Length of Array of Symbolic Variables

Hello! I have an array of several symbolic variables that I want to loop through. However, when I use the length function, it says the length of the array is 1. Does anyone know the issue? Thank you!

[Alex Mcaulley]

Can you upload your variable in a mat file? It should work

syms a b c d
A = [a;b;c;d];
length(A)
ans =
     4

[René]

Apparently, the length and size functions don't work if the symbolic variables are dependent. For instance,

syms t

syms a(t) b c d e

A = [a;b;c;d];
B = [b;c;d;e];

lA = length(A)
sA = size(A)

lB = length(B)
sB = size(B)

returns

lA =

     1


sA =

     1     1


lB =

     4


sB =

     4     1

Answer 1

[Dyuman Joshi]

@René Hochdahl, that is because a(t) is not a symbolic variable, but a symbolic function (see below).

As a(t) is a symbolic function, A is also defined as a symbolic function. symfun objects, like function handles, are 1x1 in size.

syms t

syms a(t) b c d e



A = [a;b;c;d];
B = [b;c;d;e];

lA = length(A)

lA = 1

sA = size(A)

sA = 1×2

1 1

lB = length(B)

lB = 4

sB = size(B)

sB = 1×2

4 1

whos

  Name        Size            Bytes  Class     Attributes

  A           1x1                 8  symfun              
  B           4x1                 8  sym                 
  a           1x1                 8  symfun              
  b           1x1                 8  sym                 
  c           1x1                 8  sym                 
  cmdout      1x33               66  char                
  d           1x1                 8  sym                 
  e           1x1                 8  sym                 
  lA          1x1                 8  double              
  lB          1x1                 8  double              
  sA          1x2                16  double              
  sB          1x2                16  double              
  t           1x1                 8  sym                 

[Walter Roberson]

This is correct.

If you have a symbolic function in an expression and you are not invoking the expression with specific parameters, then the datatype of the result of the expression is almost always symfun -- and symfun are scalar .

It is not possible to have a [] array that has a symfun as an indexable component.

syms a(t)
C = [a, 5/(t-1)]

C(t) = 

C(2)

ans = 

C(1)

Error using indexing
Division by zero.

You can see that C(2) is not indexing C at location 2, and is instead invoking C with parameter 2, and C(1) is not indexing C at location 1, and is instead invoking C with parameter 1. C was created as a single function that returns an array, not as an array of functions or an array in which the first element is a function and the second is a non-function.