How to substitute a value for a symbolic variable (in Python, using SymPy)
Task
If we’ve defined a symbolic variable and used it in a formula, how can we substitute a value in for it, to evaluate the formula? This is often informally called “plugging in” a value.
Related tasks:
Solution
This answer assumes you have imported SymPy as follows.
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from sympy import * # load all math functions
init_printing( use_latex='mathjax' ) # use pretty math output
Let’s assume we’ve defined a variable and created a formula, as covered in how to create symbolic variables.
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var( 'x' )
formula = x**2 + x
formula
$\displaystyle x^{2} + x$
We can substitute a value for $x$ using the subs function.
You provide the variable and the value to substitute.
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formula.subs( x, 8 ) # computes 8**2 + 8
$\displaystyle 72$
If you had to substitute values for multiple variables, you can use
multiple subs calls or you can pass a dictionary to subs.
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var( 'y' )
formula = x/2 + y/3
formula
$\displaystyle \frac{x}{2} + \frac{y}{3}$
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formula.subs( x, 10 ).subs( y, 6 )
$\displaystyle 7$
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formula.subs( { x: 10, y: 6 } )
$\displaystyle 7$
Content last modified on 24 July 2023.
See a problem? Tell us or edit the source.
Contributed by Nathan Carter (ncarter@bentley.edu)