Determining the number of occurrences of each unique element in a vector

Question

How can I determine the relative frequency of a value in a MATLAB vector?

vector = [ 2 2 2 2 1 1 1 2 2 1 1 1 2 2 2 2 1 2 ];

What function will return the number of occurrences of each unique element?

Answer 1

You can use unique in combination with histc to get the relative frequency.

A=[1,2,3,1,2,4,2,1]; %#an example vector
unqA=unique(A);

This gives the unique elements as unqA=[1,2,3,4]. To get the number of occurances,

countElA=histc(A,unqA); %# get the count of elements
relFreq=countElA/numel(A);

This gives countElA=[3,3,1,1] and relFreq=[0.3750, 0.3750, 0.1250, 0.1250], which is the relative frequency of the unique elements. This will work for both integers and floating points.

Answer 2

For the most general case where you have a vector of floating point values, you can use the functions UNIQUE and ACCUMARRAY:

[uniqueValues,~,uniqueIndex] = unique(vector);
frequency = accumarray(uniqueIndex(:),1)./numel(vector);

Answer 3

You can use the function tabulate. See this example with your vector.

vector = [ 2 2 2 2 1 1 1 2 2 1 1 1 2 2 2 2 1 2 ];
tabulate(vector);
  Value    Count   Percent
      1        7     38.89%
      2       11     61.11%

If you need it in percent order, do:

t = tabulate(vector);
t = sortrows(t, 3)