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Turn logical matrix into string vector

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Haris
Haris on 17 Oct 2020
Commented: Yvan Lengwiler on 9 May 2021
Hi. I have the logical matrix idx and the string array vec.
idx = logical([0 1 0; 0 0 1; 1 0 0; 1 0 0; 0 1 0])
vec = ["A","B","C"]
Each row of idx indicates which letter (from vec) should be assigned to that row.
The desired result is:
result = ["B";"C";"A";"A";"B"]
Is there a way to 'apply' matrix idx to a string matrix, or something like the following:
temp = repmat(vec, [size(idx,1), 1])
temp(idx) %This returns something but not as expected

Accepted Answer

Walter Roberson
Walter Roberson on 1 May 2021
Edited: Walter Roberson on 1 May 2021
idx = logical([0 1 0; 0 0 1; 1 0 0; 1 0 0; 0 1 0])
idx = 5×3 logical array
0 1 0 0 0 1 1 0 0 1 0 0 0 1 0
vec = ["A","B","C"]
vec = 1×3 string array
"A" "B" "C"
vec(idx(:,1)*1 + idx(:,2)*2 + idx(:,3) * 3).'
ans = 5×1 string array
"B" "C" "A" "A" "B"
vec(idx*(1:size(idx,2)).').'
ans = 5×1 string array
"B" "C" "A" "A" "B"

More Answers (3)

Haris
Haris on 17 Oct 2020
Btw, here is one solution:
arrayfun(@(i) vec(idx(i,:)),1:size(idx,1))'
But I was wondering if there is something that does not use any for-loop directly or indirectly.

Bruno Luong
Bruno Luong on 1 May 2021
Edited: Bruno Luong on 1 May 2021
Assuming idx has one 1 per row
idx = logical([0 1 0; 0 0 1; 1 0 0; 1 0 0; 0 1 0]);
vec = ["A","B","C"];
[r,c]=find(idx);
result(r)=vec(c)
result = 1×5 string array
"B" "C" "A" "A" "B"

Yvan Lengwiler
Yvan Lengwiler on 9 May 2021
(I wrote my answer as a comment above. Sorry. Here it is as a formal answer.)
Of course, even if a solution does not appear to use a loop as seen in the Matlab program code, there will be a loop in the ultimapte machine code anyway. Not having a code in the Matlab program can improve legibility of the text for humans, but it is not obvious that it improves the speed of execusion.
Anyway, here is a solution:
idx = logical([0 1 0; 0 0 1; 1 0 0; 1 0 0; 0 1 0]);
vec = ["A","B","C"];
tempvec = repmat(vec', [size(idx',2), 1]);
tempidx = idx'; tempidx = tempidx(:);
result = tempvec(tempidx)'

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