Differentiation with respect to variable which has been changed to numeric type

Question

Ali Baig 2017-11-7

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评论： Ali Baig 2017-11-8

Hello, consider the given expression. Here x and y are symbolic variables and X and Y are numeric matrices of same size. All other variables are scalars.

RVector = ( (x - X).^2 + (y - Y).^2 + (alpha_C * d_C).^2 ) .^q;

When I execute the the following lines

x = X;
y = Y;
R_0Vector = double((subs(RVector)));

x and y becomes numeric variables. (Right?) Now, after the execution of above lines, how can I differentiate RVector with respect to x and y?

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Answer 1

Walter Roberson 2017-11-7

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The easiest way is to avoid the problems associated with assigning a new value to a symbolic variable that has been used in other expressions. Do not do the assignments x = X and y = Y. Instead do

R_0Vector = double(subs(Rvector, {x, y}, {X, Y}));

and then x and y will still be symbolic, allowing you to continue on with

diff(Rvector, x)