Clever Geek Handbook

Using SUM and UNIQUE to Count the Values ​​of Values ​​in a Subset of a Matrix

So, suppose such a matrix is:

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

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

20 0
30 1
40 2

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

+3


source to share


3 answers


A really unique option is a good option:

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


Separating this last line:

  • arrayfun (fun, array) applies fun to each element of the array and puts it into a new array that it returns.
  • This function is a function @(y)length(x(x(:,1)==y & x(:,2)==1))

    that finds the length of the part of x where a condition is met x(:,1)==y & x(:,2)==1)

    (called boolean indexing). So for each of the unique elements, it finds a string in X, where the first is the unique element and the second is one.
+2


source


Try this (as pointed out in this answer ):



>>> [c,~,d] = unique(a(a(:,2)==1))
c =
    30
    40

d =
    1
    3

>>> counts = accumarray(d(:),1,[],@sum)
counts =
    1
    2

>>> res = [c,counts]
+1


source


You have an array of distinct integers in 'array'

the function tabulate

sorts unique values ​​and counts events.

table = tabulate(array)

find your unique account in the col column of table 2.

+1


source






More articles:


All Articles