# How to convert an nXn matrix into n/numXn/num matrix containing the sum of elelments

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Itzik Ben Shabat on 27 Apr 2013
Hi, suppose I have a matrix M which is nXn size. i wish to turn this matrix into a smaller matrix M1 which is n/numXn/num size. each element of M1 will contain the sum of numXnum elements of the original M. for example: M1(1,1)=sum(M(1:num,1:num))
is there a function in matlab that does this in an efficient way or am i forced to use a for loop and handle the case where n/num is not an integer? thanks

Cedric Wannaz on 27 Apr 2013
Edited: Cedric Wannaz on 27 Apr 2013
There is no function, up to my knowledge, that does this automatically (as you can see here, I already posted a similar question [in a different context] that was not answered). It is, however, not too complicated to build a little piece of code for this.
So, If I understand well, you want to compute some block-statistics on M (nxn), using a block size that might not be an integer divider of n.
Let's discuss a simple case:
>> n = 10 ;
>> bSize = 3 ; % 3x3 blocks.
>> M = randi(30, 10, 10) % Rand int M for the example.
M =
25 5 20 22 14 9 23 26 11 3
28 30 2 1 12 21 8 8 25 2
4 29 26 9 23 20 16 25 18 16
28 15 29 2 24 5 21 8 17 24
19 25 21 3 6 4 27 28 28 29
3 5 23 25 15 15 29 11 9 4
9 13 23 21 14 29 17 6 23 18
17 28 12 10 20 11 5 8 23 15
29 24 20 29 22 18 5 19 12 1
29 29 6 2 23 7 8 15 18 11
The first thing to note is that there are multiple ways to split 10 in blocks of 3 or almost 3.
3, 3, 3, 1
3, 3, 2, 2
3, 2, 3, 2
4, 3, 3
etc
and you'll want one or the other way depending on whether you need to have as many 3x3 blocks as possible, or if you want to balance the number of elements in each block.
So you have to choose how you want to define block sizes along both dimensions (you can CIRCSHIFT them to make the distribution of block sizes a little more homogeneous). I'll choose 4,3,3 along both dimensions for the example.
Now one way to achieve what you want is to build a matrix of block IDs, that we use then to summarize M on blocks, i.e
1 1 1 1 2 2 2 3 3 3
1 1 1 1 2 2 2 3 3 3
1 1 1 1 2 2 2 3 3 3
1 1 1 1 2 2 2 3 3 3
4 4 4 4 ...
4 4 4 4 ...
4 4 4 4 ...
7 7 7 7 ...
7 7 7 7 ...
7 7 7 7 ...
We can build such a matrix as follows:
>> base = floor((0:n-1) * bSize/n)
base =
0 0 0 0 1 1 1 2 2 2
>> [JJ, II] = meshgrid(base, bSize*base) ;
>> blockIDs = 1 + II + JJ
blockIDs =
1 1 1 1 2 2 2 3 3 3
1 1 1 1 2 2 2 3 3 3
1 1 1 1 2 2 2 3 3 3
1 1 1 1 2 2 2 3 3 3
4 4 4 4 5 5 5 6 6 6
4 4 4 4 5 5 5 6 6 6
4 4 4 4 5 5 5 6 6 6
7 7 7 7 8 8 8 9 9 9
7 7 7 7 8 8 8 9 9 9
7 7 7 7 8 8 8 9 9 9
Now we can obtain any block-stat. using ACCUMARRAY, where we use M as values, and blockIDs as indices. For the sum and the mean per block, for example:
>> blockSum = accumarray(blockIDs(:), M(:))
blockSum =
275
196
183
190
156
156
235
119
122
>> blockMean = accumarray(blockIDs(:), M(:), [], @mean)
blockMean =
17.1875
16.3333
15.2500
15.8333
17.3333
17.3333
19.5833
13.2222
13.5556
Hope it helps!

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