Doing Math with Python by Amit Saha

Author:Amit Saha
Language: eng
Format: mobi, epub, azw3, pdf
Publisher: No Starch Press, Inc.
Published: 2015-08-25T04:00:00+00:00

Plotting Expressions Input by the User

The expression that you pass to the plot() function must be expressed in terms of x only. For example, earlier we plotted y = 2x + 3, which we entered to the plot function as simply 2x + 3. If the expression were not originally in this form, we’d have to rewrite it. Of course, we could do this manually, outside the program. But what if you want to write a program that allows its users to graph any expression? If the user enters an expression in the form of 2x + 3y – 6, say, we have to first convert it. The solve() function will help us here. Let’s see an example:

>>> expr = input('Enter an expression: ')

Enter an expression: 2*x + 3*y - 6

➊ >>> expr = sympify(expr)

➋ >>> y = Symbol('y')

>>> solve(expr, y)

➌ [-2*x/3 + 2]

At ➊, we use the sympify() function to convert the input expression to a SymPy object. At ➋, we create a Symbol object to represent 'y' so that we can tell SymPy which variable we want to solve the equation for. Then we solve the expression to find y in terms of x by specifying y as the second argument to the solve() function. At ➌, this returns the equation in terms of x, which is what we need for plotting.

Notice that this final expression is stored in a list, so before we can use it, we’ll have to extract it from the list:

>>> solutions = solve(expr, 'y')

➍ >>> expr_y = solutions[0]

>>> expr_y

-2*x/3 + 2

We create a label, solutions, to refer to the result returned by the solve() function, which is a list with only one item. Then, we extract that item at ➍. Now, we can call the plot() function to graph the expression. The next listing shows a full graph-drawing program:

'''

Plot the graph of an input expression

'''

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