Substitute symbolic sub-expression in expression

Question

Martin Androvich 2020-6-3

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/541397-substitute-symbolic-sub-expression-in-expression

评论： Valeri Aronov 2021-6-27

I have an equation

eq = Tnext - 2*Tn + Tprev == - sigma*Tinf^4*h^2 - alpha*Tinf*h^2 + sigma*Tn^4*h^2 + alpha*Tn*h^2

in which I want to replace the sub-expression

sigma*Tinf^4*h^2 - alpha*Tinf*h^2

with

C = sigma*Tinf^4*h^2 - alpha*Tinf*h^2

such that it becomes

eq = Tnext - 2*Tn + Tprev == - C + sigma*Tn^4*h^2 + alpha*Tn*h^2

I have tried the following:

eq_to_sub = sigma*Tinf^4*h^2 - alpha*Tinf*h^2;
syms C
eq = subs(eq, eq_to_sub, C)
subsx = @(u,w0,w0str) feval(symengine,'subsex',u,[char(w0) '=' w0str]);
eq = subsx(eq, eq_to_sub, 'C')

which doesnt change the equation at all. Weirdly, both methods work if the sub-expression (to substitute) is limited to

eq_to_sub = sigma*Tinf^4;

resulting in

Tnext - 2*Tn + Tprev == Tn*h^2*alpha - C*h^2 + Tn^4*h^2*sigma + (-h^2)*Tinf*alpha

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Valeri Aronov 2021-6-27

Note that the monomials in C have different signs, but the same monomials in eq have the same sign. C is not found in eq because of it.

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Answer 1

Jyotsna Talluri 2020-6-8

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https://ww2.mathworks.cn/matlabcentral/answers/541397-substitute-symbolic-sub-expression-in-expression#answer_447965

在 MATLAB Online 中打开

Enclose the subexpresssion in braces as it contains more than one term.

eq = Tnext - 2*Tn + Tprev == - (sigma*Tinf^4*h^2 + alpha*Tinf*h^2) + sigma*Tn^4*h^2 + alpha*Tn*h^2;
C = sigma*Tinf^4*h^2 + alpha*Tinf*h^2;
q = feval(symengine,'subsex',eq,[char(C) '=C']);