How can I count the number of elements of a given value in a matrix?

Question

Does anyone know how to count the number of times a value appears in a matrix?

For example, if I have a 1500 x 1 matrix M (vector) which stores the values of weekdays (1 - 7), how could I count how many Sundays (1), Mondays(2), ... , Saturdays(7) are stored in M?

Answer 1

Have a look at Determine and count unique values of an array.

Or, to count the number of occurrences of 5, simply do

sum(your_matrix == 5)

Answer 2

One way you can perform this operation for all the values 1 through 7 at once is to use the function ACCUMARRAY:

>> M = randi(7,1500,1);  %# Some random sample data with the values 1 through 7
>> dayCounts = accumarray(M,1)  %# Will return a 7-by-1 vector

dayCounts =

   218       %# Number of Sundays
   200       %# Number of Mondays
   213       %# Number of Tuesdays
   220       %# Number of Wednesdays
   234       %# Number of Thursdays
   219       %# Number of Fridays
   196       %# Number of Saturdays

Answer 3

Here's a list of all the ways I could think of to counting unique elements:

M = randi([1 7], [1500 1]);

Option 1: tabulate

t = tabulate(M);
counts1 = t(t(:,2)~=0, 2);

Option 2: hist/histc

counts2_1 = hist( M, numel(unique(M)) );
counts2_2 = histc( M, unique(M) );

Option 3: accumarray

counts3 = accumarray(M, ones(size(M)), [], @sum);
%# or simply: accumarray(M, 1);

Option 4: sort/diff

[MM idx] = unique( sort(M) );
counts4 = diff([0;idx]);

Option 5: arrayfun

counts5 = arrayfun( @(x)sum(M==x), unique(M) );

Option 6: bsxfun

counts6 = sum( bsxfun(@eq, M, unique(M)') )';