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solving equation with if statements

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antoine 2020-1-17

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评论： antoine 2020-2-22

采纳的回答： Walter Roberson

在 MATLAB Online 中打开

Hello

I am trying to solve the following equation given input time and values (y) vectors.

I tried the following

syms  theta kappa alpha 
if time < alpha
    eqn = exp(-kappa*time)==y;
else
    eqn = exp(-kappa)*exp(-theta*(time-alpha))==y;
end
vars = [theta kappa alpha];
S=solve(eqn);

but it raises an error...

Conversion to logical from sym is not possible.

I tried to convert alpha into double (i.e. double(alpha) ) in the if statement but it did not work...

Any suggestions?

the data look like :

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采纳的回答

Walter Roberson 2020-1-17

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syms  theta kappa alpha time y real
eqn = y == piecewise(time < alpha, exp(-kappa*time), exp(-kappa)*exp(-theta*(time-alpha)));
vars = [theta kappa alpha];
S = solve(eqn, kappa, 'returnconditions', true);    %solve for which variable ??

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antoine 2020-1-18

在 MATLAB Online 中打开

using strictly the code you suggested;

load('values_bupr1.mat')
load('days_bupr1.mat')
 
 
y=values_bupr_nodup;
time=days_bupr_nodup;
 
syms  theta kappa alpha Time real
eqn = piecewise(Time < alpha, exp(-kappa*Time), exp(-kappa)*exp(-theta*(Time-alpha)));
residue = sum((subs(eqn, Time, time) - y).^2);
residueF = matlabFunction(residue, 'vars', {[theta kappa alpha]}, 'file', 'residue.m');  %must write to file!
Now residueF is a function handle to minimimize, such as
kappa_guess = mean(-log(y)./time);
theta_guess = randn;
alpha_guess = randn;
p0 = [theta_guess, kappa_guess, alpha_guess];
[best_parameters, best_residue] = fminsearch(residueF, p0);

I have the following error

Error using symengine
Unable to evaluate to Boolean.
Error in sym/mupadmexnout (line 1057)
        out = mupadmex(fcn,args{:});
Error in sym/matlabFunction>optimize (line 468)
    [tvalues,f,tnames] = mupadmexnout('symobj::optimizeWithIntermediates',f{:});
Error in sym/matlabFunction>writeMATLAB (line 443)
[f,tvalues,tnames] = optimize(f,optim);
Error in sym/matlabFunction (line 183)
    g = writeMATLAB(funs,file,varnames,outputs,body, opts.Optimize, opts.Sparse, opts.Comments);
Error in SOLVER_3VARS (line 14)
residueF = matlabFunction(residue, 'vars', {[theta kappa alpha]}, 'file', 'residue.m');  %must write to file!

Walter Roberson 2020-1-20

Yup, but that gives residue in the range of 62, whereas 5087.59871387777093, 0.00292734523205026836, 803477.437304047635 gives you residue in the range of 39.02 which is significantly better. Given that model, alpha should be large, over 100,000.

antoine 2020-1-20

ahaha.. I get it. thank you!

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antoine 2020-1-18

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time and y are vectors (see plot above for data input). all suggestions are welcome.

I can share the data if it helps

thank you ++

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Walter Roberson 2020-1-18

Unless time and y are vectors of length 3 exactly, it seems unlikely that there would be any solutions.

It looks to me as if what you have should be a modeling task to estimate parameters rather than a set of simultaneous equations.

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antoine 2020-1-18

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good not know...Not sure I have the skills to do that.

I really appreciated your input and advices.

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antoine 2020-2-22

dear Walter

Just realized I forgot to thank you for your help

Problem is fixed! I got my equations working.

(I am know stuck with other timer/loop function : see 'time-dependant iteration through a loop?' if you have time...)

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