Popular recipes tagged "iteration"

Easily Write Nested Loops (Python)

2012-02-18T01:34:55-08:00

Python recipe 578046 by Stephen Chappell (iteration, nested).

The "nest" generator function in this module is provided to make writing nested loops easier to accomplish. Instead of writing a for loop at each level, one may call "nest" with each sequence as an argument and receive items from the sequences correctly yielded back to the caller. A test is included at the bottom of this module to demonstrate how to use the code.

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

Reversed Ranges (Python)

2014-03-29T12:43:26-07:00

Python recipe 576801 by Tal Einat (iteration, range, reverse). Revision 16.

A simple function for efficiently iterating over ranges in reverse.

This is equivalent to reversed(range(...)) but somewhat more efficient.