Can you guess what the output of this program is?
// Program A
fn main() {
let a = 100;
let b = 1000;
println!("{}", a * (b/a));
println!("{}", a * (a/b));
}
What about this one?
// Program B
fn main() {
let a = 1000;
let b = 100;
println!("{}", a * (b/a));
println!("{}", (a * b)/a);
}
ANSWERS
Program A
1000 0
Visit PlaygroundProgram B
0 100
Visit PlaygroundDiscussion
Integers don’t have decimal places, so rounding after division works differently than what you might be used to when doing a mathematical calculation.
Program A contains two important mathematical expressions, a * (b / a) and a * (a / b).
In the first expression, things work as expect. Given a = 100 and b = 1000, here is how those variables expand:
a * (a / b) = x100 * (1000 / 100) = x100 * 10 = x1000 = x
In the second expression, things work the same, but different.
a * (a / b) = x100 * (100 / 1000) = x100 * 0 = x0 = x
When dealing with integers, there are no fractions, so the result of dividing by a number that is greater than the nominator is 0. Formally speaking, integer division returns the integer quotient.
Acknowledgements
Thanks to Marc Brooker for the reminder.