A very fast way to find elements and their indices? (Is ismember fast?)
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A very fast way to find elements and their indices? (Is ismember fast?)
This would be my example:
% input: create arrays "a" and "b"
a = {'A23'};
l = 'A' : 'Z';
rl = reshape(l,size(l,2),1);
for i = 1 : size(rl,1)
for j = 1 : 10000
f{i,j} = sprintf('%s%d',rl(i), j);
end
end
b = reshape(f,[],1);
% still part of the input: randomly replace 50 values of "b" with {'A23'}
r = round((size(b,1)-1) .* rand(50,1) + 1);
b(r) = a;
% How can I make this part way much faster ?
tic
[~,idx2] = ismember(string(b),string(a));
b_filtered = b(find(idx2),:);
toc
4 Comments
Steven Lord
on 18 Nov 2022
If you're trying to generate random integer values in an interval, use randi instead of rand.
Accepted Answer
Bruno Luong
on 18 Nov 2022
Edited: Bruno Luong
on 18 Nov 2022
You overly complicate your code for nothing, and yes ismember if fast.
Not sure if your a is always single element or just in this example.
% input: create arrays "a" and "b"
a = {'A23'};
l = 'A' : 'Z';
rl = reshape(l,size(l,2),1);
for i = 1 : size(rl,1)
for j = 1 : 10000
f{i,j} = sprintf('%s%d',rl(i), j);
end
end
b = reshape(f,[],1);
% still part of the input: randomly replace 50 values of "b" with {'A23'}
r = round((size(b,1)-1) .* rand(50,1) + 1);
b(r) = a;
% How can I make this part way much faster ?
tic
[~,idx2] = ismember(string(b),string(a));
b_filtered = b(find(idx2),:);
toc
tic
tf = ismember(b,a);
b_filtered = b(tf,:);
toc
% only when a is scalar
tic
b_filtered = b(strcmp(b, a{1}));
toc
3 Comments
Bruno Luong
on 18 Nov 2022
AFAIK ismembc work on numbers, not char array or string, and the second argumentr must be sorted. So it is NOT applicable in your case.
Personally I know this function but I never use it since it is undocumented and I never need to draw the last ounce of speed for ismember.
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