# How to substitute multiple values in symbolic matrix?

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Amit Kumar on 29 Nov 2013
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?

Andrei Bobrov on 29 Nov 2013
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)
Deepayan Bhadra on 5 Dec 2016
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 on 8 Dec 2016
>> 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

Youssef El Seblani on 31 Mar 2018
Edited: Walter Roberson on 31 Mar 2018
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)
Walter Roberson on 31 Mar 2018
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.
Youssef El Seblani on 8 Apr 2018

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