[Python tutorial] 7. Data type - List

List

• ordered
• changeable
• allow duplicate values

Note
Python에는 array가 없고 list를 대신 사용할 수 있다.
대량의 데이터는 NumPy를 사용한다.

thislist = ["apple", "banana", "cherry", "apple", "cherry"] # allow duplicate values
print(thislist)
['apple', 'banana', 'cherry', 'apple', 'cherry']

List Length: len()

print(len(thislist)) # 5

Data Types - any data types

list1 = ["apple", "banana", "cherry"]
list2 = [1, 5, 7, 9, 3]
list3 = [True, False, False]
list4 = ["abc", 34, True, 40, "male"]

Constructor: list(())

thislist = list(("apple", "banana", "cherry")) # list(())

Access Items

The first item index: 0

thislist = ["apple", "banana", "cherry"]

print(thislist[1])    # banana

# Negative Indexing: -1, the last item
print(thislist[-1])    # cherry

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]

# Range of Indexes: 마지막 숫자는 제외
print(thislist[2:5])    # ['cherry', 'orange', 'kiwi']

# items from the beginning
print(thislist[:4])     # ['apple', 'banana', 'cherry', 'orange']

# to the end
print(thislist[2:])     # ['cherry', 'orange', 'kiwi', 'melon', 'mango']

# Range of Negative Indexes
print(thislist[-4:-1])    # ['orange', 'kiwi', 'melon']

Check if Item Exists: if...in...

thislist = ["apple", "banana", "cherry"]

if "apple" in thislist: # True
  print("Yes, 'apple' is in the fruits list")

Change List Items

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "mango"]

# Change Item Value
thislist[1] = "blackcurrant"

Change a Range of Item Values

# 2 items -> 2 items
thislist = ["apple", "banana", "cherry", "orange", "kiwi", "mango"]
thislist[1:3] = ["blackcurrant", "watermelon"]
# ["apple", "blackcurrant", "watermelon", "orange", "kiwi", "mango"]

#  1 item -> 2 items: + 1
thislist[1:2] = ["grape", "pear"]
# ["apple", "grape", "pear", "watermelon", "orange", "kiwi", "mango"]

# 2 items -> 1 item: -1
thislist[1:3] = ["banana"]
# ["apple", "banana", "watermelon", "orange", "kiwi", "mango"]

Add List Items

thislist = ["apple", "banana", "cherry"]

# Append Items: append()
thislist.append("orange")
# ['apple', 'banana', 'cherry', 'orange']

# Insert Items: insert()
thislist.insert(1, "melon")
# ['apple', 'melon', 'banana', 'cherry', 'orange']

Extend List: extend()

Add Any Iterable

thislist = ["apple", "banana", "cherry"]
tropical = ["mango", "pineapple", "papaya"]
thislist.extend(tropical)
# ['apple', 'banana', 'cherry', 'mango', 'pineapple', 'papaya']

thislist = ["apple", "banana", "cherry"]
thistuple = ("kiwi", "orange")
thislist.extend(thistuple)
# ['apple', 'banana', 'cherry', 'kiwi', 'orange']

Remove List Items

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "mango"]

# Remove Specified Item: remove()
thislist.remove("banana")
# ["apple", "cherry", "orange", "kiwi", "mango"]

# Remove Specified Index: pop()
thislist.pop(1)
# ["apple", "orange", "kiwi", "mango"]

# Remove the last item
thislist.pop() 
# ["apple", "orange", "kiwi"]

# del keyword
del thislist[0]
# ["orange", "kiwi"]

# Clear the List: clear()
thislist.clear()
# []

# Delete the entire list: del
del thislist

Loop Lists

thislist = ["apple", "banana", "cherry"]

for x in thislist:
  print(x)

apple
banana
cherry

Loop Through the Index Numbers: range() & len()

thislist = ["apple", "banana", "cherry"]
for i in range(len(thislist)): # [0, 1, 2]
  print(thislist[i])

apple
banana
cherry

While Loop: while...len()...

thislist = ["apple", "banana", "cherry"]
i = 0
while i < len(thislist):
  print(thislist[i])
  i = i + 1

apple
banana
cherry

List Comprehension

thislist = ["apple", "banana", "cherry"]
[print(x) for x in thislist]

apple
banana
cherry

for.. statement

fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
newlist = []

for x in fruits:
  if "a" in x:
    newlist.append(x)

print(newlist)
# ['apple', 'banana', 'mango']

list comprehension

fruits = ["apple", "banana", "cherry", "kiwi", "mango"]

newlist = [x for x in fruits if "a" in x]
# ['apple', 'banana', 'mango']

Syntax

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

fruits = ["apple", "banana", "cherry", "kiwi", "mango"]

newlist = [x for x in fruits if x != "apple"]
# ['banana', 'cherry', 'kiwi', 'mango']

fruits = ["apple", "banana", "cherry", "kiwi", "mango"]

newlist = [x for x in fruits] # condition: option
# ['apple', 'banana', 'cherry', 'kiwi', 'mango']

newlist = [x for x in range(10)] # any iterable
# [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

newlist = [x for x in range(10) if x < 5]
# [0, 1, 2, 3, 4]

Expression

fruits = ["apple", "banana", "cherry", "kiwi", "mango"]

newlist = [x.upper() for x in fruits]
# ['APPLE', 'BANANA', 'CHERRY', 'KIWI', 'MANGO']

fruits = ["apple", "banana", "cherry", "kiwi", "mango"]

newlist = ['hello' for x in fruits]
# ['hello', 'hello', 'hello', 'hello', 'hello']

fruits = ["apple", "banana", "cherry", "kiwi", "mango"]

newlist = [x if x != "banana" else "orange" for x in fruits]
# ['apple', 'orange', 'cherry', 'kiwi', 'mango']

Sort Lists: sort()

thislist = ["orange", "mango", "kiwi", "pineapple", "banana"]
thislist.sort()
# ['banana', 'kiwi', 'mango', 'orange', 'pineapple']

thislist = [100, 50, 65, 82, 23]
thislist.sort()
[23, 50, 65, 82, 100]

Sort Descending: reverse = True

thislist = ["orange", "mango", "kiwi", "pineapple", "banana"]
thislist.sort(reverse = True)
# ['pineapple', 'orange', 'mango', 'kiwi', 'banana']

Customize Sort Function: key = function

def myfunc(n):
  return abs(n - 50) # how close the number is to 50

thislist = [100, 50, 65, 82, 23]
thislist.sort(key = myfunc) # [50, 0, 15, 32, 27] -> [50, 65, 23, 82, 100]
# [50, 65, 23, 82, 100]

Case Insensitive Sort: str.lower

thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.sort() # case sensitive
# ['Kiwi', 'Orange', 'banana', 'cherry']

thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.sort(key = str.lower)
# ['banana', 'cherry', 'Kiwi', 'Orange']

Reverse Order: reverse()

thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.reverse()
# ['cherry', 'Kiwi', 'Orange', 'banana']

Copy Lists

thislist = ["apple", "banana", "cherry"]

# copy()
mylist = thislist.copy()

# list()
mylist = list(thislist)

copy()는 shallow copy (리스트 객체의 참조를 복사). deepcopy()는 객체 자체를 복사

Join Lists

list1 = ["a", "b", "c"]
list2 = [1, 2, 3]

# +
list3 = list1 + list2
# ['a', 'b', 'c', 1, 2, 3]

# append()
for x in list2:
  list1.append(x) # join list2 into list1

# extend()
list1.extend(list2) # add list2 at the end of list1

List Methods