# find a vector in a big vector

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Rica on 21 Feb 2013
hi! how could i write this:
%
index=[find(a=2);find(a=4);find(a=6);find(a=8)....find(a=20)]
in a compact manner?
thank you

José-Luis on 21 Feb 2013
Edited: José-Luis on 21 Feb 2013
vals = (2:2:20)';
your_mat = cell2mat(arrayfun(@(x) {find(a==x)},vals,'uniformoutput',false));
And if you don't care about the order:
vals = (2:2:20)';
[ia ib] = ismember(a,vals);
Jan on 21 Feb 2013
Arrayfun with an anonymous function can be much (e.g. 10 times) slower than a corresponding simple FOR loop.

Andrei Bobrov on 21 Feb 2013
a = randi(80,50,1);
v = (2:2:20).';
[i1,i2] = ismember(a,v);
i3 = find(i1);
[~,jj] = sort(i2(i1));
index = i3(jj);

Jan on 21 Feb 2013
Edited: Jan on 21 Feb 2013
Please compare the runtime of this simple loop with the compact ARRAYFUN function with an anonymous function:
search = [2,4,6,8,20];
C = cell(1, length(search));
for iSearch = 1:length(search)
C{iSearch} = find(a == search(iSearch));
end
result = cat(2, C{:});
If compactness of the code really matters, write this as a function.