Matplotlib plot format, x**2 as x^2

donnieh

So when I plot, I noticed matplot doesn't like x^2 and other similar formats. Instead it wants Python based math expressions such as x**2.

How do I plot using x^2 notation instead of x**2? Another example is it only wants exp(x) instead of e^(x).

x = arange(-self.x_window, self.x_window, self.res)
		try:
			y = eval(self.tf.text.lower())
			plt.plot(x, y, linewidth=3)
		except:
			console.hud_alert('syntax error','error',1.00)
			return

donnieh

In my code, should I just detect for ^ or e and replace it with ** or exp, respectively?

By the way, the end goal here is to allow a user to enter "familiar" math notation into a text field and allow it to be processed. x**4 for example is not familiar notation.

Rosanne

I guess you'd have to use regular expressions to preprocess the user input before you plot. You have to be careful how you do it though, because e^ should become exp and not e**.

A quick and dirty solution would be to print a line to tell the user to use ** instead of ^ etc.

ccc

formula = formula.replace('e^(', 'exp(').replace('^', '**')

donnieh

Too smooth.

donnieh

Ok Mr. smarty pants lol. How about this one....

What if a user puts in 5x ( as in 5 times x) or any NUMBER next to x? Matplot only wants 5*x. How can I use string.replace to squeeze that * between them?

MartinPacker

I would bid for permitting both notations - in your final code. After all users are quite likely to come across both in their lives. (I certainly have.) :-)

ccc

regex is probably better here but...

def x_times(formula):
    if 'x ' in formula:
        for i in xrange(10):
            formula = formula.replace('{}x '.format(i),
                                     '{} * '.format(i))
    return formula

donnieh

@Martin

Yes, agreed. Thank you.

The exp( ) example above inherently allows for both. Also, what ever the solution is for the 5x example should allow for both 5x and 5*x as well.

Rosanne

Also, think about e^x or e^-x or e^4 (without the brackets). For that you definitely need regular expressions.

import re

formula = re.sub(r'e\^(-?[x0-9.]+?)', 'exp(\1)', formula)

Something like that, haven't tested it, but should give you a start if you want to do something with re.