Python for Programmers: Enumerate Function

Welcome to the Enumerate Function lesson!

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Python doesn't have the common C-style for loop: for (i = 0; i < max; i++). Instead, it only has a "for-each" loop: for value in values. That's great, except when we need to know both the value and its index in the list.
The enumerate built-in solves that problem. It takes an iterable like ["a", "b"] and returns a list of tuples with the indexes and values, like [(0, 'a'), (1, 'b')].

my_list = ["a", "b"]
list(enumerate(my_list))

Result:

In the next example, we use enumerate to find the index of 'a' in our list.

data = ["c", "b", "a"]
found_index = None
for index, value in enumerate(data):
  if value == "a":
    found_index = index
found_index

Result:

enumerate works on any iterable, not just lists. For example, we can use it with strings. Normally, indexing into a string gives us a one-character string containing the character at that index.
>
"cat"[1]
Result:
'a'
Using enumerate on a string gives us a list of tuples holding the index and the character, similar to what we saw above for lists.
>
list(enumerate("cat"))
Result:

list(enumerate("do"))

Result:

[(0, 'd'), (1, 'o')]

The next example defines a treasure map. It's a list of strings, with each string representing one row in the map. We can enumerate the rows (strings in the list) and columns (characters in each string) to find the treasure, marked by X.

treasure_map = [
  "........",
  "....#..#",
  "#....#..",
  "....X.#.",
  ".#......",
  "......#.",
  "#.......",
  "........",
]

for (row_coordinate, row) in enumerate(treasure_map):
  for (col_coordinate, c) in enumerate(row):
    # X marks the spot!
    if c == "X":
      location = (row_coordinate, col_coordinate)

location

Result:

(3, 4)

We can also enumerate a dictionary's keys. That gives us the keys in their original insertion order. As usual, each one is packed into a tuple along with its index.

cat_ages = {
  "Ms. Fluff": 4,
  "Keanu": 2,
}
list(enumerate(cat_ages))

Result:

[(0, 'Ms. Fluff'), (1, 'Keanu')]

Here's a code problem:
The code below defines a list of users. Each user is a dictionary with a "name" key. Write a user_index function that takes a user's name, finds that user in the list, and returns their list index. If no user has that name, it should return None.
def user_index(users, name):
for (idx, user) in enumerate(users):
if user["name"] == name:
return idx
return None
users = [
{"name": "Betty"},
{"name": "Cindy"},
{"name": "Amir"},
{"name": "Gabriel"},
{"name": "Hana"},
]

assert user_index(users, "Amir") == 2
assert user_index(users, "Betty") == 0
assert user_index(users, "Cindy") == 1
assert user_index(users, "Dalili") is None
Goal:
No errors.
Yours:
No errors.
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