Integral in matlab func

Question

Dekel Mashiach 2022-7-8

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https://ww2.mathworks.cn/matlabcentral/answers/1755860-integral-in-matlab-func

编辑： Torsten 2022-7-9

hey; I'm getting this error and don't understand why. hope someone can help...

Persistent variable 'time_k1' is undefined on some execution paths. Function 'MATLAB Function5' (#61.404.411), line 24, column 17: "time_k1" Launch diagnostic report.

here is my func:

function [theta,V,Vx,Vy] = meas(Acc,gyro,time)
persistent theta_k1 wz_k1 Vx_k1 Vy_k1 Ax_k1 Ay_k1 time_k1 
wz = gyro(3);
Ax = Acc(1);
Ay = Acc(2);
if time<0.01
    
    theta_k1 = 0;
    wz_k1    = 0;
    Vx_k1    = 0;
    Vy_k1    = 0;
    Ax_k1    = 0;
    Ay_k1    = 0;
    time_k1  = time;
    theta = 0;
    V     = 0;
    Vx = 0;
    Vy = 0;
else    
    % Integration for theta:
    dt = time - time_k1;
    theta = theta_k1 + dt*(wz + wz_k1)/2;
    
    % Integration for V:
    Vx = Vx_k1 + dt*(Ax + Ax_k1)/2;
    Vy = Vy_k1 + dt*(Ay + Ay_k1)/2;
    V = sqrt(Vx^2+Vy^2);
    
    % Store for next step:
    theta_k1 = theta; 
    wz_k1 = wz;
    Vx_k1 = Vx;
    Vy_k1 = Vy;
    Ax_k1 = Ax;
    Ay_k1 = Ay;
    
end

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Answer 1

Torsten 2022-7-8

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https://ww2.mathworks.cn/matlabcentral/answers/1755860-integral-in-matlab-func#answer_1003040

编辑：Torsten 2022-7-8

You get this error because you didn't assign a value to the variable "time" when you called "meas" or a value >= 0.01.

If you give values to Acc, gyro and time (as I can see, Acc is an array of size at least 2 and gyro is an array of size at least 3) and then call "meas", you will have no problems.

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Steven Lord 2022-7-8

I don't think that's the root cause of the problem. Remember that if this function is called with a non-scalar value of time, all the elements of time must be less than 0.01 for that if statement's body to execute. If time is the variable over which you're integrating, integral will call your function with a vector of values for that variable unless you tell it your function is 'ArrayValued'.

Therefore the code that assigns time to the variable time_k1 doesn't execute but the code in the else does, which assumes time_k1 has been assigned a value. In MATLAB that may not be a problem, as the persistent variable will have been initialized to [], but from the formatting of the message I suspect you're trying to generate code (using MATLAB Coder?) and the code generation process can be stricter than MATLAB.

If you are calling integral the persistent variables are probably not the right approach to use. There's no guarantee that the time variable in a subsequent call will always contain values greater than the values of the time variable in subsequent calls, if integral needs to evaluate the integrand multiple times in an interval to accurately compute the value of the integral on that interval. If you show us the mathematical formula for the function you're trying to integrate we may be able to offer some guidance.