How to get numerical value from symbolic variables in matlab?

Question

ABTJ 2019-3-25

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/452119-how-to-get-numerical-value-from-symbolic-variables-in-matlab

回答： Walter Roberson 2019-3-25

I am studying energy and power signals in MATLAB from the book "Signals & systems laboratory with MATLAB". I am trying to implement the example code at top of page123

code is follow:

syms t T
x=heaviside(t)-heaviside(t-1);
d=int(abs(x)^2,t,-T,T);
Px=limit((1/(2*T))*d,T,inf)
Ex=limit(d,T,inf)

I am expecting a result in numeric form, as shown in book to be Px=0 and Ex=1, but when I run my code, I get the result as a big equation as follows:

Px=limit(int(abs(heaviside(t) - heaviside(t - 1))^2, t, -T, T)/T, T == Inf)/2
Ex=limit(int(abs(heaviside(t) - heaviside(t - 1))^2, t, -T, T), T == Inf, Real)

How can I get a numeric result?

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Answer 1

Walter Roberson 2019-3-25

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/452119-how-to-get-numerical-value-from-symbolic-variables-in-matlab#answer_367154

MATLAB is having trouble with the abs(x)^2 layer.

We as human(-ish)es can analyze and see that in context, the input arguments are always going to be real valued. With real-valued arguments, abs(x)^2 is going to be the same as x^2 . If you change to x^2 instead of abs(x)^2 then you will get a definite result.