Using find to determine equality not of scalars, but vectors

Hi, I have two vectors say x=[1 2 3 4 5 ] and y=[2 4].

I would like to find the elements of y in x. Usually, if y is a scalar, a simple find(x==2) would do teh trick, or even x(x==2).

Is there something similar that would work like find(x==y) or x(x==y), without using a loop?

Answer 1

[John D'Errico]

This is not really the province of find. Yes, you can break an egg with a hammer, but it is not really the right tool for the purpose. (Ok, yes, the scrambled eggs made that way will be high in fiber. But they tend to be a little too crunchy for my tastes.) Instead, you need to learn about the set membership tools in MATLAB.

First, I'll make the problem slightly more interesting, since using the numbers 1:5 may confuse things.

x=[1:3:21]

x = 1×7

1 4 7 10 13 16 19

y=[2:2:20]

y = 1×10

2 4 6 8 10 12 14 16 18 20

There are clearly some elements in y that are not in x, and vice versa.

help ismember

 ISMEMBER True for set member.
    LIA = ISMEMBER(A,B) for arrays A and B returns an array of the same
    size as A containing true where the elements of A are in B and false
    otherwise.
 
    LIA = ISMEMBER(A,B,'rows') for matrices A and B with the same number
    of columns, returns a vector containing true where the rows of A are
    also rows of B and false otherwise.
 
    [LIA,LOCB] = ISMEMBER(A,B) also returns an array LOCB containing the
    lowest absolute index in B for each element in A which is a member of
    B and 0 if there is no such index.
 
    [LIA,LOCB] = ISMEMBER(A,B,'rows') also returns a vector LOCB containing
    the lowest absolute index in B for each row in A which is a member
    of B and 0 if there is no such index.
 
    The behavior of ISMEMBER has changed.  This includes:
      -	occurrence of indices in LOCB switched from highest to lowest
      -	tighter restrictions on combinations of classes
 
    If this change in behavior has adversely affected your code, you may
    preserve the previous behavior with:
 
       [LIA,LOCB] = ISMEMBER(A,B,'legacy')
       [LIA,LOCB] = ISMEMBER(A,B,'rows','legacy')
 
    Examples:
 
       a = [9 9 8 8 7 7 7 6 6 6 5 5 4 4 2 1 1 1]
       b = [1 1 1 3 3 3 3 3 4 4 4 4 4 9 9 9]
 
       [lia1,locb1] = ismember(a,b)
       % returns
       lia1 = [1 1 0 0 0 0 0 0 0 0 0 0 1 1 0 1 1 1]
       locb1 = [14 14 0 0 0 0 0 0 0 0 0 0 9 9 0 1 1 1]
 
       [lia,locb] = ismember([1 NaN 2 3],[3 4 NaN 1])
       % NaNs compare as not equal, so this returns
       lia = [1 0 0 1], locb = [4 0 0 1]
 
    Class support for inputs A and B, where A and B must be of the same
    class unless stated otherwise:
       - logical, char, all numeric classes (may combine with double arrays)
       - cell arrays of strings (may combine with char arrays)
       -- 'rows' option is not supported for cell arrays
       - objects with methods SORT (SORTROWS for the 'rows' option), EQ and NE
       -- including heterogeneous arrays derived from the same root class
 
    See also ISMEMBERTOL, INTERSECT, UNION, UNIQUE, UNIQUETOL, SETDIFF,
             SETXOR, SORT, SORTROWS.

    Documentation for ismember
       doc ismember

    Other uses of ismember

       categorical/ismember      datetime/ismember    sym/ismember
       cell/ismember             duration/ismember    tabular/ismember
       codistributed/ismember    gpuArray/ismember    tall/ismember
       dataset/ismember          mtree/ismember

[yinx,xloc] = ismember(y,x)

yinx = 1×10 logical array
   0   1   0   0   1   0   0   1   0   0

xloc = 1×10

0 2 0 0 4 0 0 6 0 0

So which elements of y were found in x?

y(yinx)

ans = 1×3

4 10 16

That makes sense. Next, where were they found?

xloc(yinx)

ans = 1×3

2 4 6

So at locations [2,4,6] in the x vector.

You will also find the setdiff tool to be useful at times, as well as intersect and union. Spend some time familiarizing yourself with many of these fundamental tools in MATLAB. It will be well worth the time invested.

[Stephen23]

And here are all of the basic set functions:

https://www.mathworks.com/help/matlab/set-operations.html

Note the contents on the LHS of the page: learn to navigate this so that you can find information by yourself.

Answer 2

[Harsh Mahalwar]

Hi Josue,

You can use the following matlab script to find the elements of y in x without using a loop.

y = [2 4];

x = [1 2 3 4 5];

findLoc = arrayfun(@(i) find(x==i,1),y);

Here are some references,

Apply function to each element of array on GPU - MATLAB arrayfun - MathWorks India

Thanks and regards,

Harsh Mahalwar