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 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 }
 function(n,i){for(i=n;n%--i;);return i<2}
 DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE Version 2, December 2004 Copyright (C) 2011 Mathieu 'p01' Henri 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.
 { "name": "isPrimeNumber", "description": "Check if a number is prime in 41bytes.", "keywords": [ "isPrimeNumber", "prime", "number" ] }
Expected values: false,true
Actual value:

### Yaffle commented Jul 31, 2011

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

### 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}`
Owner

### 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}`
Owner

### 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 commented Aug 4, 2011

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

### DivineGod commented Oct 3, 2011

 `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 commented Feb 27, 2012

 If javascript could better handle large numbers this could be done in 30 bytes via Fermat's little theorem: ``````function(n){return(1<

### 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 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).