Python3

Question 1

Give an example of type-hinting

Answer 1

def some_func(nums: List(int), val: int) -> int:

type hinting, afaik, is only for method params and returns, not variables

Question 2

1-liner to iterate over a list (actually, any iterable like list, tuple, string), with both an index and value

Answer 2

for idx, val in enumerate(iterable):

Fun: for idx, val in enumerate(iterable, start = 2):
THIS WILL CHOOSE THE INDEX TO START AT, NOT THE ITEM TO START AT (e.g. if you choose 1, the result will be 1-indexed)

Question 3

For debugging, use this trick to see everything in an iterator

Answer 3

print(list(iterator))

Question 4

How would you use the python repl to see if a type is an iterator?

Answer 4

if we want to see if enumerate(nums) is an iterator, we need to see if its type has the ‘__iter__’ and ‘__next__’ method.

hasattr(enumerate(nums), ‘__iter__’) and hasattr(enumerate(nums), ‘__next__’) == True

Question 5

How do you filter a list?

Answer 5

filtered_list = list(filter(lambda : somefunc(), your_list))

make sure to do list(filter()) lest you wind up with a “filter object” which seems to be a one time iterable (so if you iterate over it again, you’re gonna have a problem)

Question 6

python3 get the max int

Answer 6

import sys

sys.maxsize

Question 7

turn a string into a list

Answer 7

list(some_str)

Question 8

type hinting for a list

Answer 8

from typing import List

def some_func(strs: List[str]) -> List[str]:

Question 9

construct a list using a list comprehension

Answer 9

https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions

List comprehensions provide a concise way to create lists. Common applications are to make new lists where each element is the result of some operations applied to each member of another sequence or iterable, or to create a subsequence of those elements that satisfy a certain condition.

For example, assume we want to create a list of squares, like:

>>>
>>> squares = []
>>> for x in range(10):
...     squares.append(x**2)
...
>>> squares
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
Note that this creates (or overwrites) a variable named x that still exists after the loop completes. We can calculate the list of squares without any side effects using:

squares = list(map(lambda x: x**2, range(10)))
or, equivalently:

squares = [x**2 for x in range(10)]
which is more concise and readable.

A list comprehension consists of brackets containing an expression followed by a for clause, then zero or more for or if clauses. The result will be a new list resulting from evaluating the expression in the context of the for and if clauses which follow it. For example, this listcomp combines the elements of two lists if they are not equal:

> > > [(x, y) for x in [1,2,3] for y in [3,1,4] if x != y]
[(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
and it’s equivalent to:

>>>
>>> combs = []
>>> for x in [1,2,3]:
...     for y in [3,1,4]:
...         if x != y:
...             combs.append((x, y))
...
>>> combs
[(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
Note how the order of the for and if statements is the same in both these snippets.

If the expression is a tuple (e.g. the (x, y) in the previous example), it must be parenthesized.

Question 10

The best way to create the 4 directions in a matrix

Answer 10

TODO

Question 11

the best way to create the 8 directions (including diagonals) in a matrix

Answer 11

TODO

Question 12

TIP: BE VERY CAREFUL ABOUT _____ when nesting loops

Answer 12

indentation!!!
if you can , pull out functionality into helper functions to prevent issues in indentation, especially if you reset variables at the END of a loop`

Question 13

How is a list implemented under the hood? What does this mean about its common operations?

Answer 13

todo

Question 14

How can you get classic C-like array behavior from python (including allocating an array and constant time lookup once it’s allocated)

Answer 14

todo

Question 15

What are the built in sort operations in python? (both in-place and make-a-copy)

Answer 15

todo

Question 16

How is sort implemented in python under the hood?

Answer 16

todo

Question 17

How do you use a different kind of sort in python? Is it an override? A different import?

Answer 17

todo

Question 18

What is the performance of len(List)? Why?

Answer 18

todo

Question 19

What are some common immutable types?

Answer 19

string, int, etc.

NOTE: is it POSSIBLE to access the pass-by-reference of these? If so, how? If not, are there workarounds?

Question 20

What are some common mutable types?

Answer 20

list, etc.

Question 21

Review your type-hinting cheat sheet

Answer 21

https://mypy.readthedocs.io/en/stable/cheat_sheet_py3.html

Question 22

Does type hinting throw errors for wrong types?

Answer 22

NO. This is an explicit nongoal from the PEP standard. Type hinting can help with certain static analyzers, but I’m not sure which tools are available for this.

You must still validate!!

Question 23

Do python lists have constant time access like arrays do?

Answer 23

todo

Question 24

class Node:
  def \_\_init\_\_(self, val):
    self.val = val
    self.next = None

def some_func(anode):
  anode.val = 15

What does the following code output given the definitions above? Why?

> > headA = Node(10)
some_func(headA)
print(headA.val)
ANS

Answer 24

15 will be printed because objects are passed by reference in python3. This means that objects are mutable within methods.

Question 25

define “argument”

Answer 25

the instance of a variable that is passed into a function at runtime

Question 26

define “parameter”

Answer 26

the representation of what kind of arguments a function/method takes. It is seen in the definition of a function/method.

Question 27

sum two lists using lambda

Answer 27

first = [1, 2, 3, 4, 5]
second = [6, 7, 8, 9, 10]
three = list(map(lambda x,y: x+y,first,second))
print(three)

Output
[7, 9, 11, 13, 15]