AFAIK there is no such thing in stock function in MATLAB, the main reason is MATLAB function calling mechanism will kill the performance with user-defined comparison.
However it is not hard to write your own quicksort algorithm. My implementation below has another drawback since swapping elements requires deep-copy (?) which is slow.
% Example:
s=arrayfun(@(x) sprintf("%d",x), randi(200,1,20))
ss=qsort(s, @(x,y) sign(double(x)-double(y)))
(EDIT: optimize qsort function)
function [A, is] = qsort(A, cmpfun)
% function [As, is] = qsort(A, cmpfun)
%
% A: vector array or vector cell array
% cmpfun: comparison function for form @(x,y)
% return -1 if x<y
% return +1 if x>y
% return 0 otherwise
% where x and y are (cell)elements of A
% OUTPUTS:
% As: A, but sorted, i.e. so that cmpfun(A(i),A(j)) <= 0 for any i<=j
% is: index vector so that A(is) is equal to As.
%
% Example:
% s=arrayfun(@(x) sprintf("%d",x), randi(200,1,20))
% ss=qsort(s, @(x,y) sign(double(x)-double(y)))
n = length(A);
is = 1:n;
if nargin < 2
cmpfun = @(x,y) sign(x-y);
end
is = qsort_helper(A, cmpfun, is);
A = A(is);
end % qsort
%%
% quicksort recursive engine
function idx = qsort_helper(A, cmpfun, idx)
if ~isempty(idx)
[p, idx] = partition(A, cmpfun, idx);
idx(1:p-1) = qsort_helper(A, cmpfun, idx(1:p-1));
idx(p+1:end) = qsort_helper(A, cmpfun, idx(p+1:end));
end
end
%%
function [p, idx] = partition(A, cmpfun, idx)
% idx(1:n) is selft-permutation such that
% pivot = A(idx(p));
% all(A(idx(1:p-1))<=pivot) && all(A(idx(p+1:n))>=pivot)
n = size(idx,2);
p = 1;
if ~isempty(idx)
% pivot index: strategy median-of-three
% A(idx(mid)) <= A(idx(1)) <= A(idx(n))
mid = floor((1 + n) / 2);
if cmpfun(A(idx(mid)),A(idx(n))) > 0
idx = swap(idx, n, mid);
end
if cmpfun(A(idx(mid)),A(idx(1))) > 0
idx = swap(idx, 1, mid);
end
if cmpfun(A(idx(1)),A(idx(n))) > 0
idx = swap(idx, 1, n);
end
pivot = A(idx(p));
i = p+1;
j = n;
% Loop to satisfied pivot condition
while true
while i<n && cmpfun(A(idx(i)),pivot) <= 0
i = i + 1;
end
while j>1 && cmpfun(A(idx(j)),pivot) >= 0
j = j - 1;
end
if i < j
idx = swap(idx, i, j);
i = i + 1;
j = j - 1;
else
% i is first element of right segment
if j > 1
% Last swap to put the pivot as the last element of the left subdivision
idx = swap(idx, 1, j);
p = j;
end
return
end
end
end
end
%%
function idx = swap(idx, i, j)
% swapt two elements idx, indexes by i and j
temp = idx(i);
idx(i) = idx(j);
idx(j) = temp;
end
