 32-bit variable. Is it appropriate to put it in a 32-bit variable? For 32-bit variables, this value will overflow.

printf("%d "，c)； It is an integer output.

printf("%ld "，c)； Output as a long integer.

These two obviously have different functions. If a number is greater than an integer (usually 32 bits), obviously %d can't be used, so we have to use %ld.