012-trey-hunner-zip-and-enumerate.md

Trey Hunner: 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
>>>

Now you're ready to read this awesome looping tutorial. Read it now, then come back here and do the exercises.

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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