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Lists

Lists

In Python, a list is...

  • a sequences of objects
  • mutable
  • a workhorse of Python data structures
  • examples:
integers = [3, 186, 4431, 74400, 1048443]
[1, 9, 8]
a = ["apple", "orange", "pear"]
a[1] #=> 'orange'
a[1] = 7
a #=> ['apple', 7, 'pear']

b = []
b.append(1.618)
b.append(1.414)
b #=> [1.618, 1.414]

# using list() constructor
list("characters") #=> ['c', 'h', 'a', 'r', 'a', 'c'', t', 'e', 'r', 's']

c = ['bear',
    'giraffe',
    'elephant',
    'caterpillar',] # notice trailing comma!

Negative indices

  • index from end of sequences using negative numbers
  • last element is at index -1
  • supported in lists, strings, tuples
r = [1, -4, 10, -16, 15]
r[-1] #=> 15
r[-2] #=> -16

slicing

  • extended form of indexing for referring to portion of list or other sequence
  • syntax: a_list[start:stop]
    • start and stop indices are optional
integers = [3, 186, 4431, 74400, 1048443]
integers[1:3] #=> [186, 4431]
integers[1:-1] #=> [186, 4431, 74400]

### using only stop index
integers[2:] #=> [4431, 74400, 1048443]

### using only the start
integers[:2] #=> [3, 186]

### using neither (import idiom for copying a list, since it results in a new object)
integers[:] #=> [3, 186, 4431, 74400, 1048443]

integers_copy = integers
integers_copy is integers #=> True

integers_sliced_copy = integers[:]
integers_sliced_copy is integers #=> False
integers_sliced_copy == integers #=> True

Copying with slice

s = [1, 2, 3, 4, 5]
r = s[:]
  • new list object is created
  • however, elements within it are references to same objects referred to by original
  • if list's objects are mutable and modified, the changes will be observed in both lists

Only References Are Copied with slice

Only References Are Copied

In reference to copying with slice:

s = [1, 2, 3, 4, 5]
r = s[:]
  • new list object is created
  • however, elements within it are references to same objects referred to by original
  • if list's objects are mutable and modified, the changes will be observed in both lists

Other Ways to Copy List

s = [3, 186, 4431, 74400, 1048443]

u = s.copy()
u is s #=> False

v = list(s)
v is s #=> False

💡 IMPORTANT 💡 all of above copying techniques perform shallow copies

  • example:
a = [[1, 2], [3, 4]]
b = a[:]
a is b #=> False
a == b #=> True

a[0] is b[0] #=> True

### rebinding value of first element
a[0] = [8, 9]
a #=> [8, 9]
b #=> [1, 2]

### mutating object at second element
a[1].append(5)
a[1] #=> [3, 4, 5]
b[1] #=> [3, 4, 5]

# final state of each list
a #=> [[8, 9], [3, 4, 5]]
b #=> [[1, 2], [3, 4, 5]]

Note: for making deep copies, look to use copy module in Python standard library

Repetition Using Multiplication Operator

c = [21, 37]
d = c * 4
d #=> [21, 37, 21, 37, 21, 37, 21, 37]

### most useful for initializing list of a given size
[0] * 9 #=> [0, 0, 0, 0, 0, 0, 0, 0, 0]

###### NOTE: when using mutable objects as elements, be wary of same trap above with copying
s = [ -1, +1 ] * 5
s #=> [[-1, 1], [-1, 1], [-1, 1], [-1, 1], [-1, 1]]

s[2].append(7)
s #=> [[-1, 1, 7], [-1, 1, 7], [-1, 1, 7], [-1, 1, 7], [-1, 1, 7]]

list.index()

  • find location of object in list
  • returns index of first list element which is equal to passed argument
  • receive ValueError when searching for value that isn't present in list
  • counting matching elements can be another means of searching:
w = "sentence with words and stuff and junk".split()
w.count("and") #=> 2
  • can use in or not in operators to search for membership and non-membership, respectively
37 in [1, 78, 9, 37, 34, 53]

del

  • remove element from list by index
    • results in shortening the list
u = "jackdaws love my big sphinx of quartz".split()
u #=> ['jackdaws', 'love', 'my', 'big', 'sphinx', 'of', 'quartz']

del u[3]
u #=> ['jackdaws', 'love', 'my', 'sphinx', 'of', 'quartz']

list.remove()

  • remove element from list by value
    • results in shortening the list
  • receive ValueError when searching for value that isn't present in list
u #=> ['jackdaws', 'love', 'my', 'sphinx', 'of', 'quartz']

u.remove('jackdaws')
u #=> ['love', 'my', 'sphinx', 'of', 'quartz']

### `remove` is basically equivalent to using `index` with `del`
del u[u.index('quartz')]

list.insert()

  • insert item into list
  • accepts item and index of new item
a = 'I accidentally the whole universe'.split()
a #=> ['I', 'accidentally', 'the', 'whole', 'universe']
a.insert(2, "destroyed")
a #=> ['I', 'accidentally', 'destroyed', 'the', 'whole', 'universe']

Concatenating Lists Misc(?)

  • Concatenating Lists
    • using addition operator results in new list without modifying operands (source lists)
    • using augmented assignment operator modifies assignee in place
    • using extend method works the same as augmented assignment
# addition operator
m = [2, 1, 3]
n = [4, 7, 11]
k = m + n
k #=> [2, 1, 3, 4, 7, 11]

# augmented assignment
k += [18, 29, 47]
k #=> [2, 1, 3, 4, 7, 11, 18, 29, 47]

# using `extend`
k.extend([76, 129, 199])
k #=> [2, 1, 3, 4, 7, 11, 18, 29, 47, 76, 129, 199]

list.reverse() and list.sort()

  • common operations that modify list in place
g = [1, 11, 21]
g.reverse()
g #=> [21, 11, 1]

d = [17, 41, 29]
d.sort()
d #=> [17, 29, 41]

More About sort

  • sort accepts two optional arguments: key and reverse
    • reverse: when true, results in descending order
d #=> [17, 29, 41]
d.sort(reverse=True)
d #=> [41, 29, 17]
  • key: accepts any callable object that accepts single parameter
  • items are passed to callable and sorted according to its return value
  • examples:
# using `len` function
h = 'not perplexing do handwriting family where I illegibly know doctors'.split()
h #=> ['not', 'perplexing', 'do', 'handwriting', 'family', 'where', 'I', 'illegibly', 'know', 'doctors']
h.sort(key=len)
h #=> ['I', 'do', 'not', 'know', 'where', 'family', 'doctors', 'illegibly', 'perplexing', 'handwriting']

Reversing and Sorting Into Copies

  • reversed() and sorted()
    • "out-of-place" equivalents of reverse and sort
    • reversed() returns reverse iterator
    • sorted() returns newly-sorted list
x = [4, 9, 2, 1]
y = sorted(x)
y #=> [1, 2, 4, 9]
x #=> [4, 9, 2, 1]

p = [9, 3, 1, 0]
q = reversed(p)
q #=> <list_reverseiterator object at 0x10c68ea90>
list(q)
[0, 1, 3, 9]
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