Would you use num%2 or num&1 to check if a number is even?

Question

Well, there are at least two low-level ways of determining whether a given number is even or not:

 1. if (num%2 == 0) { /* even */ } 
 2. if ((num&1) ==          


        

Answer 1

Both approaches are not obvious especially to someone who is new to programming. You should define an inline function with a descriptive name. The approach you use in it won't matter (micro optimizations most likely won't make your program faster in a noticeable way).

Anyway, I believe 2) is much faster as it doesn't require a division.

Answer 2

Any modern compiler will optimise away the modulo operation, so speed is not a concern.

I'd say using modulo would make things easier to understand, but creating an is_even function that uses the x & 1 method gives you the best of both worlds.

Answer 3

It's been mentioned a number of times that any modern compiler would create the same assembly for both options. This reminded me of the LLVM demo page that I saw somewhere the other day, so I figured I'd give it a go. I know this isn't much more than anecdotal, but it does confirm what we'd expect: x%2 and x&1 are implemented identically.

I also tried compiling both of these with gcc-4.2.1 (gcc -S foo.c) and the resultant assembly is identical (and pasted at the bottom of this answer).

Program the first:

int main(int argc, char **argv) {
  return (argc%2==0) ? 0 : 1;
}

Result:

; ModuleID = '/tmp/webcompile/_27244_0.bc'
target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:32:32"
target triple = "i386-pc-linux-gnu"

define i32 @main(i32 %argc, i8** nocapture %argv) nounwind readnone {
entry:
    %0 = and i32 %argc, 1       ; <i32> [#uses=1]
    ret i32 %0
}

Program the second:

int main(int argc, char **argv) {
  return ((argc&1)==0) ? 0 : 1;
}

Result:

; ModuleID = '/tmp/webcompile/_27375_0.bc'
target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:32:32"
target triple = "i386-pc-linux-gnu"

define i32 @main(i32 %argc, i8** nocapture %argv) nounwind readnone {
entry:
    %0 = and i32 %argc, 1       ; <i32> [#uses=1]
    ret i32 %0
}

GCC output:

.text
.globl _main
_main:
LFB2:
  pushq %rbp
LCFI0:
  movq  %rsp, %rbp
LCFI1:
  movl  %edi, -4(%rbp)
  movq  %rsi, -16(%rbp)
  movl  -4(%rbp), %eax
  andl  $1, %eax
  testl %eax, %eax
  setne %al
  movzbl  %al, %eax
  leave
  ret
LFE2:
  .section __TEXT,__eh_frame,coalesced,no_toc+strip_static_syms+live_support
EH_frame1:
  .set L$set$0,LECIE1-LSCIE1
  .long L$set$0
LSCIE1:
  .long 0x0
  .byte 0x1
  .ascii "zR\0"
  .byte 0x1
  .byte 0x78
  .byte 0x10
  .byte 0x1
  .byte 0x10
  .byte 0xc
  .byte 0x7
  .byte 0x8
  .byte 0x90
  .byte 0x1
  .align 3
LECIE1:
.globl _main.eh
_main.eh:
LSFDE1:
  .set L$set$1,LEFDE1-LASFDE1
  .long L$set$1
ASFDE1:
  .long LASFDE1-EH_frame1
  .quad LFB2-.
  .set L$set$2,LFE2-LFB2
  .quad L$set$2
  .byte 0x0
  .byte 0x4
  .set L$set$3,LCFI0-LFB2
  .long L$set$3
  .byte 0xe
  .byte 0x10
  .byte 0x86
  .byte 0x2
  .byte 0x4
  .set L$set$4,LCFI1-LCFI0
  .long L$set$4
  .byte 0xd
  .byte 0x6
  .align 3
LEFDE1:
  .subsections_via_symbols

Answer 4

You conclusion about performance is based on the popular false premise.

For some reason you insist on translating the language operations into their "obvious" machine counterparts and make the performance conclusions based on that translation. In this particular case you concluded that a bitwise-and & operation of C++ language must be implemented by a bitwise-and machine operation, while a modulo % operation must somehow involve machine division, which is allegedly slower. Such approach is of very limited use, if any.

