Some basic data structures in python

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Matlab post

We often have a need to organize data into structures when solving problems.

1 the list

A list in python is data separated by commas in square brackets. Here, we might store the following data in a variable to describe the Antoine coefficients for benzene and the range they are relevant for [Tmin Tmax]. Lists are flexible, you can put anything in them, including other lists. We access the elements of the list by indexing:

c = ['benzene', 6.9056, 1211.0, 220.79, [-16, 104]]
print c[0]
print c[-1]

a,b = c[0:2]
print a,b

name, A, B, C, Trange = c
print Trange
benzene
[-16, 104]
benzene 6.9056
[-16, 104]

Lists are “mutable”, which means you can change their values.

a = [3, 4, 5, [7, 8], 'cat']
print a[0], a[-1]
a[-1] = 'dog'
print a
3 cat
>>> [3, 4, 5, [7, 8], 'dog']

2 tuples

Tuples are immutable; you cannot change their values. This is handy in cases where it is an error to change the value. A tuple is like a list but it is enclosed in parentheses.

a = (3, 4, 5, [7, 8], 'cat')
print a[0], a[-1]
a[-1] = 'dog'
3 cat
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment

3 struct

Python does not exactly have the same thing as a struct in Matlab. You can achieve something like it by defining an empty class and then defining attributes of the class. You can check if an object has a particular attribute using hasattr.

class Antoine:
    pass

a = Antoine()
a.name = 'benzene'
a.Trange = [-16, 104]

print a.name
print hasattr(a, 'Trange')
print hasattr(a, 'A')
benzene
True
False

4 dictionaries

The analog of the containers.Map in Matlab is the dictionary in python. Dictionaries are enclosed in curly brackets, and are composed of key:value pairs.

s = {'name':'benzene',
     'A':6.9056,
     'B':1211.0}

s['C'] = 220.79
s['Trange'] = [-16, 104]

print s
print s['Trange']
{'A': 6.9056, 'C': 220.79, 'B': 1211.0, 'name': 'benzene', 'Trange': [-16, 104]}
[-16, 104]
s = {'name':'benzene',
     'A':6.9056,
     'B':1211.0}

print 'C' in s
# default value for keys not in the dictionary
print s.get('C', None)

print s.keys()
print s.values()
False
None
['A', 'B', 'name']
[6.9056, 1211.0, 'benzene']

5 Summary

We have examined four data structures in python. Note that none of these types are arrays/vectors with defined mathematical operations. For those, you need to consider numpy.array.

Copyright (C) 2013 by John Kitchin. See the License for information about copying.

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