find minumum indices of one array in another array

I have two arrays as below:

A= [ 47 100 153 207 261 314 368 422 474 527 581 635 687 741 794 847 900 954 1007 1060];

B=[301 602 903];

I'm trying find the indexA = [5 11 17] such that the corresponding elements in A(indexA)=[261 581 900] is the largest element that smaller than B. i.e.

A(indexA(i)) <=B(i) and is the largest element in A that smaller than B(i).

Here is my code using for loop:

for i=1:length(B)

indexA=length(find(A<=B(i));

end

Is there more simple way without using for loop?

Answer 1

[Fangjun Jiang]

You have the assumption that A is incremental. If it is always true, it can be done like this.

%%
A= [ 47 100   153     207     261   314    368   422   474   527  581 635   687    741   794   847  900  954   1007 1060];
B=[301 602 903];
indexA=sum(A'<=B)
indexA =
     5    11    17
     

If A is not always incremental, you could do this. The one thing that you might not have been aware of is called "implict expansion", like C=A'-B in this case. In MATLAB® R2016b and later, you can directly use operators instead of bsxfun(), since the operators independently support implicit expansion of arrays with compatible sizes.

%%
A= [ 47 100   153     207     261   314    368   422   474   527  581 635   687    741   794   847  900  954   1007 1060];
B=[301 602 903];
C=A'-B;
C(C>=0)=nan;
[~,indexA]=max(C)
indexA =
     5    11    17

[Limei Cheng]

Thx a lot. This one is about 4 times faster than my for loop solution

.

Answer 2

[Bruno Luong]

A= [ 47 100   153     207     261   314    368   422   474   527  581 635   687    741   794   847  900  954   1007 1060];
B=[301 602 903];

[~,idxA]=histc(B,A)

Result

idxA =

     5    11    17

[Limei Cheng]

This method is the fastest. However, some times it may miss the last element in A for different cases when I test the code. ie, the result can be idxA = [5 11 0] in some cases.

[Bruno Luong]

easy to fix

[~,idxA]=histc(B,[A Inf])

[Limei Cheng]

good job. Thx a lot. It's really fast.

[Limei Cheng]

Can I use the histc function inside a parfor loop?