find count of elements in a matrix of two columns

Question

I have a matrix

and I need the resultant matrix contains only

    1        3 (number of elements that related to element 1 (2,3,4))
    2        2 (number of elements that related to element 2 (1,4))
    3        1 (number of elements that related to element 3 (1))

[What have you tried?](http://mattgemmell.com/2008/12/08/what-have-you-tried/) — Jonathan Pitre, Jun 22 '12 at 11:55
here is a similar question: [How can I count the number of elements of a given value in a matrix?](http://stackoverflow.com/questions/2880933/how-can-i-count-the-number-of-elements-of-a-given-value-in-a-matrix) — Amro, Jun 23 '12 at 07:52

score 2 · Answer 1 · answered Jun 21 '12 at 13:44

The easiest way is to use accumarray :

  accumarray(A(:,1),A(:,2),[],@numel);

Taken from Matlab help:

accumarray groups elements from a data set and applies a function to each group. A = accumarray(subs,val) creates an array A by accumulating elements of the vector val using the elements of subs as indices. The position of an element in subs determines which value of vals it selects for the accumulated vector; the value of an element in subs determines the position of the accumulated vector in the output.

In our case, we need to group all of the elements and count their number. numel does the counting.

By the way, you don't need the second column of your data at all:

  accumarray(A(:,1),zeros(size(A(:,1))),[],@numel)

even simpler: `accumarray(A(:,1),1)` (since the default function is `@sum`) — Amro, Jun 23 '12 at 07:37

score 0 · Answer 2 · edited Jul 23 '12 at 06:36

0

use find(a(:,1)==1) to find row numbers with first column value as 1

edited Jul 23 '12 at 06:36

Luke

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answered Jun 21 '12 at 13:42

magarwal

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similarly u can use find(a(:,1)==2) and find(a(:,1)==3) – magarwal Jun 21 '12 at 13:43

score 0 · Answer 3 · answered Jun 21 '12 at 14:09

This is another approach using the array indexing and unique to generate counts:

%demo array
a=[1 1 1 2 2 3]';
%get unique elements
b=unique(a);
%count and put into matrix
result=[b sum((a(:,ones(1,length(b))))'==b(:,ones(1,length(a))),2)];

result =

     1     3
     2     2
     3     1

score 0 · Answer 4 · answered Jun 23 '12 at 07:44

You can use the tabulate function from the Statistics toolbox:

>> tabulate(A(:,1))
  Value    Count   Percent
      1        3     50.00%
      2        2     33.33%
      3        1     16.67%

Another solution using HISTC:

vals = unique(A(:,1));
counts = histc(A(:,1), vals);

The resulting matrix:

>> result = [vals counts]
result =
    1     3
    2     2
    3     1

This is different from ACCUMARRAY/TABULATE in that it does not assume that the elements start at 1. To understand what I mean, modify A as: A(:,1) = A(:,1)+10; before running every solution.

find count of elements in a matrix of two columns

4 Answers4