using hasSymType(expression, 'constants') returns true when no constants

Question

Andrew 2023-9-29

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编辑： Paul 2023-9-29

When trying to find if my expression has constants, hasSymType() always returns true. For example

syms s;
hasSymType(s*2,'constant')

returns true.

children() seems to separate out the terms into it's components as well. I would expect the following code to return [s*2] but it returns [s 2].

syms s;
children(s*2)

What am I missing?

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Answer 1

Paul 2023-9-29

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Hi Andrew,

Both of those examples seem to be in accordance with doc hasSymType and children, except that children returns a cell array, not an array of sym.

syms s
hasSymType(s*2,'constant')
ans = logical
   1
syms s
children(s*2)
ans = 1×2 cell array
    {[s]}    {[2]}

What is the reason expect different results?

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Andrew 2023-9-29

编辑：Andrew 2023-9-29

Well clearly I misinterpreted the docs. My next question would be how might I figure out if there is a constant term in my expression?

For example, I have the polynomial expression f(s)=as^n+bs^(n-1)...cs+d (where n is the order of the polynomial, and a,b,c,d are constants) How would I find out if d is zero or not. Or in other words how would I find out if there is a non-zero s^0 term?

Paul 2023-9-29

编辑：Paul 2023-9-29

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For polynomials we can use coeffs

syms a b c d s

f(s) = a*s^3 + b*s^2 + c*s + d

f(s) =

[cfs,term] = coeffs(f(s),s,'all') % make sure to use 'all'

cfs =

term =

cfs(end)

ans =

d

f(s) = a*s^3 + b*s^2 + c*s

f(s) =

[cfs,terms] = coeffs(f(s),s,'all') % make sure to use 'all'

cfs =

terms =

cfs(end)

ans =

0

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Answer 2

Walter Roberson 2023-9-29

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Internally, inside the symbolic engine, s*2 is coded as a data structure

_mult(DOM_IDENT('s'), DOM_INT(2))

and taking children() of that strips off the

_mult

layer, resulting in the multiple outputs DOM_IDENT('s') and DOM_INT(2) . The interface layer knows to wrap the multiple outputs into a cell array. So the output is {s sym(2)}

2*s is not an atomic entity: it is an expression that can be decomposed into its parts. One of those parts is a constant, which is why hasType() succeeds.