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problem with taking Differentiate from variable with counter !

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shahin hashemi
shahin hashemi on 26 Apr 2018
Commented: shahin hashemi on 26 Apr 2018

dear all

i need to write a code that have x symbolic variable that contain x=[x1 x2 x3 .... xn] and also x is depended to variable t (time).at last i want to Differentiate from x. for 3 or 4 x it is easy to write code like below :

>> syms x1(t) x2(t) x3(t)
>> x=[x1 x2 x3]
x(t) =
[ x1(t), x2(t), x3(t)]
>> diff(x,t)
ans(t) =
[ diff(x1(t), t), diff(x2(t), t), diff(x3(t), t)]

now when i want to write code that contain 30 x it is difficult to this in this way

i also try this but it doesnt work :

>> syms t
>> x(t) = sym('x%d',[1 4])
x(t) =
[ x1, x2, x3, x4]
>> diff(x,t)
ans(t) =
[ 0, 0, 0, 0]

is there any way to do this in this way and if it is i also need a way to call the specific x

for example :

for i=1:3
for j=1:3
xij(t)=...
end 
end 

i know writing in this way is wrong but just for clearing my idea i really appreciated if someone could help me

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Accepted Answer

Birdman
Birdman on 26 Apr 2018
Edited: Birdman on 26 Apr 2018
Well, the approach would be as follows:
n=30;
x=sym(zeros(1,n));%preallocation
syms t
for i=1:n
x(i)=str2sym(sprintf('x%d(t)',i));
end
Now, you can easily take derivative of x as
diff(x,t)
and observe the result you want.

  3 Comments

shahin hashemi
shahin hashemi on 26 Apr 2018
ty for your attention mr birdman but i got this error :
Undefined function or variable 'str2sym'
i use matlab R2017a
Birdman
Birdman on 26 Apr 2018
Then use sym instead of str2sym:
n=30;
x=sym(zeros(1,n));%preallocation
syms t
for i=1:n
x(i)=sym(sprintf('x%d(t)',i));
end
shahin hashemi
shahin hashemi on 26 Apr 2018
Ty for your answer mr birdman i search and figure out that str2sym add to matlab in 2017b
i use sym and i got my answer but is there any benefit in useing this command is stead of sym

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