Python Dictionary - Implementation

# Python Dictionaries
## Introduction to Python Dictionaries
## What is a dictionary? A dictionary is one of the main data structure (structures we use to store data e.g list, sets, tuple etc) we use in python which is a lot like a list, with one major difference: Items in a **list** are accessed using **numbers.**

mylist[0]

Items in a **dictionary** are accessed using strings of **text.**

mydict["word"]

In other words, a dictionary is a data structure in which each piece of data (a *value*) is associated with a word (a *key*). Let's look at an example:

profile = {
			'name': 'Hootie',
			'occupation': '90s icon',
			'location': 'VH1'	
		  }

In the above example, `profile` is a dictionary. The first *key* is 'name', and it is associated with a *value*: 'Hootie'. You could access each part of the profile like this:

print profile['name']
# the result is: 'Hootie'

print profile['occupation']
# the result is: '90s icon'

print profile['location']
# the result is: 'VH1'

Therefore, a dictionary is often a more descriptive data structure than a list.
## Creating a dictionary You can write a dictionary by hand, using curly brackets. This creates an empty dictionary called `spanish`:

spanish = {}

This creates a dictionary that could translate english to spanish:

spanish = {
           'hello': 'hola'
		  }

Just like you would use a word to look up a definition in a real-life dictionary, in this example we could use the word 'hello' to look up its equivalent in spanish. 'hello' is the key which we use to look up the value that we're after. We can add more items to our dictionary, separating them by commas:

spanish = {
           'hello': 'hola',
           'bye': 'adios',
           'what is up?': 'que pasa?',
           'niice': 'que rico'
		  }

Notice that there is no comma after the last item in the list.
## Accessing items in a dictionary We can access the data in our dictionary by writing the name of our dictionary, `spanish`, followed by square brackets containing the word we wish to look up:

print spanish['hello']
# the result is: 'hola'

Again this is very similar to the way we looked up items in a list...

print mylist[0]

...except now we are using a word instead of a number.
## Looping through a dictionary Just like you can loop through a list to access each item in a list, you can loop through a dictionary to access each item, if you need to. The code for looping through the dictionary `spanish` is:

for key in spanish:
	print spanish[key]
	
# the result would be...
'hola'
'adios'
'que pasa?'
'que rico'

Each time through the loop, 'key' will be equal to one of the keys in our dictionary. (The key is the word on the lefthand side of our dictionary -- in our example, the keys are the english words.) The first time through the loop, `key` will be equal to the string 'hello'. The second time through the loop, `key` will be equal to the string 'bye'. And so on. So, the computer understands to execute the following code:

print spanish['hello']
print spanish['bye']
print spanish['what is up?']
print spanish['niice']
	
# the result would be...
'hola'
'adios'
'que pasa?'
'que rico'

## More operations: ### Editing items in a dictionary We can edit the values in our dictionary:

spanish['hello'] = "diga" 

print spanish["hello"]
# the result is: 'diga'

### Adding items to the dictionary Or we can add new items to our dictionary

spanish['where'] = 'donde'

### Is the word ____ in my dictionary? Sometimes, we might to check to see if a certain key exists in our dictionary. Python makes this easy:

if "hello" in spanish:
	print "Yes, I know how to say hello in Spanish"
else:
	print "No, I don't know how to say hello in Spanish."

In our spanish dictionary, "hello" does exist as a key, so this code would reply: ` "Yes, I know how to say hello in Spanish" `
# Dictionaries in practice Let's look at an example of how a dictionary might be used in practice. Let's imagine a census of the United States. In a census, the government gathers information about each citizen, and combines it into a large data set. The information about each person could be modeled as a dictionary (as we did in our first example):

Person = {
			name: "Hootie",
			occupation: "90s icon",
			location: "VH1"	
		  }

But, of course, in the US, there's not just one person, there are many people! The census data, then, would probably be stored as a **list** of all the people in the country. Each item in the list would be a dictionary describing one person. This is how these datatypes can combine to create useful models of the world. Let's first imagine what our list would look like:

us_census = [ 
				person1, 
				person2, 
				person3
			]

But, we need to replace each person with a dictionary describing them --

us_census = [
				{
					name: "Hootie",
					occupation: "90s icon",
					location: "VH1"	
		  		},
				{
					name: "Alanis Morissette",
					occupation: "90s icon",
					location: "MTV"	
		  		},
				{
					name: "Axl Rose",
					occupation: "90s icon",
					location: "Where are they now?"	
		  		}
			]

OK, we have a census of 3 people (strangely, all located on media waves...). To access values now will involve a bit more navigation! Let's construct the code piece by piece. First we need to choose which person we want to get information about -- meaning which item in the `us_census` list we want to access. Let's choose the first person in the list:

us_census[0]

This is familiar to you already -- we are accessing an item in a list using its index number. Then we can access data about that person:

us_census[0]['name']

This would return the name of the first person in our census.

print us_census[0]['name']
# the result is: 'Hootie'

### Why learn to combine dictionaries and lists? This is the way that the data of the world is stored! As we start our data visualizations, we'll look online for sources of data. We may use climate data, sports data, or political data, but most of the data we'll find will be a combination of lists and dictionaries -- often many levels deep.