62 lines
1.4 KiB
Matlab
62 lines
1.4 KiB
Matlab
function P = perms(V)
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%PERMS All possible permutations.
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% PERMS(1:N), or PERMS(V) where V is a vector of length N, creates a
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% matrix with N! rows and N columns containing all possible
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% permutations of the N elements.
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%
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% This function is only practical for situations where N is less
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% than about 10 (for N=11, the output takes over 3 gigabytes).
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%
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% Class support for input V:
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% float: double, single
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% integer: uint8, int8, uint16, int16, uint32, int32, uint64, int64
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% logical, char
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%
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% See also NCHOOSEK, RANDPERM, PERMUTE.
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% Copyright 1984-2015 The MathWorks, Inc.
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[~,maxsize] = computer;
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n = numel(V);
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% Error if output dimensions are too large
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if n*factorial(n) > maxsize
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error(message('MATLAB:pmaxsize'))
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end
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V = V(:).'; % Make sure V is a row vector
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n = length(V);
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if n <= 1
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P = V;
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else
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P = permsr(n);
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if isequal(V, 1:n)
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P = cast(P, 'like', V);
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else
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P = V(P);
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end
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end
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%----------------------------------------
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function P = permsr(n)
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% subfunction to help with recursion
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P = 1;
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for nn=2:n
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Psmall = P;
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m = size(Psmall,1);
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% P = zeros();
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P = sparse(nn*m,nn);
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P(1:m, 1) = nn;
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P(1:m, 2:end) = Psmall;
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for i = nn-1:-1:1
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reorder = [1:i-1, i+1:nn];
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% assign the next m rows in P.
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P((nn-i)*m+1:(nn-i+1)*m,1) = i;
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P((nn-i)*m+1:(nn-i+1)*m,2:end) = reorder(Psmall);
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end
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end |