# How to quickly find out the repeat integers in an array?

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Benson Gou on 3 Jun 2021
Commented: Benson Gou on 4 Jun 2021
Dear All,
I have an array A which contains repeat intergers (entries). For example, A = [4 20 5 4 7 5 9 5 31]. I want to find out the repeat elements {4, 5} and their locations.
Thanks.
Benson

Akira Agata on 3 Jun 2021
I believe it's time to use accumarray function!
A = [4 20 5 4 7 5 9 5 31];
B = accumarray(A',1);
pt = find(B >= 2);
loc = arrayfun(@(x) find(x == A),pt,'UniformOutput',false);
tResult = table(pt,loc,'VariableNames',{'Value','Location'});
The result is like this:
>> tResult
tResult =
2×2 table
Value Location
_____ _________
4 {[ 1 4]}
5 {[3 6 8]}
Walter Roberson on 3 Jun 2021
At the time I was composing my own reply, I didn't think anyone else would give an accumarray solution ;-)
There are a few differences between the above and my first solution:
• my version supports negative integers
• I "compress" the range of values in a sense: if the mininum integer happens to be large, a straight accumarray could waste a lot of space, whereas my version does not use up any space between 0 and the lowest integer
• but both versions potentially waste a lot of space: if for example A was [1 1e6+1 2e6+1] then both of them waste a lot of memory
• we handle the matches differently. Akira's version searches through the data once plus one time for every unique value; my version searches through the data once

Walter Roberson on 3 Jun 2021
A = [4 20 5 4 7 5 9 5 31]
A = 1×9
4 20 5 4 7 5 9 5 31
mA = min(A(:)) - 1;
At = A(:) - mA;
locs = accumarray(At, (1:length(At)), [], @(V) {V.'});
dups = cellfun(@length,locs) > 1;
info = [num2cell(At(dups)+mA), locs(dups)]
info = 2×2 cell array
{[ 4]} {[ 1 4]} {[20]} {[3 6 8]}
... but most people might find that too obscure. Less obscure might be
[G, uA] = findgroups(A);
info = cell(0,2);
for K = 1 : max(G)
matches = find(G == K);
if length(matches) > 1
info(end+1,:) = {uA(K), matches};
end
end
info
info = 2×2 cell array
{[4]} {[ 1 4]} {[5]} {[3 6 8]}
Benson Gou on 4 Jun 2021
Hello, Walter,
Thanks a lot for your great help. You have a good weekend!
Benson

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