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January 11th, 2017, 01:49 AM  #1 
Member Joined: Oct 2016 From: labenon Posts: 33 Thanks: 4  Need to get the nth binary permutation.
This is the last missing piece I need to complete my compression algorithm, new one. Let's say I have 4 bits with 2 bits set as 1, 0011. The total number of permutations for this number is 0011, 0101, 0110, 1001, 1010, 1100, 6 cases. This can be computed using the calculation. 4! / ((2!)(42)!) = 6 Now I want to be able to find the nth sequence, for instance 1st number is 0011, second number is 0101. So if I say n=5, I want to be able to get the 5th permutation sequence 1010 from the initial 0011. How do I do this? 
January 11th, 2017, 11:19 AM  #2 
Senior Member Joined: Sep 2015 From: USA Posts: 1,763 Thanks: 905 
If you are really using blocks of 4 bits I'd just precalculate everything and use a lookup table. Given available memory these days I'd definitely do this for up to 8 bits, and probably do this for up to 16 bits. 

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binary, nth, permutation 
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