9 List Comprehensions

List comprehension is an elegant way to define and create lists based on existing lists. It offers a shorter syntax when you want to create a new list based on the values of an existing list.

9.1 Example

Iterating through a string Using for Loop

['N', 'I', 'B', 'A', 'F']

Iterating through a string Using List Comprehension

['N', 'I', 'B', 'A', 'F']

If you noticed, ‘NIBAF’ is a string, not a list. This is the power of list comprehension. It can identify when it receives a string or a tuple and work on it like a list.

9.2 Example

Based on a list of fruits, you want a new list, containing only the fruits with the letter “a” in the name.

Without list comprehension you will have to write a for statement with a conditional test inside:

['apple', 'banana', 'mango']

This can be done in one line of code With list comprehension:

['apple', 'banana', 'mango']

9.3 The Syntax

new_list = [expression for item in iterable if condition == True]

The return value is a new list, leaving the old list unchanged.

9.4 Key Points to Remember

List comprehension is an elegant way to define and create lists based on existing lists.
List comprehension is generally more compact and faster than normal functions and loops for creating list.
However, we should avoid writing very long list comprehensions in one line to ensure that code is user-friendly.
Remember, every list comprehension can be rewritten in for loop, but every for loop can’t be rewritten in the form of list comprehension.

9.5 Exercise

Create the list [1,2,3,4,5] using list comprehension.
Use a list comprehension that iterates over a_list, prints a list composed of each value in a_list multiplied by 10.
```
a_list = list(range(1, 11))
```
Use a list comprehension that iterates over a_list, prints a list composed of odd numbers from 1 to 9.
Using a list comprehension which iterates over a_list and whose output expression accesses a value from a dictionary, print a list composed of the text form of each even number from 2 to 10, e.g., [ 'two', 'four', 'six', 'eight', 'ten' ].

a_dictionary = { 1 : 'one', 2 : 'two', 3 : 'three', 4 : 'four', 5 : 'five', 6 : 'six', 7 : 'seven', 8 : 'eight', 9 : 'nine', 10 : 'ten'}
Prompt the user for a number, which will be returned from input as a string data type. Strings can be iterated over like lists, such that the loop repeats for each character in the string. Using a list comprehension which iterates over the user-entered string and whose output expression accesses values from a_dictionary, print a list of the text form of each digit from the user-entered string, e.g., Enter a number: 195 return [ 'one', 'nine', 'five' ]
Using the same user-entered number and list comprehension as above, also print the text form of the digits in a single string (using the string join function) with a dash between each text form of the digit, e.g.,
```
one-nine-five
```
Write a list comprehension that builds a list containing only the names with at least 4 characters. list9 = ['SBP', 'NIABF', 'HoK', 'PSPC', 'DPC']
Write a list comprehension that builds a list containing only even numbers over 40.
```
numbers = list (range (10,50,5))
```
Add % in front and end of every single word in a given string str: “Fall is Awesome in Sakardu” only using list comprehensions. Desired Output

['%Fall%','%is%','%Awesome%','%in%','%Sakardu%']
Use list comprehension to make a list of the first letter of each word in the following list:

wordList = ["this", "is", "an", "apple"]

A string is given:

msg = "Long Live Pakistan!"

Write a list comprehension that prints a list

['L', 'o', 'n', 'g', 'L', 'i', 'v', 'e', 'P', 'a', 'k', 'i', 's', 't', 'a', 'n']

Do not print out the space or the !.