find a vector in a big vector

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

Answer 1

[José-Luis]

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);

[Rica]

there is a problem with cell2mat!

[Rica]

i solve it in this way:

 %
vals = (2:2:20)';
your_mat = (arrayfun(@(x) {find(a==x)},vals,'uniformoutput',false));
gg=cat(1,your_mat{:});
result=cell2mat(gg);

thank you!

[José-Luis]

My pleasure

[Jan]

Arrayfun with an anonymous function can be much (e.g. 10 times) slower than a corresponding simple FOR loop.

Answer 2

[Andrei Bobrov]

a = randi(80,50,1);
v = (2:2:20).';

[i1,i2] = ismember(a,v);
i3 = find(i1);
[~,jj] = sort(i2(i1));
index = i3(jj);

Answer 3

[Jan]

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.