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

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

[Stephen23]

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.

[Abdulllah]

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.

Answer 1

[KSSV]

Read about ismember, ismembertol and knnsearch.

[Abdulllah]

Hello KSSV, I still need to use loop if I use ismember or knnsearch. The length of the array cc is very large and takes hours to comlete.

[KSSV]

If you use loop, it takes time. The functions ismember and knnsearch will be fast.

[Abdulllah]

ismember or knnsearch gives indeces or logical number. In that case, I am unable to konw which number of gl should be muiltplied with which number. The multiplication of gl and gr is the acutal operation to do with them.

[KSSV]

On knowing indices, you can get the points.

[Abdulllah]

I use

[Lia,Locb] =ismember(cccc,lessnum);

Because, everly element is present (which I already know) so Lia is 1, the whole vector. Locb gives number of matches of each number but it do not give indices for them. For example, if I use

A = [5 3 4 2]; 
B = [2 4 4 4 6 8];
[Lia,Locb] =ismember(A,B);

The answer is

Lia =
  1×4 logical array
   0   0   1   1
Locb =
     0     0     2     1

So, how do I know, which 2 locations?

[Abdulllah]

KInldy, reply when you are free.

[KSSV]

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)

[Stephen23]

" 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

[KSSV]

Yes... mistake..

Answer 2

[Image Analyst]

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.