joequery
11/13/2013 - 5:59 PM

## Python enumerate()

Python enumerate()

``````>>> # Suppose we have this list of tuples, each of length two.
... # Element 1 is a persons first name, element 2 is their last name
...
>>> names = [('Joseph', 'McCullough'), ('Luke', 'PolishLastName')]
>>> # Now, if we do a normal for loop, the item per each iteration is
... # a tuple
...
>>> for name in names:
...     print("first: " + name[0])
...     print("last: " + name[1])
...
first: Joseph
last: McCullough
first: Luke
last: PolishLastName
>>> # First, let's look at tuple assignment. You can order-wise assign elements
... # of a tuple to variables
...
>>> joseph = names[0]
>>> joseph
('Joseph', 'McCullough')
>>> first,last = joseph
>>> first
'Joseph'
>>> last
'McCullough'
>>> # Because of this, when we set up our foor loop, we can do the same
...
>>> for first,last in names:
...     print("first: " + first)
...     print("last: " + last)
...
first: Joseph
last: McCullough
first: Luke
last: PolishLastName
>>> # Let's take a look at what enumerate does. It's a function you call on a list
...
>>> mylist = ['cat', 'dog', 'fish', 'thing']
>>> list(enumerate(mylist))
[(0, 'cat'), (1, 'dog'), (2, 'fish'), (3, 'thing')]
>>> # So you see it takes each element of the list, makes a 2-length tuple with
... # the first element the index of the element, and second element the value
... # So now we have what we need to iterate over a list and keep track of its index
...
>>> for i,animal in enumerate(mylist):
...     print("The animal: " + animal)
...     print("The index: " + str(i))
...
The animal: cat
The index: 0
The animal: dog
The index: 1
The animal: fish
The index: 2
The animal: thing
The index: 3
>>> # The main use case of using enumerate like this is to alter list elements
... # during iteration. Observe that through a normal loop, we cannot alter
... # the elements outside of the loop body
...
>>> mylist
['cat', 'dog', 'fish', 'thing']
>>> for animal in mylist:
...     animal = "LOL"
...
>>> mylist
['cat', 'dog', 'fish', 'thing']
>>> # But, by referencing the element through list indexing, we can change the
... # element
...
>>> for i, animal in enumerate(mylist):
...     mylist[i] = animal + "hahahah"
...
>>> mylist
['cathahahah', 'doghahahah', 'fishhahahah', 'thinghahahah']``````