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Do you know your bitwise operators?

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Re: Do you know your bitwise operators?

#21

Post by letney »

dalke wrote: post #10
gamegoblin wrote: post #3

Earlier quoted context omitted.

I keep getting various parse errors like Expected assignment, comment or return statement but " " found. When trying to get rid of parse errors, I did something like "return x" and it says Solution did not return correct output for x=1. Expected output: 1, got: 0 Which is clearly not right. Just tried doing "return 1" and still got Solution did not return correct output for x=1. Expected output: 1, got: 0

I get the same thing as you, and I get the same thing when I have "return 0." The puzzle is annoying. For what it's worth, here's my answer: 1>>(x&~(~x+1))&~(1>>x) . (I didn't figure out that the tester was broken before I developed my answer.) I tested it with positive integers on Python. I haven't tried it with negatives in Javascript. The '(x&~(~x+1))' is a work-around to get 'x&(x-1)', which is 0 if x is a power…

The work-around for 'x&(x-1)' I found is one-character longer: '(x&(~x+x+x))'

Re: Do you know your bitwise operators?

#22

Post by norswap »

This is making me crazy, does someone have the solution?

The best I could come up with is:

function one_bit(x) { return 1 >>> (x & (x + ~0)) }

("x + ~0" is equivalent to "x - 1")

But this doesn't work when x is 0 All other case should be fine, afaik.

Re: Do you know your bitwise operators?

#23

Post by joshvm »

conductr wrote: post #4

This is stated to be a puzzle. But, when does this actually get used? Are there use cases of bit manipulation in JS? I've never understood when using bit operators could be necessary or even useful. Of course, I'm speaking from my experience building web apps; ruby, python, js, php, etc. I don't have much other programming experience

Embedded applications use bitwise ops all the time. Particularly for performing changes to registers.

Example, a microcontroller has an 8 bit status register. Each bit controls some hardware function. You wish to change bit 5, but you can't change anything else because it would cause unwanted side effects. So this rules out simply saying myregister = 0b00001000.

Common solution is to make a macro like:

#define setbit(BYTE,BIT) BYTE |= (1 Thus you can say I want to set bit 5 in my register by calling setbit(foo, 5) while keeping the rest of the byte intact. There are analogous macros for clearing and toggling bits.

Or you might want to perform an operation based on a particular bit in a byte, e.g. byte 5 is a status flag that tells you when a timer has overflowed.

#define getbit(BYTE, BIT) !!(BYTE & ~(1 Then you can say:

if( getbit(myregister, 5)){ // do something }

Fun fun :)

Re: Do you know your bitwise operators?

#24

Post by ejk314 »

norswap wrote: post #22

This is making me crazy, does someone have the solution? The best I could come up with is: function one_bit(x) { return 1 >>> (x & (x + ~0)) } ("x + ~0" is equivalent to "x - 1") But this doesn't work when x is 0 All other case should be fine, afaik.

That would work if the `>>>` operator worked as expected, but actually it just looks at the first 5 bits of the second variable so that `a>>>b` is effectively equivalent to `a>>>(b&31)`. His description of it is wrong.

http://jsfiddle.net/Ayzkt/2/

Re: Do you know your bitwise operators?

#25

Post by ejk314 »

ejk314 wrote: post #24
norswap wrote: post #22

This is making me crazy, does someone have the solution? The best I could come up with is: function one_bit(x) { return 1 >>> (x & (x + ~0)) } ("x + ~0" is equivalent to "x - 1") But this doesn't work when x is 0 All other case should be fine, afaik.

That would work if the `>>>` operator worked as expected, but actually it just looks at the first 5 bits of the second variable so that `a>>>b` is effectively equivalent to `a>>>(b&31)`. His description of it is wrong. http://jsfiddle.net/Ayzkt/2/

Here's an ugly solution.

https://gist.github.com/anonymous/8364074

Re: Do you know your bitwise operators?

#28

Post by Someone »

That will get fairly hairy, given that JavaScript doesn't have integers. x should be expected to be a IEEE 754 double (can one discriminate between NaNs in JavaScript?), but might also be a string (does "@" have one bit set?, or might it be EBCDIC?), an array, an object, null or undefined (do those have a bit pattern?)

Re: Do you know your bitwise operators?

#29

Post by warmfuzzykitten »

fourspace wrote: post #6
warmfuzzykitten wrote: post #2

Validator seems broken.

I certainly can't get it to work. For the first problem, I tried: function one_bit(x) { return x & (x-1) == 0 } Only to see this error: Expected "}" or comment but "&" found. WAT

That returns true or false, which I believe strictly speaking aren't numeric in Javascript. By definition of problem, probably should be:

function one_bit(x) { return x & (x-1) == 0 ? 1 : 0; }

Re: Do you know your bitwise operators?

#30

Post by amenghra »

ejk314 wrote: post #25
ejk314 wrote: post #24

Earlier quoted context omitted.

That would work if the `>>>` operator worked as expected, but actually it just looks at the first 5 bits of the second variable so that `a>>>b` is effectively equivalent to `a>>>(b&31)`. His description of it is wrong. http://jsfiddle.net/Ayzkt/2/

Here's an ugly solution. https://gist.github.com/anonymous/8364074

Nice. Find a shorter one now (afaik, the best score is 12 and it's like in golf, smaller scores are better).
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