Introduction to Python Lists

Posted by PythonStacks on October 22, 2020

Python List is a highly flexible and powerful data structure. They are found in other languages, often referred to as arrays. A List is an ordered sequence of information, accessible by index.

 

In Python, square brackets [ ] indicate a list and individual elements in the list are separated by commas. Here is an example of a list that contains names of friends :

friends = ['hermione', 'Grimoire', 'Harry', 'John']
print(friends)

Program Output

['hermione', 'Grimoire', 'Harry', 'John']

 

Changing elements in a list.

The mutability of data_types determines whether you can or cannot change its elements after creating it.

Lists are mutable and assigning to an element at an index, changes the value.

L = [3, 5, 9, 4]
L[1] = 0           # L is now [3, 0, 9, 4]

 Note that L is now [3, 0, 9, 4].

 

Accessing Elements in a List

The first value in the list is at index 0, the second value is at index 1, the third value is at index 2, and so on.

languages = ['python', 'java', 'julia', 'C++']

 

Element python java julia C++
Index 0 1 2 3

 

print(languages[2])         # julia

Indexes can be only integer values, not floats.

Python will give you an IndexError error message if you use an index that exceeds the number of values in your list value.

print(languages[5])        # IndexError


# program output
Traceback (most recent call last):
  File "lists.py", line 3, in <module>
    print(languages[5])        
IndexError: list index out of range

 

Negative Indexes

Though indexes start at 0 and go up, you can also use negative integers for the index. The index value -1 refers to the last element in a list, the value -2 refers to the second-to-last element in a list, and so on :

languages = ['python', 'java', 'julia', 'C++']

print(languages[-1])        # C++
print(languages[-2])        # julia

 

Generating sublists in python | Slicing

Slicing is the process of generating sub-lists from lists via indexes

languages = ['python', 'java', 'julia', 'C++', 'C#', 'Javascript']

sub_languages = languages[1:5]
print(sub_languages)                # ['java', 'julia', 'C++', 'C#']

The first integer in [1:5] i.e 1 is the index where the slice begins and it ends at the fourth (4) index i.e 5 - 1

 

More Examples

languages = ['python', 'java', 'julia', 'C++', 'C#', 'Javascript']

languages[3:5]                       # ['C++', 'C#']
languages[:3]                       # ['python', 'java', 'julia']
languages[1:]                       # ['java', 'julia', 'C++', 'C#', 'Javascript']
languages[0:1]                      # ['python']

 

Length of a list.

The built-in len()  function will return the number of values/elements that are in a list passed to it.

L = ['john', 3, 5, 8, True]
len(L)                     #  5

 

Iterating Over a list

There are many cases in programming that will require you to go over a list or some ordered collection of data.

A simple example is a program that prints every element in a list.

friends = ['john', 'doe', 'harry', 'potter', 'downey']

for f in friends:
	print(f)

The f is a variable, you call it any name. it could be for friend in friends or for i in friends.

Program Output

john
doe
harry
potter
downey

 

Another program example is to calculate the sum of all elements in a list given all its elements are integers or perhaps floats.

figures = [5, 5, 4, 6, 1]

sum = 0

for number in figures:
	sum += number

print(sum)                # 21


sum is a variable that keeps track of the sum as the loop goes over the list. The last line prints the final sum when the loop is done.

 

 


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