Unique elements in each column of an array (Matlab) -


i might bit rusty matlab, maybe answer question more trivial imagine @ moment. have searched online efficient solutions , haven't found any, try here.

i have large matrix, y here:

n = 1e6; k = 20; n = 100; y = ceil(n * rand(k, n));

for each column of array count number of unique elements. loop slow:

tic r = zeros(n, 1); ii = 1:n   r(ii) = numel(unique(y(:, ii))); end toc

looking vectorised, faster version.

david's answer seems correct , fast too. thank you!

n = 1e6; k = 20; n = 100; y = ceil(n * rand(k, n));  tic r1 = zeros(n, 1); ii = 1:n   r1(ii) = numel(unique(y(:, ii))); end toc  tic r2 = sum(diff(sort(y)) ~= 0) + 1; toc  all(r1' == r2)

try this

sum(diff(sort(y))~=0)+1

which uses functions vectorise. performance seems better for loop case, i'd imagine larger problems proposed solution worse due memory limitation.

with n=1e5, method takes ~7.5s on computer, , proposal takes ~0.05s. n=1e6, timings ~75s , ~0.75s.


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