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arraysmatlab
sasha
sasha

Reputation: 97

MATLAB: comparing all elements in three arrays

I have three 1-d arrays where elements are some values and I want to compare every element in one array to all elements in other two.

For example:

a=[2,4,6,8,12]
b=[1,3,5,9,10]
c=[3,5,8,11,15]

I want to know if there are same values in different arrays (in this case there are 3,5,8)

Upvotes: 4

Views: 4011

Answers (4)

Joe Serrano
Joe Serrano

Reputation: 424

c(ismember(c,a)|ismember(c,b)),

ans =
     3     5     8

I think this works for all matrices.

Upvotes: 3

gnovice
gnovice

Reputation: 125854

The answer given by AB is correct, but it is specific for the case when you have 3 arrays that you are comparing. There is another alternative that will easily scale to any number of arrays of arbitrary size. The only assumption is that each individual array contains unique (i.e. non-repeated) values:

>> allValues = sort([a(:); b(:); c(:)]);  %# Collect all of the arrays
>> repeatedValues = allValues(diff(allValues) == 0)  %# Find repeated values

repeatedValues =

     3
     5
     8

If the arrays contains repeated values, you will need to call UNIQUE on each of them before using the above solution.

Upvotes: 6

AVB
AVB

Reputation: 4024

Leo is almost right, should be 

unique([intersect(a,[b,c]), intersect(b,c)])

Upvotes: 5

Leo Alekseyev
Leo Alekseyev

Reputation: 13483

Define what you mean by compare. If the arrays are of the same length, and you are comparing equality then you can just do foo == bar -- it's vectorized. If you need to compare in the less than/greater than sense, you can do sign(foo-bar). If the arrays are not the same length and/or you aren't comparing element-wise -- please clarify what you'd like the output of the comparison to be. For instance,

foo = 1:3;
bar = [1,2,4];
baz = 1:2;
sign(repmat(foo',1,length([bar,baz])) - repmat([bar, baz],length(foo),1))
# or, more concisely:
bsxfun(@(x,y)sign(x-y),foo',[bar,baz])

does what you ask for, but there is probably a better way depending on what you want as an output.

EDIT (OP clarified question): To find common elements in the 3 arrays, you can simply do:

>> [intersect(a,[b,c]), intersect(b,c)]
ans =
     8     3     5

Upvotes: 1

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