var(I(:)); Means?
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Hi, I have this problem
signal_var = var(I(:));
What's the meaning of the code below?
var(I(:));
Thanks a lot :)
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Answers (2)
the cyclist
on 5 May 2012
I(:) treats the array I as a column vector (instead of, say, maybe the MxN array that it was). The var() function takes the variance of that column vector. [See "help var" for details.]
(That assumes that you did not define var as a variable name earlier in the code. If var is a variable, then this code is indexing into that variable.)
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Image Analyst
on 5 May 2012
Superb, normally var() will take the mean of each column of a 2D array and return a row vector where each element is the variance of the corresponding column. The programmer didn't want that - he wanted the variance of the entire array, not a column-by-column variance. So to do that you can use the (:) construct, which basically takes all the elements in an array (of any dimension) and strings them all together in a single 1D list of numbers (called a "column vector"). Once you've done that, then the var() function will operate on the whole array, not column-by-column. Now it will return a single number rather than a 1D row vector of the column variances. Does that explain it well?
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