reading-notes

List Comprehensions

Create Lists
[expression for-clause 0-or-for/if-clauses]
creates a new list with the results from evaluation the expression on the for and if clauses

[ expression for item in list if conditional ]
Old Way

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

List Comprehension way

  new_list = [expression(i) for i in old_list if filter(i)]

if filter(i) - apply a filter with an if

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

*result* = [*transform* *iteration* *filter* ] - The * operator is used to repeat. The filter part answers the question if the item should be transformed.

Create a 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]

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]

multiply parts of list list1 = [3,4,5]

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

first letter of each word

  listOfWords = ["this","is","a","list","of","words"]

  items = [ word[0] for word in listOfWords ]

  print items

lower/upper case

  >>> [x.lower() for x in ["A","B","C"]]
  ['a', 'b', 'c']

  >>> [x.upper() for x in ["a","b","c"]]
  ['A', 'B', 'C']

print out the numbers from a given string

  string = "Hello 12345 World"
  numbers = [x for x in string if x.isdigit()]
  print numbers

  >> ['1', '2', '3', '4', '5']

Parse a file using list comprehension

  fh = open("test.txt", "r")

  result = [i for i in fh if "line3" in i]

  print result

Create a function and name it double: def double(x): return x*2

If you now just print that function with a value in it, it should look like this: »> print double(10) 20

>>> [double(x) for x in range(10)]

print double
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

You can put in conditions:

>>> [double(x) for x in range(10) if x%2==0]
[0, 4, 8, 12, 16]

You can add more arguments:

>>> [x+y for x in [10,30,50] for y in [20,40,60]]
[30, 50, 70, 50, 70, 90, 70, 90, 110]

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