Count items in one array in another array matlab

Question

Given the values in C, I want to count how many times I encounter each of these values in the vector B. For example, if C = [1 2], I'd like to count how many "1"s are in B and how many "2"s are in B by checking the elements of array C.

However, the code I have seems to count up to 5, but what the true output should be is that the number of 1s is 2 and the number of 2s is 3.

The code I wrote is shown below. Any help would be certainly appreciated.

My code :

 B = [ 1 2 1 4 5 2 2 ]
 C = [ 1 2 ]

 lb = length(B)
 lc = length(C)

for i = 1:length(B)
    for j = 1:length(C)
      if B(1,i)==C(1,j)
          c=c+1
          A(i)=c
      end 
    end
end

rayryeng · Accepted Answer · 2015-05-27 16:35:52Z

3

Santhan Salai gave the corrected answer if you want to stick with loops. However, for efficiency, you could achieve what you want with bsxfun and sum if you don't want to loop:

>> B = [ 1 2 1 4 5 2 2 ];
>> C = [ 1 2 ];
>> D = sum(bsxfun(@eq, B, C.'), 2)

D =

     2
     3

bsxfun is a nice tool that allows you to broadcast data across singleton dimensions. In our case, I used the equals functions to check for element-wise equality and I transposed C so that it becomes a column vector. What will be produced is a matrix where we have as many rows as there are in C and as many columns as there are in B. Each row of this output matrix tells you which elements were equal to each corresponding element in C. C is broadcasted such that we get a matrix of a bunch of [1;2] columns mashed together for as many elements as there are in B and B is broadcasted such that we get replicates of B with as many rows as there are in C. By doing element-wise equality, we would produce a logical matrix where each element for a row tells you which elements were equal to the corresponding value of C. All you have to do now is sum over the columns to get how many elements were equal to each value in C, hence the sum call over the second dimension.

If you want overkill, you can also use accumarray. Assuming that B and C only consist of integers, do:

>> B = [ 1 2 1 4 5 2 2 ];
>> C = [ 1 2 ];
>> D = accumarray(B.', 1);    

D =

     2
     3
     0
     1
     1

This however computes the frequency of all unique values in B, and so you'd just want the first two entries in D.

One more alternative is to use histc:

>> B = [ 1 2 1 4 5 2 2 ];
>> C = [ 1 2 ];
>> D = histc(B, C)

D =

   2     3

What you are basically computing is a histogram of B at specified bins, which are denoted at C.

edited May 27, 2015 at 16:35

answered May 26, 2015 at 15:58

rayryeng

105k22 gold badges201 silver badges206 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

rayryeng Over a year ago

@SanthanSalai - Thank you! Also good job debugging the loop code. I was about to correct it, but you already did it for me lol.

Luis Mendo Over a year ago

Great answer! Just a comment: accumarray will only work for integer values (unless you precede it with unique, third output)

rayryeng Over a year ago

@LuisMendo ohhh good point! I assumed the input would be integer. I'll modify my post. Thanks!

Santhan Salai · Accepted Answer · 2015-05-26 15:59:47Z

1

Using arrayfun could be one alternate to your loop

out = arrayfun(@(x) sum(ismember(B,x)), C)

Here is your corrected loop

count = zeros(1,numel(C));
for ii = 1:numel(C)
    for jj = 1:numel(B)
        if (C(ii) == B(jj))
            count(ii) = count(ii) + 1;
        end
    end
end

edited May 26, 2015 at 15:59

answered May 26, 2015 at 15:52

Santhan Salai

3,89821 silver badges29 bronze badges

Collectives™ on Stack Overflow

Count items in one array in another array matlab

2 Answers 2

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related