How to count number of terms in a symbolic expression?
13 Ansichten (letzte 30 Tage)
Ältere Kommentare anzeigen
Mehdi
am 2 Dez. 2023
Kommentiert: Walter Roberson
am 3 Dez. 2023
I have a long symbolic expression composed of many terms. How is it possible to count the total number of terms of my expression and then choose the ith term?
syms x y
z=x*y+cos(x*y)*x+x*sin(x)+sin(x)*cos(x)-x-y+2;
for example here the z is composed of 7 terms and the 3rd term is x*sin(x).
0 Kommentare
Akzeptierte Antwort
Walter Roberson
am 3 Dez. 2023
Bearbeitet: Walter Roberson
am 3 Dez. 2023
syms x y
z=x*y+cos(x*y)*x+x*sin(x)+sin(x)*cos(x)-x-y+2;
terms = children(z)
nterms = length(terms)
terms{3}
children(z, 3)
2 Kommentare
Walter Roberson
am 3 Dez. 2023
Notice the "third" term as far as MATLAB is concerned is not x*sin(x)
MATLAB re-arranges terms according to algebraic equivalences, into its own preferred order. The rules for ordering are not published, and can be contextual.
Walter Roberson
am 3 Dez. 2023
Further example:
syms x
f = 6 + x - 3 - 2 - 1
as far as the symbolic toolbox is concerned this is the same as
f = x
The toolbox always folds rational and symbolic floating point numbers
5 + 2*sqrt(sym('3.2'))
will get completely executed to scalar
5 + 2*sqrt(sym(3.2))
will execute to an expression. sym(3.2) is converted to sym(32)/sym(10) and sqrt of a rational is not fully evaluated
Weitere Antworten (1)
VBBV
am 3 Dez. 2023
Bearbeitet: VBBV
am 3 Dez. 2023
syms x y
z='x*y+cos(x*y)*x+x*sin(x)+sin(x)*cos(x)-x-y+2'
C = strsplit(z,{'+','-'})
length(C)
C{3} % 3rd term
4 Kommentare
Walter Roberson
am 3 Dez. 2023
Bearbeitet: Walter Roberson
am 3 Dez. 2023
Consider
syms x y
z='x*y+cos(x-y)*x+x*sin(x)+sin(x)*cos(x)-x-y+2'
C = strsplit(z,{'+','-'})
length(C)
C{3} % 3rd term
The pattern matching has to be a lot more complicated to extract expressions. Indeed, it can be proven that it cannot be done using traditional regular expressions -- not unless you are willing to impose a maximum nesting depth (and use terribly messy expressions.) Some programming languages such as perl add regexp constructs that extend the pattern capabilities to make it possible; if MATLAB has those facilities I am overlooking them.
Siehe auch
Kategorien
Mehr zu Number Theory finden Sie in Help Center und File Exchange
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!