Question

I’m stucked on the chapter 3.3 “Math functions” of “Think Python”.

It tells me to import math (through the interpreter).
Then print math and that I should get something like this:
<module 'math' from '/usr/lib/python2.5/lib-dynload/math.so'>

Instead I get <module 'math' <built-in>>

Anyway that’s not the problem. Though I wasn’t able to find a ‘math.so’ file in my python folder. The most similar file is named test_math.

The problem is that I’m supposed to write this:
>>> ratio = signal_power / noise_power

>>> decibels = 10 * math.log10(ratio)

>>> radians = 0.7

>>> height = math.sin(radians)

When I write the first line it tells me this:
Traceback <most recent call last>:
File "<stdin>", line 1, in <module>
NameError: name 'signal_power' is not defined

On the book says “The first example uses log10 to compute a signal-to-noise ratio in decibels (assuming that signal_power and noise_power are defined).”

So I assume that the problem might be that I didn’t defined ‘signal_power’, but I don’t know how to do it and what to assign to it…

This is the first time that I feel that this book is not holding my hand and I’m already lost. To be honest I don’t understand this whole chapter.

By the way, I’m using Python2.7 and Windows XP. I may copy and paste the whole chapter if anyone feels that I should do it.
Python is my first language and I already tried to learn it using “Learn Python the hard way” but got stucked on chapter 16. So I decided to use “Think Python” and then go back to “Learn Python the hard way”.

Answer 1

You indeed need to assign a value to both signal_power and noise_power. The author likely left them out because the values are arbitrary. Even when supplied with exact values in a text, you should also play around with the values. After all, there’s not much point in having you type anything in if the results on the screen are the same as on the page.

signal_power = 100
noise_power = 17

This particular example defines the mathematical relationship between variables. If it were presented as such, it wouldn’t be such a mystery that exact values were left out, as they would be supplied by someone when using the formula. The same holds for the code sample.