Operations on Lists

The last unit showed us how to use a for loop on a list. Lets revise, how does a for loop look like?

>>> for i in range(0,9):
    print i

This would result in:

0
1
2
3
4
5
6
7
8

A for loops make our life easier by reducing the number of lines of code. It makes our work more efficient and quick. Let's apply this on a list:

listExample = ['apple', 'berry', 'cherry']
for i in range(len(listExample)):  # Using the `len()` function
    print listExample[i]

This will print:

apple
berry
cherry

Note: As a for loop iterates over a range of items, we use the range() function over the length of the list. This helps us to iterate our for loop from 0 to the length of the list.

What else can we do with Lists?

Finding the existence of values in Lists

One thought that often comes to the mind is, what if the list we have is huge and we need to check the presence of a particular value? Will we have to run a for loop over the entire list just for this value? Not exactly! Python has a solution for this as well. The in and not in operators. These are used in expressions where the result is Boolean, i.e., True or False.

>>> listExample = ['apple', 'berry', 'cherry']
>>> 'apple' in listExample
True
>>> 'blueberry' in listExample
False
>>> 'blueberry' not in listExample
True

Augmented Assignment Statements

This is another cool feature in Python, to reduce your code and increase efficiency. Often, while using assignment operators, we repeat the variable name to add, subtract, divide, get remainder or multiple new values. This can be made easy.

>>> data = 0
>>> data = data + 1
>>> data
1
>>> data += 1
>>> data
2

As you can see, using += in the adding operator was a shortcut than writing the entire variable name again. This is a table for all the augmented assignment statements in Python

PRO TIP: Adding on to what we read earlier, the += operator can be used for String and List concatenation, while the *= operator can be used for String and List replication. Try it!

Assignments with Lists

Lets consider an example. We have a List of characteristics of a person, like the name, age, height, weight and we wish to assign it to different variables. One way to do this would be:

>>> student = ['Alex', 29, 179, 150]
>>> name = student[0]
>>> age = student[1]
>>> height = student[2]
>>> weight = student[3]

Or, we could do this:

>>> student = ['Alex', 29, 179, 150]
>>> name, age, height, weight = student
>>> name
Alex

Lists allow us to do multiple assignments, iff the size of the lists and variables is the same. Otherwise, it throws a ValueError.

>>> student = ['Alex', 29, 179, 150]
>>> name, age, height, weight, sex = student
ValueErrorTraceback (most recent call last)
<ipython-input-16-219fff1c0b23> in <module>()
      1 student = ['Alex', 29, 179, 150]
----> 2 name, age, height, weight, sex = student

ValueError: need more than 4 values to unpack

DO IT YOURSELF: Try swapping two values using the multiple assignment method!