Alg-Code: Daily Notes

Friday, February 25, 2011

Daily Notes - Python

# The "collections" and "itertools" module in Python:

Checkout an example:

http://stackoverflow.com/questions/4980414/python-dividing-a-list-of-lists-to-groups

Python-org Introduction for Collections: http://docs.python.org/library/collections.html

Python Itertool and Generator: http://heather.cs.ucdavis.edu/~matloff/Python/PyIterGen.pdf

# Besids, a very good example for using the Regex for "hashing"

<http://stackoverflow.com/questions/260056/hashtable-dictionary-map-lookup-with-regular-expressions>

  >>> regex_dict = { re.compile(r'foo.') : 12, re.compile(r'^FileN.*$') : 35 }
  >>> regex_dict['food']
  12
  >>> regex_dict['foot in my mouth']
  12
  >>> regex_dict['FileNotFoundException: file.x does not exist']
  35

# Sort for dictionary tuples --- excellent example:

down vote favorit

I've got a python list of dictionaries:

  mylist = [
  {'id':0, 'weight':10, 'factor':1, 'meta':'ABC'},
  {'id':1, 'weight':5, 'factor':1, 'meta':'ABC'},
  {'id':2, 'weight':5, 'factor':2, 'meta':'ABC'},
  {'id':3, 'weight':1, 'factor':1, 'meta':'ABC'}
  ]

Whats the most efficient/cleanest way to order that list by weight then factor (numericaly). The resulting list should look like:

  mylist = [
  {'id':3, 'weight':1, 'factor':1, 'meta':'ABC'},
  {'id':1, 'weight':5, 'factor':1, 'meta':'ABC'},
  {'id':2, 'weight':5, 'factor':2, 'meta':'ABC'},
  {'id':0, 'weight':10, 'factor':1, 'meta':'ABC'},

  mylist.sort(key=lambda d: (d['weight'], d['factor']))     # note: this is using the key- corresponding value to sort the dic

  import operator
  mylist.sort(key=operator.itemgetter('weight', 'factor'))

Below will work as well:

  def cmp_dict(x, y):
      weight_diff = y['weight'] - x['weight']
      if weight_diff == 0:
          return y['factor'] - x['factor']
      else:
          return weight_diff
  
myList.sort(cmp_dict)