How to integrate a function w.r.t. a variable that is not the input variable to the function

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liz
liz 2016-9-8
回答： Michael Wang 2016-9-14
Hi,
I have 2567x1 double variable which is a Strain, with a corresponding time variable.
I have a function:
function [y]=mystrainrate(reflectedstrain)
c=4980;
l_s=6*10^-3;
y=c*(reflectedstrain)/l_s;
end
I want the integral of y with respect to time between 0 and a specified time. But I want to plot it, so essentially I want a loop to find the integral up to each time index from 1 to 2567 and create another 2567x1 variable of the integrals.
But, I can't work out how to integrate a function with respect to a variable that is not the input variable to the function.
For example, I currently have this code:
for i=1:length(reflectedstrain)
nominalstrain(i,1)=integral(@mystrainrate,0,time(i));
end
But obviously this integrates the strain rate function w.r.t. the strain and not w.r.t the time.
Any help would be appreciated. Thanks.

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回答（1 个）

Michael Wang
Michael Wang 2016-9-14
As you have mentioned, the function you are using as the input of the ‘integral’ function should be a function that w.r.t time ( i.e. y(t) or in other form y(reflectedstrain(t)) ).
The ‘ integral ’ function in this case will not work due to the relation between "reflectedstrain" and time "t" are not constant so the input function varies from time to time. I would recommend you to calculate the integral value numerically (e.g. integral_y = y (i)*delta_t ) assuming you can calculate the time difference "delta_t" between time steps.
You could also try to integrate the “y” value using ‘ trapz ’ function.
Hope these help!

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