In this lesson, you will learn how to use dictionaries to accomplish certain tasks. 

Using Dictionaries

Sometimes we want to get a key and remove it from the dictionary. Imagine we were running a raffle, and we have this dictionary mapping ticket numbers to prizes:
```py
raffle = {223842: "Teddy Bear", 872921: "Concert Tickets", 320291: "Gift Basket", 412123: "Necklace", 298787: "Pasta Maker"}
```

When we get a ticket number, we want to return the prize and also remove that pair from the dictionary, since the prize has been given away. We can use `.pop()` to do this. Just like with `.get()`, we can provide a default value to return if the key does not exist in the dictionary:

```py
print(raffle.pop(320291, "No Prize"))
# Prints "Gift Basket"
print(raffle)
# Prints {223842: "Teddy Bear", 872921: "Concert Tickets", 412123: "Necklace", 298787: "Pasta Maker"}
print(raffle.pop(100000, "No Prize"))
# Prints "No Prize"
print(raffle)
# Prints {223842: "Teddy Bear", 872921: "Concert Tickets", 412123: "Necklace", 298787: "Pasta Maker"}
print(raffle.pop(872921, "No Prize"))
# Prints "Concert Tickets"
print(raffle)
# Prints {223842: "Teddy Bear", 412123: "Necklace", 298787: "Pasta Maker"}
```

`.pop()` works to delete items from a dictionary, when you know the key value.


Delete a Key

Once you have a dictionary, you can access the values in it by providing the key. For example, let's imagine we have a dictionary that maps buildings to their heights, in meters:

```py
building_heights = {"Burj Khalifa": 828, "Shanghai Tower": 632, "Abraj Al Bait": 601, "Ping An": 599, "Lotte World Tower": 554.5, "One World Trade": 541.3}
```
Then we can access the data in it like this:
```py
print(building_heights["Burj Khalifa"]) # Prints 828
print(building_heights["Ping An"]) # Prints 599
```


Get A Key

Sometimes we want to operate on all of the keys in a dictionary. For example, if we have a dictionary of students in a math class and their grades:
```py
test_scores = {"Grace":[80, 72, 90], "Jeffrey":[88, 68, 81], "Sylvia":[80, 82, 84], "Pedro":[98, 96, 95], "Martin":[78, 80, 78], "Dina":[64, 60, 75]}
``` 

We want to get a roster of the students in the class, without including their grades. We can do this with the built-in `list()` function:
```py
print(list(test_scores))
# Prints ["Grace", "Jeffrey", "Sylvia", "Pedro", "Martin", "Dina"]
```

Dictionaries also have a `.keys()` method that returns a `dict_keys` object. A `dict_keys` object is a _view_ object, which provides a look at the current state of the dictionary, without the user being able to modify anything. The `dict_keys` object returned by `.keys()` is a set of the keys in the dictionary. You cannot add or remove elements from a `dict_keys` object, but it can be used in the place of a list for iteration:

 ```py
for student in test_scores.keys():
  print(student)
```
will yield:
```
Grace
Jeffrey
Sylvia
Pedro
Martin
Dina
```


Get All Keys

We saw in the last exercise that we had to add a key:value pair to a dictionary in order to avoid a KeyError. This solution is not sustainable. We can't predict every key a user may call and add all of those placeholder values to our dictionary! 

Dictionaries have a `.get()` method to search for a value instead of the `my_dict[key]` notation we have been using. If the key you are trying to `.get()` does not exist, it will return `None` by default:

```py
building_heights = {"Burj Khalifa": 828, "Shanghai Tower": 632, "Abraj Al Bait": 601, "Ping An": 599, "Lotte World Tower": 554.5, "One World Trade": 541.3}

#this line will return 632:
building_heights.get("Shanghai Tower")

#this line will return None:
building_heights.get("My House")
```

You can also specify a value to return if the key doesn't exist. For example, we might want to return a building height of 0 if our desired building is not in the dictionary:

```py
print(building_heights.get('Shanghai Tower', 0)) # Prints 632
print(building_heights.get('Mt Olympus', 0)) # Prints 0
print(building_heights.get('Kilimanjaro', 'No Value')) # Prints 'No Value'
```


Safely Get a Key

You can get both the keys and the values with the `.items()` method. Like `.keys()` and `.values()`, it returns a `dict_list` object. Each element of the `dict_list` returned by `.items()` is a tuple consisting of:
```py
(key, value)
```
so to iterate through, you can use this syntax:
 ```py
biggest_brands = {"Apple": 184, "Google": 141.7, "Microsoft": 80, "Coca-Cola": 69.7, "Amazon": 64.8}

for company, value in biggest_brands.items():
  print(company + " has a value of " + str(value) + " billion dollars. ")
```

which would yield this output:
```
Apple has a value of 184 billion dollars.
Google has a value of 141.7 billion dollars.
Microsoft has a value of 80 billion dollars.
Coca-Cola has a value of 69.7 billion dollars.
Amazon has a value of 64.8 billion dollars.
```


Get All Items

Let's say we have our dictionary of building heights from the last exercise:
```py
building_heights = {"Burj Khalifa": 828, "Shanghai Tower": 632, "Abraj Al Bait": 601, "Ping An": 599, "Lotte World Tower": 554.5, "One World Trade": 541.3}
```

What if we wanted to know the height of the Landmark 81 in Ho Chi Minh City? We could try:
```py
print(building_heights["Landmark 81"])
```
But `"Landmark 81"` does not exist as a key in the `building_heights` dictionary! So this will throw a KeyError:
```
KeyError: 'Landmark 81'
```

One way to avoid this error is to first check if the key exists in the dictionary:
```py
key_to_check = "Landmark 81"

if key_to_check in building_heights:
  print(building_heights["Landmark 81"])
```

This will not throw an error, because `key_to_check in building_heights` will return `False`, and so we never try to access the key.



Get an Invalid Key

Dictionaries have a `.values()` method that returns a `dict_values` object (just like a `dict_keys` object but for values!) with all of the values in the dictionary. It can be used in the place of a list for iteration:

 ```py
test_scores = {"Grace":[80, 72, 90], "Jeffrey":[88, 68, 81], "Sylvia":[80, 82, 84], "Pedro":[98, 96, 95], "Martin":[78, 80, 78], "Dina":[64, 60, 75]}

for score_list in test_scores.values():
  print(score_list)
```
will yield:
```
[80, 72, 90]
[88, 68, 81]
[80, 82, 84]
[98, 96, 95]
[78, 80, 78]
[64, 60, 75]
```

There is no built-in function to get all of the values as a list, but if you _really_ want to, you can use:
```py
list(test_scores.values())
```
However, for most purposes, the `dict_values` object will act the way you want a list to act. 


Get All Values

Now that we know how to create a dictionary, we can start using already created dictionaries to solve problems.

In this lesson, you'll learn how to:
- Use a key to get a value from a dictionary
- Check for existence of keys
- Iterate through keys and values in dictionaries



In this lesson, you've learned how to go through dictionaries and access keys and values in different ways. Specifically, you have seen how to:

- Use a key to get a value from a dictionary
- Check for existence of keys
- Remove a key: value pair from a dictionary
- Iterate through keys and values in dictionaries
