symbolic substitution symbol by symbol

Question

kevin k 2015-10-15

编辑： Walter Roberson 2015-10-15

Hi

I was trying to replace symbol by symbol with the code:

syms u v x y
N=u+3*v;
u=2*x;
v=2*y;
N=subs(N,[u,v],[2*x,2*y]);

The result I got :

N =
u + 3*v

where is my mistake?

Answer 1

Martin Schätz 2015-10-15

You should not define the u=2*x; v=2*y; because then you get right anwer from subs. It wil simplify the answer and it will be true. Try to run this:

clear all, close all, clc
syms u v x y 
N=u+3*v;
N=subs(N,[u v],[2*x 2*y])

and then this:

clear all, close all, clc
syms u v x y 
u=2*x; v=2*y; 
N=u+3*v

You will get right answer both times.