How to substitute multiple values in symbolic matrix?

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Amit Kumar
Amit Kumar 2013-11-29
评论： Youssef El Seblani 2018-4-8
Hi, How to substitute multiple values in symbolic matrix? With function subs, we can substitute only single value in expression. Writing nested subs(subs(...)) can work but it becomes tedious. Here is sample code:
syms x y z w
N=[ 1y+y+z,x/2 - y/3 + z/3, x*y, x+z];
Is there any way to substitute values to w,x,y,z in single line? I guess then I have to define variables like x(1), x(2) etc. and have to use for loop to change values. Is this approach correct or is there any better way?

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采纳的回答

Andrei Bobrov
Andrei Bobrov 2013-11-29
syms x y z w
N=[ w+y+z,x/2 - y/3 + z/3, x*y, x+z]
X = sym(['[',sprintf('X(%d) ',1:4),']'])
out = subs(N,[x, y, z, w],X)
  2 个评论
Deepayan Bhadra
Deepayan Bhadra 2016-12-5
Hi,
I wanted some explanation on how the above code works. Say my N is a 6x6 matrix with 5 symbolic variables and I want to evaluate N at one go, for 5 different values of the variables. How do I modify the above answer for that? Thank you.
Karan Gill
Karan Gill 2016-12-8
That answer is wrong. Please do not use string input to sym. Instead:
>> syms x y z w
N=[ 1*y+y+z,x/2 - y/3 + z/3, x*y, x+z];
vars = [w x y z];
newVars = sym('x%d',[1 4]);
subs(N,vars,newVars)
ans =
[ 2*x3 + x4, x2/2 - x3/3 + x4/3, x2*x3, x2 + x4]
Much easier to read and understand. Look up the doc for sym to understand the command I used: https://www.mathworks.com/help/symbolic/sym.html

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更多回答（1 个）

Youssef El Seblani
Youssef El Seblani 2018-3-31
编辑：Walter Roberson 2018-3-31
clc
clear
syms x y z w
N=[ w+y+z,x/2 - y/3 + z/3, x*y, x+z]
A=sym('X',[1,4])
N(A) = subs(N,[x, y, z, w],A)
for example
N(1,2,3,4)
  2 个评论
Walter Roberson
Walter Roberson 2018-3-31
This will not work if I recall correctly. You cannot use a vector of symbolic variables after a variable name in order to define a function of multiple parameters. Instead each variable must be a separate parameter.

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