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p01/LICENSE.txt

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Created July 30, 2011 11:27
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isPrimeNumber in 41 bytes
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README.md
Raw
annotated.js
function(
n, // the number
i // placeholder counter
){
for(i=n;n%--i;); // keeps going until the modulo is falsy like n%1 for instance
return i<2
}
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index.js
function(n,i){for(i=n;n%--i;);return i<2}
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LICENSE.txt
DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
Version 2, December 2004
Copyright (C) 2011 Mathieu 'p01' Henri <http://www.p01.org/releases/>
Everyone is permitted to copy and distribute verbatim or modified
copies of this license document, and changing it is allowed as long
as the name is changed.
DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
0. You just DO WHAT THE FUCK YOU WANT TO.
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package.json
{
"name": "isPrimeNumber",
"description": "Check if a number is prime in 41bytes.",
"keywords": [
"isPrimeNumber",
"prime",
"number"
]
}
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test.html
<!DOCTYPE html>
<title>isPrimeNumber</title>
<div>Expected values: <b>false,true</b></div>
<div>Actual value: <b id="ret"></b></div>
<script>
var myFunction = function(n,i){for(i=n;n%--i;);return i<2};
document.getElementById( "ret" ).innerHTML = myFunction(140)+','+myFunction(541);
</script>
@Yaffle
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Yaffle commented Jul 31, 2011

function(n,i){for(i=2;i_i<=n&&n%i;i++);return n>1&&i_i>n}

@atk
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atk commented Jul 31, 2011

@Yaffle: you can use * instead of && in the for loop, because false coerces to zero.

Update: operator precedence failed me... sorry. My current result is:

function(n,i){for(i=1;++i<=n&&n%i;);return i==n}

@p01
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p01 commented Jul 31, 2011

Nice move putting the n%i inside the condition. Yaffle's i*i speed optimization and fix for the case for 0 and 1 were a nice touch.

Here's a 47 bytes version of Atk's golf:
function(n,i){for(i=1;n>++i&&n%i;);return i==n}

then a 55bytes version with Yaffle's improvements:
function(n,i){for(i=1;n>++i*i&&n%i;);return n<2||i*i>n}

And 52bytes with fixing the case for 0 and 1:
function(n,i){for(i=1;n>++i&&n%i;);return n<2||i==n}

@p01
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p01 commented Aug 2, 2011

:D/ reversing the loop brought this puppy down to 46 bytes with the fix for the case 0 and 1
function(n,i){for(i=n;n%--i;);return n<2||i<2}

EDIT: Actually the n<2|| is not necessary anymore due the falsiness of x%1 and x%0. And that, gentlemen, brings us to 41 bytes:
function(n,i){for(i=n;n%--i;);return i<2}

@tsaniel
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tsaniel commented Aug 4, 2011

Nice!
But I think 0 and 1 shouldn't be identified as prime numbers.
Ref: http://oeis.org/A000040

@DivineGod
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return i==1 instead of return i<2 will return correctness of function at the expense of a byte.
42 is a nice number anyway.

@mattneary
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If javascript could better handle large numbers this could be done in 30 bytes via Fermat's little theorem:

function(n){return(1<<n)%n==2}

Sadly, this function only works for numbers less than 30 due to the poor handling of math in JavaScript.

@tsaniel
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tsaniel commented Feb 28, 2012

@mattneary: I think even JavaScript could handle large numbers, your code won't work.
Because some non-prime numbers (e.g. 341) can pass the test.

@tsaniel
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tsaniel commented Feb 28, 2012

I mean the theorem cannot tell whether a number is really a prime number or not, 341 is an example(It passes the test even it is not a prime number).

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