List Comprehensions

List comprehensions provide a concise way to create lists. It consists of brackets containing an expression followed by a for clause, then zero or more for or if clauses. The expressions can be anything, meaning you can put in all kinds of objects in lists. The result will be a new list resulting from evaluating the expression in the context of the for and if clauses which follow it. The list comprehension always returns a result list.

Example :-

 new_list = []
  for i in old_list:
   if filter(i):
    new_list.append(expressions(i)) 

Syntax

The list comprehension starts with a ‘[‘ and ‘]’, square brackets, to help you remember that the result is going to be a list.

The basic syntax uses square brackets.

• new_list

The new list (result).

• expression(i)

Expression is based on the variable used for each element in the old list.

• for i in old_list

The word for followed by the variable name to use, followed by the word in the old list.

• if filter(i)

Apply a filter with an If-statement.

• new_range = [i * i for i in range(5) if i % 2 == 0]

Which corresponds to:

result = [transform iteration filter ]

The * operator is used to repeat. The filter part answers the question if the item should be transformed.

Examples :

• Create a simple list

  x = [i for i in range(10)]
  print x
  
  # This will give the output:
  [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] 
  

• Create a list using loops and list comprehension

  # You can either use loops:
  squares = []
  
  for x in range(10):
  squares.append(x**2)
   
  print squares
  [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
  
  # Or you can use list comprehensions to get the same result:
  squares = [x**2 for x in range(10)]
  
  print squares
  [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
  

• Multiplying parts of a list

  list1 = [3,4,5]
   
   multiplied = [item*3 for item in list1] 
   
  print multiplied 
   [9,12,15]