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how can we estimate memory requirements for nchoosek?

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Andy
Andy on 29 Jul 2018
Edited: dpb on 4 Aug 2018
nchoosek is a memory intensive command ... to the point it can easily kneel down your system ... or get something back as the below:
Error using nchoosek>combs (line 175) Out of memory. Type HELP MEMORY for your options.
Error in nchoosek>combs (line 174) Q = combs(v(k+1:n),m-1);
Error in nchoosek>combs (line 174) Q = combs(v(k+1:n),m-1);
Error in nchoosek (line 132) c = combs(v,k);
Error in NChooseKR (line 7) parfor i = kRange
  1 Comment
Stephen
Stephen on 29 Jul 2018
Original question:
"how can we estimate memory requirements for nchoosek?"
Number of combinations * number of elements per combination * bytes per element

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Accepted Answer

dpb
dpb on 29 Jul 2018
>> help nchoosek
nchoosek Binomial coefficient or all combinations.
...
nchoosek(V,K) where V is a vector of length N, produces a matrix
with N!/K!(N-K)! rows and K columns. Each row of the result has K of
the elements in the vector V. This syntax is only practical for
situations where N is less than about 15.
...
TMW writes documentation for a reason...
  4 Comments
dpb
dpb on 4 Aug 2018
>> nchoosek(int16(1:5),3)
ans =
10×3 int16 matrix
...
>> whos ans
Name Size Bytes Class Attributes
ans 10x3 60 int16
>> nchoosek((1:5),3)
ans =
...
>> whos ans
Name Size Bytes Class Attributes
ans 10x3 240 double
>>
Is 4:1; uint8 would be 8:1 and handle values up to 255; as implemented it may require double() first I don't know and there's still the issue of the specific recursive algorithm used that probably could be improved on.

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