'A' is an
int constant used to represent the character ‘A’. The same is true for digits, of course.
If you have a variable
c which contains a digit (say ‘9’) you can find the numerical value by
c - '0' which in this case evaluates to 9.
If you wish to read a number (such as 124) from input using
getchar(), you can do as follows. First check that the current input char you have read is a digit. Do that with the function
isdigit. It returns a nonzero value if you give it a digit, such as
'9'. To read a number then, you can have a variable
x initially set to zero, and do:
x = x * 10 + c - '0';
Do this for each input char
c you read that is a digit. If we read the digits
'4' in sequence, x will first be 0, then 1, then 12, and finally 124!
Note that the type of
c should be
int, as explained here.