Lists Flashcards
Creating lists
List = [1,2,3]
Blist = list (4,5,6)
Converting to lists
List(‘cat’)
»> [‘c’, ‘a’, ‘t’]
Or
List(a_tuple). ->. Creates list of values in a_tuple
Splitting function
Birthday = ‘1/6/1952’
Birthday.split(‘/’)
»> [‘1’, ‘6’, ‘1952’]
Get a list item by using [offset]
Change value by offset
Starts at zero
List[2] = ‘b’
»> changes the 3rd value of list to ‘b’
Get a slice to extract items by offset range
List[::2]. ->. Starts at list beginning and goes by 2
List[1:8:3]. ->. Goes from item 2 to item 9, by 3’s
Appending lists
List.append(‘value’)
-> appends ‘value’ to end of list
Extend function
Merges one list with another
A = [1,2,3] B = [6,7,8]
A.extend(B)
»> [1,2,3,6,7,8]
”+=”
Functions like extend
Merges the items in the lists
(As opposed to append, which would have added the second list as a single list item
A=[1,2]. B=[3,4]
A += B. ->. [1,2,3,4]
A.append(B). ->. [1,2,[3,4]]
Insert() function
Adds an item before any offset in a list.
List.insert(3, ‘itema’)
Adds ‘itema’ before item #4
Del [ ] function
Deletes an item by offset
Del list[2]. -> deletes the third item in list
Remove ( ) function
Removes that item wherever in the list
List.remove(‘cat’). ->. Removes ‘cat’
“LIFO”
Last in first out
Data structure or stack.
Append() followed by pop()
“FIFO”
First in, first out
Data structure or stack
Pop(0)
Index(. ) function
List.index(‘value ‘) -> tells what offset an item is
‘Value ‘ in list
Returns True if the value is in list, else False
Count(. ) function
Counts how many times a particular value occurs in a list.
List.count(‘value’)
Sort() function
Sorts the list in place, changes the list
List.sort()
List.sort(reverse =True). - > reverse sorts
Sorted() function
Returns a sorted copy of the list
List_sorted = sorted(list)
Len() function
Returns the number of items in a list
Ways to copy lists
A = [1,2,3] B = A.copy() C = list(A) D = A[:]
B, C, D are copies of A, changing A does not change the rest and vica versa
Tuple
Immutable
Follow all items with a comma, except last item
Tuple unpacking
Assigning multiple variables at once
A_tuple = (1,2,3)
a,b,c = A_tuple
-> a is 1, b is 2, etc
Tuple() conversion function
Makes tuples of other things
Tuple(list)
Dictionary
Created by placing curly brackets around comma separated key:value pairs
Dict = {key:value1, …}
dict() function
Converts two-value sequences into a dictionary. The first item in the list is the key, the second item is the value.
Dictionary keys must be unique.
Adding to a dictionary
Dict[‘added_value’] = ‘value’
Update() dictionary function
Copies the keys and values of one dictionary into another.
Pythons.update(funny_others)
Adds dictionary funny_others to pythons
Deleting an item by key with “del[]”
Del dict[‘key’]
clear() function for dictionaries
Dict.clear()
Deletes all keys and values from a dictionary
Test for a key by using in
Returns True or False
‘Chapman’ in pythons
True
Get an item by [key]
Most common use of a dictionary.
Test for key in dictionary first, if present, use:
Dict[‘key’] -> returns value
Dictionary get() function
Dict.get(‘key’) -> returns value
Can also provide an optional value, if key exists you get its value; if not, you get the optional value.
Pythons.get(‘Marx’, ‘not a python’)
Will return ‘not a python’
Get all keys by using keys() function
Dict.keys()
Returns all keys
Get all values by using values ()
Dict.values()
Returns values of all keys in dict
Get all key:value pairs by using items ()
Dict.items()
Returns all key:value pairs
Lists
Lists are mutable.
Made from zero or more elements, separated by commas, and surrounded by square brackets.
[1,2,3]
Set()
Set is like a dictionary with its values thrown away, leaving only the keys.
Use set() function
Or
Enclose one or more comma separated values in curly brackets.
zip() function
Iterates multiple sequences in parallel
Ex = creating an English French dictionary
English = []
French []
EF_Dict = dict(zip(English, French))
range()
Generates number sequences
For x in range (0,3):
Print(x)
-> creates a sequence from 0 to 2
List(range(0,3)) -> crestes a list of numbers from 0 to 2
Comprehensions
A comprehension is a compact way of creating a Python data structure from one or more iterators.
Can combine loops and conditional tests with a less verbose syntax.
List comprehension
[expression for item in iterable]
number_list = [number for number in range(1,6)
creates a list from 1-5
[expression for item in iterable if condition]
a_list = [number for number in range(1,6) if number % 2 == 1]
creates a list of odd numbers between 1 and 5
List comprehension for rows, cols, cells
rows = range(1,4)
cols = range(1,3)
cells = [(row, col) for row in rows for col in cols]
for cell in cells:
print(cell)
or
for row, col in cells:
print(row, col)
Dictionary comprehensions
{key_expression:value_expression for expression in iterable}
word =’letters’
letter_counts = {letter:word.count(letter) for letter in word}
creates a dictionary of each letter in “letters” and how many times each letter occurs.
Set comprehension
{expression for expression in iterable}
a_set = {number for number in range(1,6) if number % 3==1}
returns a list of every third number
Generator comprehensions
Tuples do not have comprehensions!
Creates a “generator comprehension”, returns a generator object.
number_thing = (number for number in range(1,6))
type(number_thing)
A generator can only be run once.
A generator is one way to provide data to an iterator.
