# How to replace two symbolic variables with two (1x3) arrays and result in a (1x3) array?

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Annsley Greathouse on 7 Feb 2023
I am trying to do an analytic derivation of a long expression and then plug the numerical values in once the derivation is done. I've given a simplified example below. The problem that I am having is that it returns a (1x9) array instead of a (1x3) array.
Is there a command to replace multiple variables at once? Right now, I have only figured out how to replace one at a time which might be causing the array issue. Additionally, is there a way to have the numerical values calculated immediately? It works with a one variable replacement, but once I add more variables, the expressions are not evaluated.
% r and s arrays that will replace x and y
r = [1 2 3] ;
s = [10 20 30] ;
% defining x and y as symbolic variables
x = sym('x','real') ;
y = sym('y','real') ;
% derivative of y*sin(x) wrt x
dx = diff(y.*sin(x),x) ;
% replacing symbolic x with r array (in dx)
dx1 = subs(dx,["x"],r) ;
% replacing symbolic y with s array (in dx1)
dxf = subs(dx1,["y"],s)
dxf = Sulaymon Eshkabilov on 7 Feb 2023
There are a few different ways to get it done. One of the possible ways is the next one:
% r and s arrays that will replace x and y
r = [1 2 3] ;
s = [10 20 30] ;
% defining x and y as symbolic variables
x = sym('x','real') ;
y = sym('y','real') ;
% derivative of y*sin(x) wrt x
dx = diff(y.*sin(x),x) ;
for ii=1:3
Dxy1(ii,:) = (subs(dx, [x, y], [r(ii), s(ii)]));
end
Dxy1
Dxy1 = %% Alt. Way
syms f(x,y)
f(x,y) = y*sin(x);
dF = diff(f(x,y), x)
dF = for ii=1:3
Dxy2(ii,:) = subs(dF, [x, y], [r(ii), s(ii)]);
end
Dxy2
Dxy2 = ### Categories

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