# Homework 4 Solutions

## Lambdas

### Question 1: Operation-inator

Dr. Doofenshmirtz is making a secret evil device that creates functions that do basic arithmetic operations using lambda functions. The operation-inator takes in an `operation_string`

and returns a function that does that specific operation. The strings map to the functions like so:

`"self"`

-> Return a function that takes an input and returns the exact same input without modifications`"add one"`

-> Return a function that takes in a number and returns the number + 1`"multiply together"`

-> Return a function that takes in 2 numbers and returns their product`"zero to self"`

-> Return a function that takes in a positive integer as input and returns a list of numbers that counts up by 1 from zero (inclusive) to the input (exclusive)

Help the good doctor implement his operation-inator by filling in the blanks with lambda functions.

See the doctests for more details.

```
def operation_inator(operation_string):
"""
>>> identity = operation_inator('self')
>>> identity(5)
5
>>> identity(6)
6
>>> add_one = operation_inator('add one')
>>> add_one(2)
3
>>> add_one(3)
4
>>> mul_together = operation_inator('multiply together')
>>> mul_together(0, 1)
0
>>> mul_together(3, 2)
6
>>> zero_to_self = operation_inator('zero to self')
>>> zero_to_self(3)
[0, 1, 2]
>>> zero_to_self(1)
[0]
"""
if operation_string == 'self':
return lambda x: x
elif operation_string == 'add one':
return lambda x: x + 1
elif operation_string == 'multiply together':
return lambda x, y: x * y
elif operation_string == 'zero to self':
return lambda x: list(range(x))
```

Use OK to test your code:

`python3 ok -q operation_inator`

### Question 2: Higher Order Lambdas

Return a lambda function that takes in a multiplier (the multiplier is a number) and returns a lambda function that will take in another number and will return the new input multiplied by the multiplier.

```
def higher_order_lambdas():
"""
Return a lambda function that takes in a multiplier and returns a lambda function that given an input will
return the input multiplied by the multiplier
>>> hol = higher_order_lambdas()
>>> doubles = hol(2)
>>> doubles(3)
6
>>> hol = higher_order_lambdas()
>>> triples = hol(3)
>>> triples(4)
12
"""
return lambda m : lambda n : m * n
```

Use OK to test your code:

`python3 ok -q higher_order_lambdas`

### Question 3: Lambdas and Currying

We can transform multiple-argument functions into a chain of single-argument, higher order functions by taking advantage of lambda expressions. This is useful when dealing with functions that take only single-argument functions. We will see some examples of these later on.

Write a function `lambda_curry2`

that will curry any two argument
function `f2`

using lambdas. See the doctest if you're not sure what this
means.

```
def lambda_curry2(f2):
"""
Returns a Curried version of a two argument function func.
>>> from operator import add, mul
>>> x = lambda_curry2(add)
>>> y = x(3)
>>> y(5)
8
>>> a = lambda_curry2(mul)
>>> b = a(3)
>>> b(5)
15
"""
return lambda arg1: lambda arg2: f2(arg1, arg2)
```

Use OK to test your code:

`python3 ok -q lambda_curry2`

## Dictionaries

### Question 4: Replace All

Given a dictionary `d`

, return a new dictionary where all occurences of `x`

as a value (not a key) is replaced with `y`

.

```
def replace_all(d, x, y):
"""
>>> d = {'foo': 2, 'bar': 3, 'garply': 3, 'xyzzy': 99}
>>> e = replace_all(d, 3, 'poof')
>>> e == {'foo': 2, 'bar': 'poof', 'garply': 'poof', 'xyzzy': 99}
True
"""
new = {}
for key in d:
if d[key] == x:
new[key] = y
else:
new[key] = d[key]
return new
```

Use OK to test your code:

`python3 ok -q replace_all`

### Question 5: Lets Listen To...

Implement the function `music_dict`

which takes in a dictionary where the keys are song titles and the value is the artist's name. The function `music_dict`

returns a dictionary where each key is now the artist's name and the values are a list of all of the songs by that artist.

```
def music_dict(songs_dict):
"""
Returns a dictionary where each key is an artist name and the
value is a list of all of the songs by that artist.
>>> songs = {"Good Days": "SZA", "Karma": "Taylor Swift", "22": "Taylor Swift", "Snooze": "SZA", "vampire": "Olivia Rodrigo"}
>>> music_dict(songs)
{'SZA': ['Good Days', 'Snooze'], 'Taylor Swift': ['Karma', '22'], 'Olivia Rodrigo': ['vampire']}
"""
artist_songs = {}
for song, artist in songs_dict.items():
if artist in artist_songs:
artist_songs[artist].append(song)
else:
artist_songs[artist] = [song]
return artist_songs
```

Use OK to test your code:

`python3 ok -q music_dict`

### Question 6: Merge Dictionaries

Implement the function `merge_dict`

. The `merge_dict`

function merges two dictionaries with the same keys together
by adding up their values for the corresponding keys and returning the resulting dictionary.

```
def merge_dict(dict1, dict2):
"""Returns a dictionary that is the result of two dictionaries being merged together.
Dictionaries are merged by adding up their values. You can assume that the same keys
appear in both dictionaries.
>>> data8 = {"midterms":1, "projects":3}
>>> data100 = {"midterms":2, "projects":3}
>>> combined_exams = merge_dict(data8, data100)
>>> combined_exams
{'midterms': 3, 'projects': 6}
>>> sunday_orders = {"pizza": 3, "hot dogs": 2, "fries": 5}
>>> monday_orders = {"pizza": 1, "hot dogs": 1, "fries": 8}
>>> combined_orders = merge_dict(sunday_orders, monday_orders)
>>> combined_orders
{'pizza': 4, 'hot dogs': 3, 'fries': 13}
"""
result_dict = {}
for work in dict1:
result_dict[work] = dict1[work] + dict2[work]
return result_dict
```

Use OK to test your code:

`python3 ok -q merge_dict`