Popular recipes tagged "map"

Classifying characters using nested conditional expressions (Python)

2017-04-27T21:26:00-07:00

Python recipe 580792 by Vasudev Ram (characters, classification, conditional_expressions, expressions, join, lambda, map).

Python has a feature called conditional expressions, similar to C's ternary operator. For example:

print n, 'is odd' if n % 2 == 1 else 'is even'

Here, the conditional expression is this part of the print statement above:

'is odd' if n % 2 == 1 else 'is even'

This expression evaluates to 'is odd' if the condition after the if keyword is True, and evaluates to 'is even' otherwise.

The Python Language Reference section for conditional expressions shows that they can be nested. This recipe shows that we can use nested conditional expressions (within a return statement in a user-defined function) to classify characters into lowercase letters, uppercase letters, or neither.

It also shows how to do the same task using map, lambda and string.join, again with a nested conditional expression, but without using return or a user-defined function.

Collection Pipeline in Python (Python)

2016-03-16T14:45:02-07:00

Python recipe 580625 by Steven D'Aprano (bash, filter, map, pipe, pipline).

A powerful functional programming technique is the use of pipelines of functions. If you have used shell scripting languages like bash, you will have used this technique. For instance, to count the number of files or directories, you might say: ls | wc -l. The output of the first command is piped into the input of the second, and the result returned.

We can set up a similar pipeline using Lisp-like Map, Filter and Reduce special functions. Unlike the standard Python map, filter and reduce, these are designed to operate in a pipeline, using the same | syntax used by bash and other shell scripting languages:

>>> data = range(1000)
>>> data | Filter(lambda n: 20 < n < 30) | Map(float) | List
[21.0, 22.0, 23.0, 24.0, 25.0, 26.0, 27.0, 28.0, 29.0]

The standard Python functional tools is much less attractive, as you have to write the functions in the opposite order to how they are applied to the data. This makes it harder to follow the logic of the expression.

>>> list(map(float, filter(lambda n: 20 < n < 30, data)))
[21.0, 22.0, 23.0, 24.0, 25.0, 26.0, 27.0, 28.0, 29.0]

We can also end the pipeline with a call to Reduce to collate the sequence into a single value. Here we take a string, extract all the digits, convert to ints, and multiply:

>>> from operator import mul
>>> "abcd12345xyz" | Filter(str.isdigit) | Map(int) | Reduce(mul)
120

An alternative way to draw parallels and meridians with basemap (Python)

2013-01-05T15:59:24-08:00

Python recipe 578399 by PG (layout, map, plots, plotting). Revision 2.

Basemap is a toolkit of matplotlib used to plot geographic maps.

With this function you can:

Draw the latitude/longitude grid easily in one line of code, specifying the lat/lon intervals.
Use rcParams, so all your figures will look more consistent.
Specify the label pad in points (instead of projection units).
Place the labels indicating which margins will be used.

Plotting maps with Polar Stereographic projection focused in a region with Basemap (Python)

2012-12-15T11:49:52-08:00

Python recipe 578379 by PG (map, matplotlib, plotting, projections, stereographic). Revision 3.

When plotting maps with either 'npstere' or 'spstere' projections with the Basemap toolkit for Matplotlib, the pole will always be placed in the center of the figure. With this function you can zoom the map to the area of interest by giving the latitude/longitudes coordinates of the bounds.

fmap(): a kind of inverse of the built-in Python map() function (Python)

2012-10-06T22:15:17-07:00

Python recipe 578281 by Vasudev Ram (composition, fmap, functional, map).

The Python map() function returns a list of the results of applying the function to the items of the argument sequence(s).

The fmap() function does the inverse, in a sense. It returns the result of applying a list of functions to a given argument.

Geocoding Lists via Google Maps (Python)

2012-05-11T05:06:27-07:00

Python recipe 578126 by Mano Bastardo (batch, coordinates, geocode, geocoding, google, google_maps, lat, latitude, list, list_comprehension, lng, longitude, map, web). Revision 2.

A simple script written as an experiment in geocoding addresses in a database. A list of addresses in the form of "100 Any Street, Anytown, CA, 10010" is passed to a Google Maps URL, and the latitude/longitude coordinates are extracted from the returned XML.

XML methods are not used in this script, but simple string searches instead.

Find function in a ram.map file (Python)

2011-12-23T21:58:01-08:00

Python recipe 577995 by Sharon (map, ram, vxworks).

The findCloseAddress() function receives an input parameter ram.map file "ramMapFile", and search inside it for the function with the closest address to the input parameter "reqAddress".

map_longest and map_strict (Python)

2011-05-06T17:35:17-07:00

Python recipe 577687 by Steven D'Aprano (iteration, map). Revision 2.

In Python 3, the map builtin silently drops any excess items in its input:

>>> a = [1, 2, 3]
>>> b = [2, 3, 4]
>>> c = [3, 4, 5, 6, 7]
>>> list(map(lambda x,y,z: x*y+z, a, b, c))
[5, 10, 17]

In Python 2, map pads the shorter items with None, while itertools.imap drops the excess items. Inspired by this, and by itertools.zip_longest, I have map_longest that takes a value to fill missing items with, and map_strict that raises an exception if a value is missing.

>>> list(map_longest(lambda x,y,z: x*y+z, a, b, c, fillvalue=0))
[5, 10, 17, 6, 7]
>>> list(map_strict(lambda x,y,z: x*y+z, a, b, c))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 9, in map_strict
ValueError: too few items in iterable

Planet Terrain Heightmap Generator (Python)

2010-03-10T16:35:37-08:00

Python recipe 576929 by Shea Kauffman (creation, generation, generator, heightmap, landscape, map, pil, planet, planted, pygame, terrain, world). Revision 9.

The process is simply: 1. Take a plane 2. Cut out a shape 3. Make it a little taller 4. Repeat

Similar to the spherical landscape algorithm by Hugo Elias.
I found a combination of Ovals and Triangles to produce the best results.

Parallel map (Python)

2009-04-02T19:41:10-07:00

Python recipe 576709 by Miki Tebeka (algorithm, map, parallel). Revision 3.

Implementation of parallel map (processes).

This is a Unix only version.

processing.Pool variation which allows multiple threads to send the same requests without incurring duplicate processing (Python)

2008-09-17T17:01:21-07:00

Python recipe 576462 by david decotigny (map, parallel, pool, processing, threads). Revision 3.

processing.Pool (http://pypi.python.org/pypi/processing) is a nice tool to "parallelize" map() on multiple CPUs. However, imagine you have X threads which send the same request Pool.map(getNthPrimeNumber, [100000, 10000000, 10000]) at (almost) the same time. Obviously, you don't want to compute X times getNthPrimeNumber for 100000, 10000000, 10000... unless you have 3.X processors available. You would like one thread to submit the 3 requests, and then the X-1 others would notice that the requests have already been submitted and will then just wait for the result. This is what this code is about: a kind of "trensient memoize" for processing.Pool::imap().