enumerate_examples.rst

The enumerate() and zip() functions

.. toctree::
   :maxdepth: 2
   :caption: gen

This page gives some basic examples of how the enumerate() and zip() functions work. It will also show the related function, izip(), provided by the itertools module.

---

Looping through a string

Much like looping through the elements in a list, we can loop through the characters in a string using a fairly straight-forward for-loop

my_string = "Hello world"

for i in my_string:
    print i,

output:

>>> run_demo_script.py

H e l l o   w o r l d

---

We can also loop through the index positions of each character in the string as follows

for i in range(len(my_string)):
    print i,

output:

>>> run_demo_script.py

0 1 2 3 4 5 6 7 8 9 10

---

What if we need to know the position and value of each character while we loop through the string?

for i in range(len(my_string)):
    char = my_string[i]
    print i, char

output:

>>> run_demo_script.py

0 H
1 e
2 l
3 l
4 o
5
6 w
7 o
8 r
9 l
10 d

---

The enumerate() function

A better way to do this is just to use the enumerate() function.

for i, char in enumerate(my_string):
    print i, char

output:

>>> run_demo_script.py

0 H
1 e
2 l
3 l
4 o
5
6 w
7 o
8 r
9 l
10 d

---

So if we wanted to know the postion of all 'l' characters in the string we could do the following Note, I am using the shorthand i, v instead of index, value. Either is find (you can call these variables what you like...)

lpost_list = []

for i, v in enumerate(my_string):
    if v == 'l':
        lpost_list.append(i)

print lpost_list

output:

>>> run_demo_script.py

[2, 3, 9]

To confirm that this works...

for i in lpost_list:
    print my_string[i],

output:

>>> run_demo_script.py

l l l

---

Note, we can also make use of this with list comprehension

lpost_list_2 = [i for i,v in enumerate(my_string) if v == 'l']

print lpost_list
print lpost_list_2

output:

>>> run_demo_script.py

[2, 3, 9]
[2, 3, 9]

---

Looping through multiple lists in tandem

What about looping through two list at the same time? Let us say we want to link the values [1, 2, 3] to the words ['one', 'two', 'three']. Since both lists are the same length, we could just do the following

num_list = [1, 2, 3]
word_list = ['one', 'two', 'three']

for i,v in enumerate(num_list):

    num_val = num_list[i]
    word_val = word_list[i]

    print num_val, word_val

output:

>>> run_demo_script.py

1 one
2 two
3 three

---

The zip() function

A useful altenative, however, is to use the zip() function

for num_val, word_val in zip(num_list, word_list):
    print num_val, word_val

output:

>>> run_demo_script.py

1 one
2 two
3 three

---

Assembling a dictionary using zip()

Of course, this type of relationship is more useful as a dictionary. Note, the following

num_dict = dict(zip(num_list, word_list))   #sets each element in num_list as a key
                                            # and corresponding value in word_list as a value

print num_dict

output:

>>> run_demo_script.py

{1: 'one', 2: 'two', 3: 'three'}

Inefficiency of zip()

The zip() function creates a new list of tuples corresponding to the positions of each element in each list given to the function. If two or more large lists are given to the function, it can use an unnecessarily large quantity of memory for this, e.g.

print zip(range(10000), range(10000,0,-1))  #not a great example of zip()

[(0, 10000), (1, 9999), (2, 9998),... (etc)..., (9997, 3), (9998, 2), (9999, 1)]

The izip() function

Based on the ineficient nature of the zip() function, it is recommended that izip() be used instead, e.g.

from itertools import izip

for i, k in izip(range(10000), range(10000,0,-1)):
    print i, k

0 10000
1 9999
2 9998

... (etc)...

9997 3
9998 2
9999 1