Automatic type conversion: the data value type on the left side of the arrow can be automatically converted to the data value type on the right side of the arrow.

Metaphor: the water in a small bottle is poured into the water in a big bottle (no problem)

char a = ' 4

int b；

b = a-' 0 '； //char is automatically converted to int type, and each character has an ascii code value. The ascii code value of "0" is 48, while the ascii code value of "4" is 52, so 52-48=4 is regarded as a normal conversion to an integer value.

Summary: Every character number must be reduced by' 0' before it can be converted into an integer, because the number of char characters exists in the form of ascii code in the computer.