Expressions with integer and floating-point values
It is quite common to use expressions involving both integer and floating-point values, for example,
a / 3 where a is float
n * 0.25 where n is int
In C, the rule for such expressions is:
If either operand of an arithmetic operator is floating-point, the calculation is done in floating-point arithmetic. The calculation is done in double unless both operands are float, in which case the calculation is done in float.
In the first example above, the integer 3 is converted to float and the calculation is done in float. In the second example, n is converted to double (since 0.25 is double) and the calculation is done in double.
How do we get the exact value of an integer division, 19/5, say? We can force a double precision calculation by writing one or both constants as double, thus: 19/5.0, 19.0/5 or 19.0/5.0. We can also use a cast, as in
(double) 19 / 5
A cast allows us to force the conversion of one type to another. Here, the int 19 is cast to double, forcing 5 to be converted to double and a double precision division is performed.
However, we must be careful with a construct like
(double) (19 / 5)
since it may not do what we think. This does NOT do a floating-point division. Since both constants are integer, the expression inside the brackets is evaluated as an integer division (which discards the remainder), giving 3; this value is converted to double, giving 3.0.