Convert to int function

#1 
   ans+= (s[i]-'0')*p;

why are we subtracting 0 every time ? and how is char type multiplied with a char type?

#2

When you performing arithmetic operation, they are actually being converted to their ascii value equivalent and then the arithmetic operation is performed.
So actually your statement is equivalent to
ans+=(asci value of s[i]-ascii value of ‘0’) * p
So s[i]-‘0’ will actually give the number/digit in integer form.