Python Data Structures

Updated and presented by: Tracy Teal

Adapted from Software Carpentry materials http://software-carpentry.org/

Starting an iPython notebook Mac ~/anaconda/bin/ipython notebook –pylab=inline PC ./run-in-vm.sh

Python Lists

SWC Tutorial: http://software-carpentry.org/4_0/python/lists/

Collections let us store many values together

The most common way we do this is with a list

We create a list in Python with

listname = ['a', 'b', 'c', 'd']

example

gases = ['He', 'Ne', 'Ar', 'Kr']

The index for lists starts with 0 instead of 1, so the first item in a list is item 0

gases[0] would return 'He'
gases[1] would return 'Ne'

You can also get items from the end of the list

gases[-1] would return 'Kr'
gases[-2] would return 'Ar'

Use

len(listname)

to get the length of the list, or how many values are in the list.

You can also change a list after you make it. If you want to change, say Kr to K you can do

gases[3] = 'K'

Now your list will be: [‘He’, ‘Ne’, ‘Ar’, ‘K’]

Lists can store values of many kinds, even other lists

helium = ['He', 2]
neon = ['Ne', 8]
gases = [helium, neon]

Now if you want do something to every item in the list, you can use a loop

gases = ['He', 'Ne', 'Ar', 'Kr']
i = 0
while i < len(gases):
   print gases[i]
   i += 1

This will print out each of the gases.

A better way to do this would be to use a ‘for’ loop

for i in gases:
   print i

This will also print out each gas.

You can use if statements to see if something in the list is true

if 'Pu' in gases:
   print 'But plutonium is not a gas'
else:
   print 'The universe is well ordered'

Some useful list methods

You can append items to the list:

gases.append('H')

You can print out how many of something is in the list:

print gases.count('He')

You can print where the item is in the list:

print gases.index('Ne')

Dictionaries

SWC Tutorial: http://software-carpentry.org/4_0/setdict/dict/

Dictionaries have key, value pairs. Here is an example of a dictionary.

>>> tel = {'jack': 4098, 'sape': 4139}
>>> tel['guido'] = 4127
>>> tel
{'sape': 4139, 'guido': 4127, 'jack': 4098}
>>> tel['jack']
4098
>>> del tel['sape']
>>> tel['irv'] = 4127
>>> tel
{'guido': 4127, 'irv': 4127, 'jack': 4098}
>>> tel.keys()
['guido', 'irv', 'jack']
>>> 'guido' in tel
True

You can loop over items in a dictionary the same way you can over items in a list.

for keys in tel:
  print tel[keys]

If you want to mix some text in with your printing

for keys in tel:
  print 'This is the number', tel[keys]

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