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

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

Answer 1

[Akira Agata]

I believe it's time to use accumarray function!

How about the following?

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]

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

Answer 2

[Walter Roberson]

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]

Hello, Walter,

Thanks a lot for your great help. You have a good weekend!

Benson