Area under curve (no function)

2014 9 9
2 个回答
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更新时间：2021 1 16
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5 个投票

Hello,
If I have a set of data such as
X = [0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.85];
rA = [-.0053, -.0052, -.0050, -.0045, -.0040, -.0033, -.0025, -.0018, -.00125, -.0010];
plot(X,-1./rA)
and I want to know the area under the curve generated in the graph, how would I do that? There is no function involved here, this is just raw data, so I know I can't use quad or any of those integral functions.
Thanks

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Ashok
Ashok 2016-7-9
% Find the number of pixels of the connected component
Bin_image - logical binary image CC = bwconncomp(Bin_image); CC numPixels = cellfun(@numel,CC.PixelIdxList); disp(numPixels);
% Find the number of pixels of the connected component
[Label,~]=bwlabel((Bin_image),8); [Label,~]=bwlabel(Bin_log_4mult,8); num=1; [row, ~] = find(Label==num); Obj_area=numel(row); display(Obj_area);
Samarth Patil
Samarth Patil 2018-6-7
how to find the area in an interval for such a plot?
Thanks a lot. It helped a lot.

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 采纳的回答

Star Strider
Star Strider 2014-9-9
编辑：Star Strider 2014-9-9

20 个投票

Use trapz. It’s designed to do exactly what you want.
Int = trapz(X, -1./rA)
produces:
Int =
306.7455

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Rick
Rick 2014-9-9
编辑：Rick 2014-9-9
now how do I know if I should do trapz(X,-1./rA) or trapz(-1./rA,X)?? Which would give the area under the curve for the plot(X,-1./rA)??
The documentation is quite unclear to me, it says
Z = trapz(X,Y) computes the integral of Y with respect to X using
the trapezoidal method. X and Y must be vectors of the same
length, or X must be a column vector and Y an array whose first
non-singleton dimension is length(X). trapz operates along this
dimension.
if X is my x-axis and -1./rA is my y-axis, does that mean I should do trapz(X,-1./rA)??
Star Strider
Star Strider 2014-9-9
‘if X is my x-axis and -1./rA is my y-axis, does that mean I should do trapz(X,-1./rA)??’
Yes. Actually, X is your independent variable and (-1./rA) is your dependent variable.
Use:
Int = trapz(X, -1./rA)
to produce:
Int =
306.7455
Saurabh Parmar
Saurabh Parmar 2020-5-25
Thank You So Much! That helped a lot.
Star Strider
Star Strider 2021-1-16
Everyone — My pleasure!

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更多回答（1 个）

Ashok
Ashok 2016-7-9

0 个投票

% Find the number of pixels of the connected component
Bin_image - logical binary image CC = bwconncomp(Bin_image); CC numPixels = cellfun(@numel,CC.PixelIdxList); disp(numPixels);
% Find the number of pixels of the connected component
[Label,~]=bwlabel((Bin_image),8); [Label,~]=bwlabel(Bin_log_4mult,8); num=1; [row, ~] = find(Label==num); Obj_area=numel(row); display(Obj_area);

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