python_dictionaries.md

This file contains some tips and tricks for using Dictionaries easier

Python dictionaries are one of the easiest ways to express relationships in python.

What is a Dictionary?

A dictionary ocntains a key-value pair. For every key in a dictionary, there is a value. Anything within a {} is a dictionary and the keys and values are seperated using :. Each key-value pair in the dictionary is seperated by a ,.

Creating a Dictionary

d={'India':'Delhi', 'France':'Paris', 'Germany':'Berlin', 'Brazil':'Brasilia', 'Spain':'Madrid'}

If you have two lists and you want to create a dictionary out of them, then use zip (refer to python_loops.md)

countries=['India', 'France', 'Germany', 'Brazil', 'Spain']
cities=['Delhi', 'Paris', 'Berlin', 'Brasilia', 'Madrid']
d=dict(zip(countries, cities))

This means that you can also use enumerate to make a dict out of position of the list:

d=dict(enumerate(countries))

Iterating through a Dictionary

To get the keys of a Dictionary:

for k in d:
    print(k)

Note: Do NOT change the Dictionary values when Iterating through like this. The better way to do that is:

for k in d.keys():
    if len(k)>5:
        del d[k]

d.keys() creates a copy of the keys making it safe to mutate the original dictionary. The above snippet also shows how to delete a key value pair from the dictionary.

The above methods only return the key. To get the values, do this:

for k, v in d.items():
    print(f"Country {k} has capital {v}")

#Do Not Do this
for k in d:
    print(f"Country {k} has capital {d[k]}")

The second method will have to rehash the keys every time, thus making it slow.
items returns the whole list meaning it uses ore memory. iteritems returns an iterator to the items, so it occupies less space.

for k, v in d.iteritems():
    print(f"Country {k} has capital {v}")

Counting with Dictionaries

#The Easiest way to count