Problem with "Conversion to logical from sym is not possible."
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Hello, i am using a really simple example for this problem, which I just can't get to work. I tried most of the examples which where suggested in similiar topics. I defined this function:
function [FDInt] = mg(x)
if x<0.8686
FDInt = 1/(0.27+exp(-x));
else
FDInt = (4/(3*sqrt(pi)))*(x^2 + pi^2/6)^(3/4);
end
end
and I call it here:
syms EF
solve(mg(EF)==10^20, EF)
I get this error message: "Error in mg (line 3) if x<0.8686 " I tried using vpa(x) or double(x) without working. I think the solution should be really easy - however, I just can't get it to work. If someone could give me a hint, that would be nice :-)
采纳的回答
Star Strider
2017-1-4
There seems to be only one value that meets that criterion. Just use an anonymous function and fzero:
mg = @(x) (x<0.8686).*(1./(0.27+exp(-x))) + (x>=0.8686).*((4./(3*sqrt(pi)))*(x.^2 + pi^2/6).^(3/4));
EF = fzero(@(x) mg(x)-1E+20, 1)
EF =
26.0470e+012
更多回答（2 个）
Walter Roberson
2017-1-4
1 个投票
You are passing a symbolic variable into a function that tests the value with < in the context of an if statement. That is not permitted, because if requires a definite decision and you cannot make a definite decision about whether a symbolic variable has a particular relationship or not.
You can use piecewise() if you have a new enough MATLAB, or you can code with the form that Star Strider shows (but that form can fail if the unselected expression turns out to be infinite)
Karan Gill
2017-1-9
Since you MATLAB isn't new enough to have piecewise, you can use heaviside as a substitute for piecewise.
FDInt = 1/(0.27+exp(-x))*heaviside(x-0.8686) + (4/(3*sqrt(pi)))*(x^2 + pi^2/6)^(3/4)*heaviside(0.8686-x)
But yes, using piecewise is simplest if you can upgrade :)
Karan (Symbolic doc)
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