Find multiple elements in an array.

I have an array a = [1 5 2 5 3 5 4 5]. I have a second array b = [2,3,4].

I want to type something like c = find(a == b), but Matlab doesn't like it that the dimensions don't agree. The answer I am looking for is c = [3,5,7].

I know I could do it with a for loop. Trying to avoid a for loop for speed concerns. Any help would be appreciated.

Answer 1

[Image Analyst]

Use ismember:

a = [1 5 2 5 3 5 4 5]
b = [2,3,4]
% Give "true" if the element in "a" is a member of "b".
c = ismember(a, b)
% Extract the elements of a at those indexes.
indexes = find(c)

Results:

a =
     1     5     2     5     3     5     4     5
b =
     2     3     4
c =
     0     0     1     0     1     0     1     0
indexes =
     3     5     7

Obviously you can do away with "c" if you want, and just have a one-liner. I did it in two steps just for tutorial purposes.

[Edward Umpfenbach]

Perfect. Thanks.

[Yu]

what if the number in a is unique, e.g a=[1,2,3,4,5,6,7,8,9], and b=[2,3,3] then, i would like to know the location of the numbers of b in a, according to you algorithm, the result will be [2,3], but i actuall would like to have the result of [2,3,3], since there are two 3 in b.

Any suggestion?

In your example, if 5 is included in b, the result is unacceptable as well. the ans will be : 2 3 4 5 6 7 8

hwo to improve?

[Supraj Akileti]

I have a follow up on this. What if the number is a = [1 5 2 5 3 5 4 2 5] and b = [5,2,4]. Acc to the above algorithm the index would be [2 3 4 6 7 8 9] but I actually would like to have the result [2,4,6,9,3,8,7]. Any suggestions?

[Image Analyst]

Supraj:

Try this:

a = [1 5 2 5 3 5 4 2 5] 
b = [5,2,4]
output = [];
for k = 1 : length(b)
  output = [output, find(a == b(k))]
end

[Shlomo Geva]

Without an explicit loop (a bit convoluted, sorry, just having fun, 8 years too late)

a = [1 5 2 5 3 5 4 2 5];

b = [5,2,4];

[~,Locb]=ismember(a,b);

[s,si]=sort(Locb);

out=si(s>0)

out =

2 4 6 9 3 8 7

>>

Answer 2

[Jos (10584)]

Use the second output of ismember:

a = [4 3 1 2 5]
b = [2 3 3] 

[tf, loc] = ismember(b,a)
% loc is [ 4 2 2] as required

Answer 3

[Shlomo Geva]

function out = findMultipleElements(a,b)

% Find multiple elements in an array

% example:

% a = [1 5 2 5 3 5 4 2 5]

% b = [5,2,4]

% result = [2,4,6,9,3,8,7] % the indexes in a of elements from b, in order

% % The convoluted (obfuscated) solution.

[~,Locb]=ismember(a,b);

[s,si]=sort(Locb);

out=si(s>0);

% % The straight shooter solution.

% out = [];

% for n = 1:length(b)

% out = [out,find(a==b(n))]

% end

end