Bitwise operation to get 1 if > 0 else 0

Question

I want to apply a bitwise operation on a value to turn it to 0 if the value is 0 or to turn it to 1 if it greater than zero.

Specifically I want a soultion in JavaScript, a way around Number(Boolean(value))

0 -> 0 1 -> 1 2 -> 1 3 -> 1 4 -> 1 ...

can I achieve this with bitwise operations? Arithmetic operations are fine, too.

Are arithmetic or logical operators allowed (e.g. + or !), or does it need to be strictly bitwise operators only ? — Paul R
– Paul R, Commented Jun 19, 2017 at 9:13
Artithmetic would be fine. Essentially I want a more eloquent way around Number(Boolean(value)) — Bernd Strehl
– Bernd Strehl, Commented Jun 19, 2017 at 9:15
Good answers already, just for fun: 1-!x should also do the trick — Me.Name
– Me.Name, Commented Jun 19, 2017 at 10:01
Number(Boolean(x)) can be written as +!!x (unary operation) — Emissary
– Emissary, Commented Jun 19, 2017 at 10:05

Bathsheba · Accepted Answer · 2017-06-19 09:13:19Z

4

!!x is the idiomatic way of doing this, for an integral type (including boolean) x.

answered Jun 19, 2017 at 9:13

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9 Comments

Not bitwise though (! is a logical operator).

Indeed, I assumed it was the "user doesn't quite know what they want" situation. And I'm too old to use ~ and ^.

You could be right - I've asked for clarification in the comments.

If used in JS for example a value of 3 will give me true i do not want a boolean, i want a Number

@Strernd "i want a Number" then do !!x|0 now you have bitwise operation :)

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slim · Accepted Answer · 2017-06-19 10:17:46Z

x && 1 does it.

for(x=0; x<5; x++) { console.log(x & 1) }

Writes:

0 1 1 1 1

This is a logical AND but Javascript's type rules appear to keep the result as a number.

But you should consider how many JS coders would instinctively understand this.

Preferring legibility to terseness, I would personally use the ternary operator:

x == 0 ? 0 : 1

... although the option you've asked to avoid is also highly legible:

Number(Boolean(x))

You seem to imply that this might be slower than the alternatives.

I see no reason why this would be slow. Number and Boolean are language built-ins that do very simple operations that we'd expect to be incredibly cheap
Premature optimisation is the root of all evil. You should only worry about this kind of micro-optimisation if your code is running unacceptably slowly. This operation would only have such an effect if it was running millions of times in a loop. If that was the case you could easily tell if another option was cheaper, just by trying it.

Note that your requirement:

turn it to 0 if the value is 0 or to turn it to 1 if it greater than zero

... does not specify what you want to happen if x is negative. This answer allows you to tune accordingly.

I was just curious about if this method would be faster than using if or casting.
@Strernd both are stupidly fast. Don't optimise for speed unless you're experiencing problems with slowness and it results in a measurable improvement.