zip-and-enumerate.md

zip and enumerate

Now we know how for loops work in Python. But for loops aren't limited to printing each item in a list, they can do a lot more.

To be able to understand for loop tricks we need to first know assigning values to multiple variables at once. It works like this:

>>> a, b = 1, 2
>>> a
1
>>> b
2
>>>

We can use () and [] around these values however we want and everything will still work the same way. [] creates a list, and () creates a tuple.

>>> [a, b] = (1, 2)
>>> a
1
>>> b
2
>>>

We can also have [] or () on one side but not on the other side.

>>> (a, b) = 1, 2
>>> a
1
>>> b
2
>>>

Python created a tuple automatically.

>>> 1, 2
(1, 2)
>>>

If we're for looping over a list with pairs of values in it we could do this:

>>> items = [('a', 1), ('b', 2), ('c', 3)]
>>> for pair in items:
...     a, b = pair
...     print(a, b)
...
a 1
b 2
c 3
>>>

Or we can tell the for loop to unpack it for us.

>>> for a, b in items:
...     print(a, b)
...
a 1
b 2
c 3
>>>

This feature is often used with Python's built-in zip() and enumerate() functions.

zip

What comes to your mind when you hear the word zip? A mechanism extensively used to tie two parts of something, e.g. shirt or jacket. Python's zip() functions does pretty much the same, it helps us tie corresponding items together.

>>> users = ["Tushar", "Aman", "Anurag", "Sohit"]
>>> uids = ["usr122", "usr123", "usr124", "usr125"]
>>> user_details = zip(uids, users)
>>> print(list(user_details))
[('usr122', 'Tushar'), ('usr123', 'Aman'), ('usr124', 'Anurag'), ('usr125', 'Sohit')]
>>>

Note that print(user_details) doesn't work as expected:

>>> print(user_details)
<zip object at 0x0000016938E311C0>
>>>

This is because zip() is an iterator, i.e. lazy: it gives the items as needed, instead of calculating them and storing them into memory all at once like a list. So the zip object cannot show its elements before the elements are used, because it hasn't computed them yet.

>>> users = ["Tushar", "Aman", "Anurag", "Sohit"]
>>> uids = ["usr122", "usr123", "usr124", "usr125"]
>>> user_details = zip(uids, users)

If the lists are of different lengths, some items from the end of the longer list will be ignored.

>>> users = ["Tushar", "Aman", "Anurag"]
>>> emails = ["[email protected]", "[email protected]", "[email protected]", "[email protected]"]
>>> users_contact = zip(users, emails)
>>> print(list(users_contact))
[('Tushar', '[email protected]'), ('Aman', '[email protected]'), ('Anurag', '[email protected]')]
>>>

Here the shortest list is users, with length 3, so zip(users, emails) only takes the first 3 emails. We do not recommend calling zip() with lists of different lengths, because ignoring items is usually not what you intended to do.

Using zip in a for loop

It is very common to for loop over a zip(), and unpack the returned tuples in the for loop. This is why we introduced unpacking in the beginning of this page. When used this way, there's no need to convert the result of zip(...) to a list.

>>> roll_nums = [20, 25, 28]
>>> students = ["Joe", "Max", "Michel"]
>>> for roll_num, student in zip(roll_nums, students):
...     print(f"Roll number of {student} is {roll_num}")
...
Roll number of Joe is 20
Roll number of Max is 25
Roll number of Michel is 28
>>>

enumerate

enumerate() is an amazing Built-in function offered by python. When used, gives us the index and the item combined.

>>> even_nums = [2, 4, 6, 8, 10, 12]
>>> for index, item in enumerate(even_nums):
...     print(f"Index of {item} is {index}")
...
Index of 2 is 0
Index of 4 is 1
Index of 6 is 2
Index of 8 is 3
Index of 10 is 4
Index of 12 is 5
>>>

It is also possible (but more difficult) to do this without enumerate():

>>> even_nums = [2, 4, 6, 8, 10, 12]
>>> for index in range(0,  len(even_nums)):
...     print(f"Index of {even_nums[index]} is {index}")
...
Index of 2 is 0
Index of 4 is 1
Index of 6 is 2
Index of 8 is 3
Index of 10 is 4
Index of 12 is 5
>>>

Here:

• range(0, len(even_nums)) gives 0,1,2,3,4,5, with the list length 6 excluded. These are the indexes of our list of length 6.
• even_nums[index] prints each element of even_nums, because index comes from the range of all indexes into that list.

Because this is complicated to think about and easy to get wrong, it is better to use enumerate().

Exercises

1. Create a program that works like this. Here I entered everything after the > prompt that the program displayed.

   Enter something, and press Enter without typing anything when you're done.
   >hello there
   >this is a test
   >it seems to work
   >
   Line 1 is: hello there
   Line 2 is: this is a test
   Line 3 is: it seems to work
   

2. Create a program that prints all letters from A to Z and a to z next to each other:

   A a
   B b
   C c
   ...
   X x
   Y y
   Z z
   

   Start your program like this:

   uppercase = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
   lowercase = 'abcdefghijklmnopqrstuvwxyz'

   Hint: how do strings behave with zip? Try it out on the >>> prompt and see.

3. Can you make it print the indexes also?

   1 A a
   2 B b
   3 C c
   ...
   24 X x
   25 Y y
   26 Z z
   

The answers are here.

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