4.7) A Final Use of The Colon Operator
Another use of the colon operator is to compress an array down to a vector. This is useful if you want to use a reduction type operator on a whole array. For example:
- Use mean to calculate the mean of an array
- Use std to calculate the standard deviation of an array
- Use min or max to calculate the min/max of an array.
What do we mean by a reduction type operator? These operators work on column or row vectors and generate a single output value. So how can we find the average value of an array?
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[matlab] a=rand(100,100); b=mean(a); size(b); ans= 1 x 100 [/matlab] We end up with a row vector where each cell is the mean of a column of "a". So we could then do: |
[matlab] a=rand(100,100); b=mean(a); c=mean(b); size(c); ans= 1 x 1 [/matlab] "c" is now the mean of the row vector "b". But we can do this more simply using the colon: |
[matlab] a=rand(100,100); b=mean( a(:) ); size(b) ans= 1 x 1 [/matlab] Here the colon operator compresses the array "a" into a vector so that "mean( a(:) )" results in a scalar value. |