How to collapse within a column vector

Hello,

I am trying to collapse a long column to a shorter one by suming across elements. For instance, if I have:

x =

1

2

3

4

5

6

7

8

9

I would like to sum up every three elements and arrive at:

y= 6

15

24

This is basically the 'collapse' function in Stata but I'm struggling to do it in Matlab. Thanks for your advide.

Answer 1

[Arthur Roué]

You can use the sum function after reshape your vector into a matrix

vX = 1:9;
Interval = 3;

mX = reshape(vX, Interval, numel(vX)/Interval)
mX =

     1     4     7
     2     5     8
     3     6     9

sum(mX)
ans =

     6    15    24

/!\ reshape() won't work if the number of elements in vX is not a multiple of Interval

[Ara Jo]

Thank you very much -- it works well!

[Ara Jo]

A related question is.. how would you do it if you had a matrix rather than a column vector? For instance, if I have a 6 by 3 matrix X

X =

1 4 7

2 5 8

3 6 9

1 4 7

2 5 8

3 6 9

How could I turn this into 3 by 3 by suming up every two elements in each column and get:

Y=

3 9 15

4 10 16

5 11 17

I tried to use reshape again and it seems the matrix is sliced in an inconvinient way.. Thank you very much for your time!

[Alan Stevens]

How about:

X = [1 4 7; 2 5 8; 3 6 9; 1 4 7; 2 5 8; 3 6 9];
X = X(:);
X = reshape(X,2,length(X)/2);
Y = sum(X);
Y = reshape(Y, 3, length(Y)/3);

[Ara Jo]

Works perfectly - thank you very much!

Answer 2

[Alan Stevens]

One possibility is:

x = 1:12;
for i = 1:length(x)/3
    p = 3*(i-1)+1;
    y(i) = sum(x(p:p+2));
end

[Ara Jo]

Great, thanks!