Python Common Operations

• Initialize integer as positive or negative infinity: {python} i: int = float('-inf'|'inf')
• Integer vs Float:
  • 10e9 is float, 10**9 is int

Collections §

Sets §

• Set difference{python} set(list1) - set(list2)
  • $O(n)$
• Set lookup {python}i in {1, 2, 3}
  • ==Constant time $O(1)$ lookup==

List §

• get index of var from list: {python}arr.index(val)
  • get all index of var from list: {python}indices = [i for i, x in enumerate(lst) if x == var]
• List Sort {python} list2 = sorted(list1)
  • $O(n\log n)$
• Deduplication{python} list1 = list(set(list1))
  • $O(n)$
• Counter {python}Counter("mississippi")
  • returns {python}{'i': 4, 's': 4, 'p': 2, 'm': 1}
  • $O(n)$
• Accumulate from itertools:{python} accumulate(list1)
  • $O(n)$
• Assign slice: {python}dp[:4] = [1, 1, 2, 5]
• Multidim ($m\times n$): `{python}arr = [[0]*n for _ in range(m)]
  • ! note the dims appear in reverse order

Dictionary (=Hashmap in python) §

• Default value for dictionary entries{python}dictionary = collections.defaultdict(list|int)
• Helps when you have a list of dicts, or accumulating a count in a dict.
• var in works for keys, not values:

d = {'a': 1, 'b': 2}
print('a' in d)   # True  → key check
print(1 in d)     # False → 1 is a value, not a k

Priority Queue §

from queue import PriorityQueue
pq = PriorityQueue()
for x in [5,2,8]:
    pq.put((-x, x))
    
_, max_val = pq.get() # pops the max item
 
pq.queue[0] # peek at the front of queue
 
print(max_val)

All Conditions True §

def is_increasing(lst):
    return all(earlier < later for earlier, later in zip(lst, lst[1:]))
 
# Example usage:
my_list = [1, 2, 3, 4, 5]
print(is_increasing(my_list))  # Output: True