find a vector in a big vector

2013 2 21
3 个回答
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0 个投票

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

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 采纳的回答

José-Luis
José-Luis 2013-2-21
编辑：José-Luis 2013-2-21

0 个投票

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);

4 个评论
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Rica
Rica 2013-2-21
there is a problem with cell2mat!
Rica
Rica 2013-2-21
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!
Jan
Jan 2013-2-21
Arrayfun with an anonymous function can be much (e.g. 10 times) slower than a corresponding simple FOR loop.

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更多回答（2 个）

Andrei Bobrov
Andrei Bobrov 2013-2-21

1 个投票

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
Jan 2013-2-21
编辑：Jan 2013-2-21

0 个投票

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.

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