Elegant way to return point a specific value occurs at in a vector
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First I'd like to thank everyone who answers these posts. I've found this forum really useful when looking up a solution to a problem.
Ok here's an example situation.
given a rather chaotic waveform D and its time vector T
I want to find the value of D where the slope is say at a maximum.
Slopes = diff(D)./diff(T) %so I would take the derivative
MaxSlope = max(Slopes) %find the max
now I want to find the value in D where that slope occurs. Is there a more elegant way of doing this than?
for index = 1:length(Slopes)
if Slopes(index) == MaxSlope;
Point = index
end
end
Value = D(Point)
Such as some way of finding out at what point in Slopes MaxSlope occurs?
2 Comments
Accepted Answer
dpb
on 19 Jul 2013
Sure...
Slopes = diff(D)./diff(T);
[MaxSlope,ix] = max(Slopes);
vMxSlope=D(ix); % duplicates loop wrt boundary condition
Might want to use ix+1 for the point owing to the reduction in length from the diff() operation; your call.
doc max
Also if the time sampling is uniform can eliminate one diff() on the whole vector and use dT=T(2)-T(1). Of course, if isn't uniform then the above is needed. Probably makes little, if any noticeable difference unless very long time series are involved.
3 Comments
Image Analyst
on 20 Jul 2013
mean() does not, for obvious reasons, since the mean is not required to be in the array like min and max are. min() and max() both return the extreme value plus the index(es) where it occurs. To find a general value, you must use the FAQ to look within a tolerances. http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F or else look for the closest value
index = find(sort(abs(data - soughtValue), 'ascend'), 1, 'first');
More Answers (2)
Image Analyst
on 19 Jul 2013
Try this:
Slopes = diff(D)./diff(T)
[maxSlope, indexOfMaxSlope] = max(Slopes)
0 Comments
per isakson
on 20 Jul 2013
Edited: per isakson
on 20 Jul 2013
Logical indexing is one of my hammers, thus I saw a nail;-)
N = 1e4;
Slopes = randn( N, 1 );
tic, Point1 = max(Slopes); ixPoint1 = find( Slopes==Point1 ); toc
tic, [ Point2, ixPoint2 ] = max( Slopes ); toc
the fourth execution returned
Elapsed time is 0.000087 seconds.
Elapsed time is 0.000036 seconds.
No doubt which construct is more elegant!
BTW: The tooltip help (Cntrl+F1) doesn't show the output arguments, only the inputs. It's too easy not to remember and overlook the second and third outputs.
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