How to use dict in python?

Question

10
5
-1
-1
-1
1
1
0
2
...

If I want to count the number of occurrences of each number in a file, how do I use python to do it?

Answer 1

1. Use collections.defaultdict so that by deafult count for anything is zero
2. After that loop thru lines in file using file.readline and convert each line to int
3. increment counter for each value in your countDict
4. at last go thru dict using for intV, count in countDict.iteritems() and print values

Answer 2

Use dictionary where every line is a key, and count is value. Increment count for every line, and if there is no dictionary entry for line initialize it with 1 in except clause -- this should work with older versions of Python.

def count_same_lines(fname):
    line_counts = {}
    for l in file(fname):
     l = l.rstrip()
     if l:
      try:
       line_counts[l] += 1
      except KeyError:
       line_counts[l] = 1
    print('cnt\ttxt')
    for k in line_counts.keys():
     print('%d\t%s' % (line_counts[k], k))

Answer 3

Read the lines of the file into a list l, e.g.:

l = [int(line) for line in open('filename','r')]

Starting with a list of values l, you can create a dictionary d that gives you for each value in the list the number of occurrences like this:

>>> l = [10,5,-1,-1,-1,1,1,0,2]
>>> d = dict((x,l.count(x)) for x in l)
>>> d[1]
2

EDIT: as Matthew rightly points out, this is hardly optimal. Here is a version using defaultdict:

from collections import defaultdict
d = defaultdict(int)
for line in open('filename','r'):
    d[int(line)] += 1

Answer 4

I think what you call map is, in python, a dictionary.
Here is some useful link on how to use it: http://docs.python.org/tutorial/datastructures.html#dictionaries

For a good solution, see the answer from Stephan or Matthew - but take also some time to understand what that code does :-)

Answer 5

l = [10,5,-1,-1,-1,1,1,0,2]
d = {}
for x in l:
  d[x] = (d[x] + 1) if (x in d) else 1

There will be a key in d for every distinct value in the original list, and the values of d will be the number of occurrences.

Answer 6

This is almost the exact same algorithm described in Anurag Uniyal's answer, except using the file as an iterator instead of readline():

from collections import defaultdict
try:
  from io import StringIO # 2.6+, 3.x
except ImportError:
  from StringIO import StringIO # 2.5

data = defaultdict(int)

#with open("filename", "r") as f: # if a real file
with StringIO("10\n5\n-1\n-1\n-1\n1\n1\n0\n2") as f:
  for line in f:
    data[int(line)] += 1

for number, count in data.iteritems():
  print number, "was found", count, "times"

Answer 7

Counter is your best friend:)
http://docs.python.org/dev/library/collections.html#counter-objects

for(Python2.5 and 2.6) http://code.activestate.com/recipes/576611/

>>> cnt = Counter()
>>> for word in ['red', 'blue', 'red', 'green', 'blue', 'blue']:
...     cnt[word] += 1
>>> cnt
Counter({'blue': 3, 'red': 2, 'green': 1})

for this :

print Counter(int(line.strip()) for line in open("foo.txt", "rb"))
##output
Counter({-1: 3, 1: 2, 0: 1, 2: 1, 5: 1, 10: 1})

Answer 8

counter.py

#!/usr/bin/env python
import fileinput
from collections import defaultdict

frequencies = defaultdict(int)
for line in fileinput.input():
    frequencies[line.strip()] += 1

print frequencies

Example:

$ perl -E'say 1*(rand() < 0.5) for (1..100)' | python counter.py
defaultdict(<type 'int'>, {'1': 52, '0': 48})

Answer 9

New in Python 3.1:

from collections import Counter
with open("filename","r") as lines:
    print(Counter(lines))