Operations on consecutive elements of a vector
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Imagine there is a vector C with n elements, c(i). Without using a loop I would like to a have new vector D whose elements are:
d(i)= c(i) if i=1
d(i)= c(i-1)-c(i) if 1<i<n
d(i)= -c(i) if i=n
I do not care about the first and last elements which I could add by concatenation, but generally I am looking for a way to perform mathematical operations on consecutive elements of a vector without using a loop. Is this possible?
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dpb
2016-2-8
If'en I interprets your requirements correctly...
d=[c(1) diff(c(1:end-1)) -c(end)];
Example:
>> c=randi(10,1,5)
c =
9 9 4 7 3
>> d=[c(1) diff(c(1:end-1)) -c(end)]
d =
9 0 -5 3 -3
>>
6 个评论
The question specifies the difference as " c(i-1)-c(i)" rather than the c(i)-c(i-1) that diff supports. A simple minus resolves the problem:
d = [c(1),-diff(c(1:end-1)),-c(end)]
dpb
2016-2-8
I had it that way first and decided probably was misstated by OP... :)
Saeid
2016-2-9
Saeid
2016-2-9
All depends on what the operation(s) is(are); sometimes it's easily vectorized, other times "not so much". filter can do many operations but not all. See
help datafun % for list to give some ideas on what's builtin
After that, it's mostly just thinking about the operations and using "time in grade" to apply Matlab syntax to the particular case.
Oh, and it helps if you're reasonably fluent in matrix algebra and have some knowledge of some of the special properties of matrices so that can recognize when magic occurs...
ADDENDUM
Oh, I am not into image processing so didn't think of it originally, but there's blockproc in that toolbox or, if you don't have access to it a submission in the File Exchange (not tried)... Apply function on subsets of array>
Saeid
2016-3-3
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