Firstly, I can't imagine a real-life C++ compiler that would interpret the language operations in such a "literal" way, i.e. by mapping them into the "equivalent" machine operations. Mostly because more often than one'd think the equivalent machine operations simply do not exist.

When it comes to such basic operations with an immediate constant as an operand, any self-respecting compiler will always immediately "understand" that both num & 1 and num % 2 for integral num do exactly the same thing, which will make the compiler generate absolutely identical code for both expressions. Naturally, the performance is going to be exactly the same.

BTW, this is not called "optimization". Optimization, by definition, is when the compiler decides to deviate from the standard behavior of abstract C++ machine in order to generate the more efficient code (preserving the observable behavior of the program). There's no deviation in this case, meaning that there's no optimization.

Moreover, it is quite possible that on the given machine the most optimal way to implement both is neither bitwise-and nor division, but some other dedicated machine-specific instruction. On top of that, it is quite possible that there won't be any need for any instruction at all, since even-ness/odd-ness of a specific value might be exposed "for free" through the processor status flags or something like that.

In other words, the efficiency argument is invalid.

Secondly, to return to the original question, the more preferable way to determine the even-ness/odd-ness of a value is certainly the num % 2 approach, since it implements the required check literally ("by definition"), and clearly expresses the fact that the check is purely mathematical. I.e. it makes clear that we care about the property of a number, not about the property of its representation (as would be in case of num & 1 variant).

The num & 1 variant should be reserved for situations when you want access to the bits of value representation of a number. Using this code for even-ness/odd-ness check is a highly questionable practice.

Answer 5

If you're going to say that some compilers won't optimise %2, then you should also note that some compilers use a ones' complement representation for signed integers. In that representation, &1 gives the wrong answer for negative numbers.

So what do you want - code which is slow on "some compilers", or code which is wrong on "some compilers"? Not necessarily the same compilers in each case, but both kinds are extremely rare.

Of course if num is of an unsigned type, or one of the C99 fixed-width integer types (int8_t and so on, which are required to be 2's complement), then this isn't an issue. In that case, I consider %2 to be more elegant and meaningful, and &1 to be a hack that might conceivably be necessary sometimes for performance. I think for example that CPython doesn't do this optimisation, and the same will be true of fully interpreted languages (although then the parsing overhead likely dwarfs the difference between the two machine instructions). I'd be a bit surprised to come across a C or C++ compiler that didn't do it where possible, though, because it's a no-brainer at the point of emitting instructions if not before.

In general, I would say that in C++ you are completely at the mercy of the compiler's ability to optimise. Standard containers and algorithms have n levels of indirection, most of which disappears when the compiler has finished inlining and optimising. A decent C++ compiler can handle arithmetic with constant values before breakfast, and a non-decent C++ compiler will produce rubbish code no matter what you do.

Answer 6

I code for readability first so my choice here is num % 2 == 0. This is far more clear than num & 1 == 0. I'll let the compiler worry about optimizing for me and only adjust if profiling shows this to be a bottleneck. Anything else is premature.

I consider the second option to be far more elegant and meaningful

I strongly disagree with this. A number is even because its congruency modulo two is zero, not because its binary representation ends with a certain bit. Binary representations are an implementation detail. Relying on implementation details is generally a code smell. As others have pointed out, testing the LSB fails on machines that use ones' complement representations.

Another point is that the second one might be a little harder to comprehend for less experienced programmers. On that I'd answer that it will probably only benefit everybody if these programmers take that short time to understand statements of this kind.

I disagree. We should all be coding to make our intent clearer. If we are testing for evenness the code should express that (and a comment should be unnecessary). Again, testing congruency modulo two more clearly expresses the intent of the code than checking the LSB.

And, more importantly, the details should be hidden away in an isEven method. So we should see if(isEven(someNumber)) { // details } and only see num % 2 == 0 once in the definition of isEven.