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symvar

Find symbolic variables in symbolic expression, matrix, or function

Syntax


symvar(s)
symvar(s,n)

Description

symvar(s) returns a vector containing all the symbolic variables in s in alphabetical order with uppercase letters preceding lowercase letters.

symvar(s,n) returns a vector containing n symbolic variables in s alphabetically closest to x. If s is a symbolic function, symvar(s,n) returns the input arguments of s in front of other free variables in s.

Input Arguments

s

Symbolic expression, matrix, or function.

n

Integer.

Examples

Find all symbolic variables in the sum:

syms wa wb wx yx ya yb
symvar(wa + wb + wx + ya + yb + yx)
ans =
[ wa, wb, wx, ya, yb, yx]
 

Find all symbolic variables in this function:

syms x y a b
f(a, b) = a*x^2/(sin(3*y - b));
symvar(f)
ans =
[ a, b, x, y]

Now find the first three symbolic variables in f. For a symbolic function, symvar with two arguments returns the function inputs in front of other variables:

symvar(f, 3)
ans =
[ a, b, x]

For a symbolic expression or matrix, symvar with two arguments returns variables sorted by their proximity to x:

symvar(a*x^2/(sin(3*y - b)), 3)
ans =
[ x, y, b]
 

Find the default symbolic variable of these expressions:

syms v z
g = v + z;
symvar(g, 1)
ans =
z
syms aaa aab
g = aaa + aab;
symvar(g, 1)
ans =
aaa
syms X1 x2 xa xb
g = X1 + x2 + xa + xb;
symvar(g, 1)
ans =
x2

More About

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Tips

  • symvar(s) can return variables in a different order than symvar(s,n).

  • symvar does treat the constants pi, i, and j as variables.

  • If there are no symbolic variables in s, symvar returns the empty vector.

  • When performing differentiation, integration, substitution or solving equations, MATLAB® uses the variable returned by symvar(s,1) as a default variable. For a symbolic expression or matrix, symvar(s,1) returns the variable closest to x. For a function, symvar(s,1) returns the first input argument of s.

Algorithms

When sorting the symbolic variables by their proximity to x, symvar uses this algorithm:

  1. The variables are sorted by the first letter in their names. The ordering is x y w z v u ... a X Y W Z V U ... A. The name of a symbolic variable cannot begin with a number.

  2. For all subsequent letters, the ordering is alphabetical, with all uppercase letters having precedence over lowercase: 0 1 ... 9 A B ... Z a b ... z.

See Also

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