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Python ​

Arrays ​

Creating ​

python
a = []
a = [1,2,3]
a = list(range(1,11)) # 1 ... 10
a = [x for x in [3, 4, 5, 6, 7] if x > 5]
a = [0] * 3 # [0,0,0]

Accessing ​

python
a = [1,2,3]
print(a[0]) # 1
print(a[-1]) # 3

Manipulating ​

Adding ​

python
a = []
a.append(1) 
a.extend([9, 11, 13])
a += [6,7,8]
a.insert(0, 2) # Insert 2 at index 0

Removing ​

python
li = ['bread', 'butter', 'eggs', 'milk']
li.pop() # returns and removes 'milk'
li.pop(2) # returns and removes 'eggs'
del li[0] # removes 'bread'
li.remove('butter')

Slicing ​

python
# a_list[start:end]
# a_list[start:end:step]
a = ['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']
a[2:5] # ['bacon', 'tomato', 'ham']
a[-3:] # ['tomato', 'ham', 'lobster']
a[-5:-2] # ['egg', 'bacon', 'tomato']
a[:4] # ['spam', 'egg', 'bacon', 'tomato']
a[2:] # ['bacon', 'tomato', 'ham', 'lobster']
a[0:6:2] # ['spam', 'bacon', 'ham']
a[6:0:-2] # ['lobster', 'tomato', 'egg']
a[::-1] # ['lobster', 'ham', 'tomato', 'bacon', 'egg', 'spam']
a[:] # Create a copy of an array

Sorting ​

python
li = [3, 1, 3, 2, 5]
li.sort() # [1, 2, 3, 3, 5]
li.reverse() # [5, 3, 3, 2, 1]
sorted(li) # returns a sorted copy of li

li = [
    ('john', 'A', 15),
    ('jane', 'B', 12),
    ('dave', 'B', 10)
]
sorted(li, key=lambda x: x[2])
sorted(li, cmp=lambda x,y: 1 if x<y else -1) # Negative return means second number is "less than"

Filtering ​

python
a = list(filter(lambda x : x % 2 == 1, range(1, 20)))

Strings ​

Strings act a lot like arrays/lists in Python, so many of the methods still apply.

Helper Functions ​

python
s = 'spam'
s in 'I saw spamalot!' # True
s not in 'I saw The Holy Grail!' # True

s = '12345' * 5 # '1234512345123451234512345'

"#".join(["John", "Peter", "Vicky"]) # 'John#Peter#Vicky'
"Hello, world!".endswith("!") # True
"Hello, world!".startswith("H") # True

Dictionaries ​

Creating ​

python
d = {}
d = {'color': 'green', 'points', 5}

Accessing ​

python
d['color']
d.get('points') # Returns None or value of points
d.get('points', 0) # Default value

d['age'] = 30

Removing ​

python
del d['age']

Iterating ​

python
for key, value in d.items():
    # do something

for key in d.keys():
    # do something

for value in d.values():
    # do something

Zipping ​

python
group_1 = ['kai', 'abe', 'ada', 'gus', 'zoe']
group_2 = ['jen', 'eva', 'dan', 'isa', 'meg']
pairings = {name:name_2 for name, name_2 in zip(group_1, group_2)}
# {'kai': 'jen', 'abe': 'eva', 'ada': 'dan', 'gus': 'isa', 'zoe': 'meg'}

Sorting ​

By Key ​

python
my_dict = {'b': 2, 'a': 1, 'c': 3}
sorted_dict_by_keys = sorted(my_dict.items())
# [('a', 1), ('b', 2), ('c', 3)]

By Value ​

dict.items() returns a tuple of the key value pairs for each iteration, so we can use sorted and use the value of the pair to return a list of the sorted pairs.

python
my_dict = {'b': 2, 'a': 1, 'c': 3}
sorted_dict_by_values = sorted(my_dict.items(), key=lambda item: item[1])
# [('a', 1), ('b', 2), ('c', 3)]

OrderedDict ​

OrderedDicts preserve the order in which the items were added, whereas normal dicts do not guarantee the order upon iteration. So you can sort the dictionary by key or value, and then pass that into OrderedDict to have a dict-like structure that preserves the inserted order.

python
from collections import OrderedDict

my_dict = {'b': 2, 'a': 4, 'c': 3}
sorted_dict_by_keys = OrderedDict(sorted(my_dict.items()))
for key, value in sorted_dict_by_keys:
    # do something

Heap ​

heapq is a module that implements a heap structure on a list of items. It's helpful for finding kth largest/smallest elements and priority queues.

python
import heapq

li = [10, 20, 15, 30, 40]

heapq.heapify(li)
# [10, 20, 15, 30, 40]

# Appending an element
heapq.heappush(h, 5)

# Pop the smallest element from the heap
min = heapq.heappop(h)

# Push a new element (5), then pop the smallest element and return
min = heapq.heappushpop(h, 5)

# Pop the smallest element, and then push new element (5)
min = heapq.heapreplace(h, 5) 

# Find the 3 largest elements
maxi = heapq.nlargest(3, h)
# [40, 30, 20]

# Find the 3 smallest elements
min = heapq.nsmallest(3, h)
# [10, 15, 20]

# Merge heaps
h3 = list(heapq.merge(h1, h2))