Question

So, presume a matrix like so:

20 2
20 2
30 2
30 1
40 1
40 1

I want to count the number of times 1 occurs for each unique value of column 1. I could do this the long way by [sum(x(1:2,2)==1)] for each value, but I think this would be the perfect use for the UNIQUE function. How could I fix it so that I could get an output like this:

20 0
30 1
40 2

Sorry if the solution seems obvious, my grasp of loops is very poor.

Answer 1

Indeed unique is a good option:

u=unique(x(:,1))
res=arrayfun(@(y)length(x(x(:,1)==y & x(:,2)==1)),u)

Taking apart that last line:

1. arrayfun(fun,array) applies fun to each element in the array, and puts it in a new array, which it returns.
2. This function is the function @(y)length(x(x(:,1)==y & x(:,2)==1)) which finds the length of the portion of x where the condition x(:,1)==y & x(:,2)==1) holds (called logical indexing). So for each of the unique elements, it finds the row in X where the first is the unique element, and the second is one.