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

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?

Answer 1

[Paul]

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?

[Andrew]

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]

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

Answer 2

[Walter Roberson]

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.

[Andrew]

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?

[Walter Roberson]

syms a b c d s
f(s) = a*s^6 + b*s^4 + c*s + d

f(s) = 

g(s) = a*s^6 + b*s^4 + c*s + 0

g(s) = 

[val1, powers1] = coeffs(f(s),s)

val1 = 

powers1 = 

constant_term1 = val1(powers1 == 1)

constant_term1 = 

d

[val2, powers2] = coeffs(g(s),s)

val2 = 

powers2 = 

constant_term2 = val2(powers2 == 1)

 
constant_term2 =
 
Empty sym: 1-by-0
 

%alternative
val3 = coeffs(f(s), s, 'all')

val3 = 

val3(end)

ans = 

d

val4 = coeffs(g(s), s, 'all')

val4 = 

val4(end)

ans = 

0

[Walter Roberson]

In the case where all of the coefficients are numeric (or convertable to double) you can use sym2poly and then look at the last entry.

[Andrew]

Thanks! Very helpful