Subs Command for Multivariable Function

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Alex Santizo
Alex Santizo 2020-2-7
评论： Walter Roberson 2020-12-11
syms x y
g=(1/2)*e^(-2*x/3).*tan(y+1);
subs(g,x,0.3);
subs(g,y,-0.7);

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回答（2 个）

Walter Roberson
Walter Roberson 2020-2-7
For the case of scalars
subs(g, [x, y], [0.3,-0.7])
For the case of non-scalars
subs(g, {x, y}, {newx, newy})
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Ashwin Shibu
Ashwin Shibu 2020-12-11
i dont think there shd be s dot after e^(-2*x/3)
Walter Roberson
Walter Roberson 2020-12-11
Dot between the e and the ^ is needed if x might be non-scalar. Dot between the ) and the *tan is needed if x and y might both be non-scalar.
That said, exp() is recommended instead of e^
g=(1/2)*exp(-2*x/3).*tan(y+1);
As a matter of style: I recommend that .* be used in any case involving variables unless you deliberately mean matrix multiplication (inner product). I would suggest * with scalars only for very simple constant multiples, such as 3*x -- but not, for example, x*y even if you know that x and y are both scalars. Using .* explicitly saves thinking by other people reading the code about whether you really meant matrix multiplication and saves programmers reading your code tracing back to prove that in every possible code path to that point, that x and y are guaranteed to be scalars.

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Vladimir Sovkov
Vladimir Sovkov 2020-2-7
If e^ is the exponent and if you want to estimate the result numerically (from the question it is not quite clear what you want), you can use
syms x y
g=(1/2)*exp(-2*x/3).*tan(y+1);
double(subs(subs(g,x,0.3),y,-0.7))
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Alex Santizo
Alex Santizo 2020-2-7
The question I am trying to solve is to evaluate that function at x & y are equal to the listed values using the subs command and by conversion into a matlab function.
Vladimir Sovkov
Vladimir Sovkov 2020-2-7
编辑：Vladimir Sovkov 2020-2-7
This case the code above should work. Though, you can do it easier and with faster computation avoiding symbolic computations as, e.g. (there are many ways to do it)
function g = myfunc (x,y)
g=(1/2)*exp(-2*x/3).*tan(y+1);
end
and call it as
g=myfunc(0.3,-0.7)
The error in your code is because Matlab is unaware what the variable "e" is. If you mean the exponent function, use "exp" instead, as in the code above. If you actually defined "e" somewhere before, the error can be with nonscalar values; if you mean the element-wise operation, use ".^" instead of "^" analogous to ".*" in the following part.

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