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Match each element of one array with each element of other array without loops

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Imran Kanjoo
Imran Kanjoo on 22 May 2019
Answered: Image Analyst on 25 May 2019
Hello,
I want to match each element of one array with elements of the other array say "cc". Then multiply with a number from the third array. I am doing using loops. The length of arrays are very large therefore it takes couple of hours. Is it possible to do wihtout loops or make it faster. Here is the code, I am doing,
uniquec=sort(unique(cc));
maxc=max(uniquec);
c35p=0.35*maxc;
g=2;
lessnum=uniquec(uniquec<=c35p);
greaternum=uniquec(uniquec>c35p);
gl=linspace(1,2,length(lessnum));
gr=linspace(2,1,length(greaternum));
newC=zeros(size(cc));
for i=1:length(gl)
newC(cc==lessnum(i))= cc(cc==lessnum(i)).*gl(i);
end
for i=1:length(gr)
newC(cc==greaternum(i))= cc(cc==greaternum(i)).*gr(i);
end

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Stephen Cobeldick
Stephen Cobeldick on 22 May 2019
Have you used the profiler to actually check which part of your code is the bottleneck ?
If CC is very large then your code has a few "features" that might prevent it from running efficiently. For example:
  • unique already returns a sorted output, so calling sort will waste time.
  • duplicating data into lessnum and greaternum likely wastes memory.
  • duplicating exactly the same equivalence operations (e.g. indexing inside the loop).
It seems that the loops could be replaced with some logical/subscript indexing. As KSSV wrote, you should read about ismember.
Imran Kanjoo
Imran Kanjoo on 22 May 2019
Hello Stephen, you are right. I should not write sort. I dont have issue with the memeory, I have 64GB RAM. lessnum and greaternum are differenet numbers you have seen. I had only this logic in mind to implement. The issues are with the loops. Every entery of the loop has to comapre the full array cc.

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Answers (2)

KSSV
KSSV on 22 May 2019
Edited: KSSV on 22 May 2019
Read about ismember, ismembertol and knnsearch.

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KSSV
KSSV on 22 May 2019
Pick non-zero numbers in Locb; it gives the positions in B.
A = [5 3 4 2];
B = [2 4 4 4 6 8];
[Lia,Locb] =ismember(A,B);
A(Lia)
Locb(Locb==0) = [] ;
B(Locb)
Stephen Cobeldick
Stephen Cobeldick on 22 May 2019
" Locb gives number of matches of each number but it do not give indices for them. "
That is incorrect. ismember does not provide any histogram/counting functionality. Its second output are the indices of the values of A in array B. Since R2013a the default is to return the index of the first instance. In your example the first instance of the value 4 is a position 2, the first instance of value 2 is at position 1.
>> Locb(Lia)
ans =
2 1
>> B(Locb(Lia))
ans =
4 2

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Image Analyst
Image Analyst on 25 May 2019
You say "Every entery of the loop has to comapre the full array cc."
You might want to take a look at pdist2(), if you have the stats toolbox.

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