Previous even number

Question

If an integer is uneven (odd), I would like to get the previous number, otherwise I would like to keep the current number. E.g. if x = 3, I would like to assign 2 to x, if x = 4, then nothing happens.

At the moment, I do the following: x = (x/2)*2, but I have heard that division is computational expensive. Does -O3 optimize this expression? I am using the c++ (Ubuntu 4.8.4-2ubuntu1~14.04) 4.8.4 compiler. x is a uint32_t.

`x = (x >> 1) << 1;` You can use shifts, but the compiler likely optimizes the multiplication and division to this anyway. — owacoder, Dec 10 '15 at 17:45
You could also use `x &= ~1`, which should compile into a single `AND` by a constant. — owacoder, Dec 10 '15 at 17:47
You could go with `(x & 0xFFFFFFFE)` but I am forced to wonder is this _really_ the bottleneck of your program and is this sort of femto-optimization the best use of your time? — Nik Bougalis, Dec 10 '15 at 17:49

Vlad from Moscow · Answer 1 · 2015-12-10T17:54:01.237

2

Try the following

x &= ~1;

I suppose that x is declared as having type int. Otherwise if the rank of type x is greater than the rank of type int then you should use an integer literal of the type of the variable x.

edited Dec 10 '15 at 17:54

answered Dec 10 '15 at 17:47

Vlad from Moscow

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Previous even number

1 Answers1