Category Archives: convenience

Introducing Absolute Ratio

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Let’s define the absolute ratio for positive numbers:

abs_ratio(x) = 1 / x when x < 1, otherwise: x

When x is smaller than 1 return 1 / x, otherwise return x. Here are a few example values:

x abs_ratio(x)
0.5 2
2 2
0.2 5
5 5

And a graph:

Absolute Ratio Graph

Another spelling for the same operator would take 2 positive numbers and give their absolute ratio:

And a graph:

Absolute ratio in 3D

Use case examples

  • Music and audio – an octave of a frequency F is 2F. More generally a harmony of a frequency F is N*F where N is a natural number. To decide if one frequency is a harmony of another we just need to get their absolute ratio and see if it’s whole. E.g. if abs_ratio(F1, F2) == 2 they’re octaves. If abs_ratio(F1, F2) is whole – they’re harmonies.
  • Computer vision – to match shapes that have similar dimensions e.g. their width is only 10% larger or smaller. We don’t care which is the bigger or smaller, we just want to know if 0.91 < W1 / W2 < 1.1 which may be easier to pronounce as abs_ratio(W1, W2) < 1.1
  • Real life – when we see 2 comparable objects we’re more likely to say one is “three times the other” vs “one third the other”. Either way in our brains both statements mean the same concept. We think in absolute ratios.
  • General case – When you want to know if X is K times bigger than Y or vice versa and you don’t care which is the bigger one.

Interesting Properties

  • abs_ratio(Y / X) == abs_ratio(X / Y)
  • log(abs_ratio(X)) = abs(log(X))
  • log(abs_ratio(Y / X)) = abs(log(Y / X)) = abs(log(Y) – log(X))
  • You can see from the above that absolute ratio is somewhat of an absolute value for log-space.

What’s next for absolute ratio

  • I’d love to hear more use cases and relevant contexts.
  • What would be the written symbol or notation?
  • How can we get this operator famous enough to be of use to mainstream minds?
  • About negative numbers and zero – right now that’s undefined as I don’t see a use case for that domain.
  • For some code and graphs in python checkout https://github.com/ubershmekel/abs_ratio

EDIT – I’m growing to like the binary form of the operator more so from now on let’s call it like this in python:

def abs_ratio(a, b):
    return a / b if a > b else b / a

A new module – import_file

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So I had this python web server and I wanted to write a script that does some maintenance. The problem was, if the maintenance script isn’t part of the “__init__.py” package tree, it couldn’t use any of the web server modules. A hacky way to get around this is to add the target module’s directory to the path:

    import sys
    sys.path.append('/usr/wherever_that_module_is')
    import your_module

Another trick is using os.chdir. Even when importing modules from the same package things can become confusing as can be learned from PEP 328, PEP 366, an abundance of stackoverflow questions on the subject and many more. I’d like to quote The Zen of Python:

    Simple is better than complex.
    There should be one-- and preferably only one --obvious way to do it.
    If the implementation is hard to explain, it's a bad idea.

I don’t believe any of these can be said about python imports, at least not for anything past the trivial case of one-folder-with-all-the-modules. The moment you want to organize your project in folders it becomes complex if not complicated and unnatural.

“import math” just works and that’s great, I just wished there was an equivalent to the banal:

#include "path/to/module"

From those inferior languages. So I wrote import_file which can be used like this:

    >>>from import_file import import_file
    >>>mylib = import_file('c:\\mylib.py')
    >>>another = import_file('relative_subdir/another.py')

It’s very similar to the imp module syntax, except the function requires one argument less. This is the way it should be imo. Enjoy import_file at google code, from the cheese shop, “easy_install import_file” or pip, etc.

Filecache

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Working on a project that needed some web scraping, I made filecache. I just wanted a simple decorator that would save the results of a function to disk. Memcache and other recipes seemed to be either overly complex or just plain memory caching (without persistence). If you want to help improve it then check out the google code filecache project.