Graphics Programs Reference
In-Depth Information
29.2 Operations with Multidimensional Arrays
Many matrix operators work with multidimensional arrays. For example,
the columnar sum of our 3
×
3
×
2 matrix,
a
,is
>> a
a(:,:,1) =
1
2
3
4
5
6
7
8
9
a(:,:,2) =
3
3
3
3
3
3
3
3
3
>> sum(a)
ans(:,:,1) =
12
15
18
ans(:,:,2) =
9
9
9
If you look carefully, you will see that the result of the sum is a 1
×
3
×
2
matrix:
>> size(sum(a))
ans =
1
3
2
This is not the same as a 3
×
2 matrix. If you want the result to be
a3
2 matrix, you can use the
squeeze
function, which gets rid of
singleton dimensions:
×
>> squeeze(sum(a))
ans =
12
9
15
9
18
9
matlab does not do an automatic squeeze whenever the result has sin-
gleton dimensions because there are times when you need the singleton
dimension to add more data.
If you want to sum over other dimensions than the rows, you give a
second parameter to the
sum
function specifying the dimension you want
to sum over. For example, to sum over columns:
>> sum(a,2)
ans(:,:,1) =
6
15
24
