Python 2.7与3.x中的函数

Question

I'm new to Python. I've been trying to familiarize myself with Numpy, Scipy, and Matplotlib, as I have a background in the sciences, and hope to make myself a more competitive candidate for work in neuroscience laboratories.

I've been browsing through the Matplotlib documentation, trying to learn by example. I will reference an example from the following URL: http://matplotlib.org/users/pyplot_tutorial.html

I am under the impression that these examples are written in Python 3.x, and that I am having trouble because I am using Python 2.7. I am using 2.7 because some of the libraries I wanted weren't available for 3.x.

The website gave an example of using subplots. Their code is as follows:

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import numpy as np
import matplotlib.pyplot as plt

def f(t):
    return np.exp(-t) * np.cos(2*np.pi*t)

t1 = np.arange(0.0, 5.0, 0.1)
t2 = np.arange(0.0, 5.0, 0.02)

plt.figure(1)
plt.subplot(211)
plt.plot(t1, f(t1), 'bo', t2, f(t2), 'k')

plt.subplot(212)
plt.plot(t2, np.cos(2*np.pi*t2), 'r--')

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This is supposed to return a Figure like this: http://i.stack.imgur.com/ejNDu.png

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When I copy the same code into IDLE, it gives me an error. On the line

t1 = np.arange(0.0, 5.0, 0.1)

, IDLE tells me that "t1" is invalid syntax.

My first question: What is the problem with using t1 as a variable?

If I copy in similar code, but with a few things tweaked, I can avoid this error. However, I am then presented with another error. When I add the line equivalent to

plt.plot(t1, f(t1), 'bo', t2, f(t2), 'k')

, IDLE presents me with the error:

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Traceback (most recent call last):
  File "<pyshell#9>", line 1, in <module>
    plt.plot(ty, f(tx), 'bo', ty, f(ty), 'k')
NameError: name 'f' is not defined

---

I'm not sure how Python expects me to define "f" separately from the function "f(t)".

My second and MAIN question: Could somebody please explain why "f" must be defined separately? How do I use functions like this correctly in Python 2.7?

If anybody needs me to explain the Numpy/Matplotlib mechanisms used here, I will do my best to explain how they work to bring about the graphs.

Answer 1

For the syntax error, check the line or two before . t1 is a perfectly valid variable name (the rules are similar to various other languages: variable names can contain letters, numbers and _ , but can't start with a number) - but if you've forgotten a close bracket or similar above, then the interpreter will get confused and tell you you can't have an assignment statement there (newlines don't count as 'end of statement' if they're inside () , {} or [] , or if the last character before the newline is \\ ).

Defining a function does give you a variable in that namespace with that name - you don't need to define f separately. It is difficult to tell without seeing your exact code, but what has likely happened is either you have defined it in a different scope (so the name isn't visible), or you have renamed the function to something other than f .

Answer 2

Try typing everything again, in a new window. 'f' isn't supposed to be defined separately.

Answer 3

That error of invalid syntax seams to appear because the missing of a new line, and an unexpected indentation.

In the IDLE, after defining the function f(t) you have to press again <return> to finish the declaration of f(t) - putting an empty line after the declaration of f(t). If you don't add that additional new line in the IDLE, you would get a syntax error because the unexpected indentation, and the definition of f(t) wouldn't be finished, so you also get the next error that says 'f' is not defined.