List 리스트

List

list: A mutable sequence* that grows or shrinks dynamically as new elements are added or removed.

*sequence: A container that stores a collection of values that can be accessed by an integer index. (string, list, …)

 

Use SQUARE BRACKETS [ ] to create a list. 

ex) values = [32, 54, 67.5, 80, 110, 44.5] # A list with 6 elements

 

Another way to create a list: 

values = [4]

sets values to the list [4]; that is, the list containing the single element 4.

 

Can access and change elements with the subscript operator ([ ]) 

Lists can hold values of any type, and mutable. (Can replace element)

ex) values[2] = 30.5 # Replace third element to 30.5

 

Attempting to access an element whose index is not within the valid index range is called an out-of-range error* or a bounds error**.

*out-of-range error: Attempting to access an element whose index is not in the valid index range; bounds error.

**bounds error: Trying to access a sequence element that is outside the legal range.

 

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Traversing Lists

There are two common ways of visiting all elements of a list.

 

1. If you need the index values

for i in range(len(values)) :

print(i, values[i])

2. If you don’t need the index values

for element in values :

print(element)

 

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List References

In Python, a list is stored elsewhere and the list variable hold a reference* to the list. 

*reference: A value that denotes the location of an object in memory. In Python, a variable whose type is a class contains a reference to an object of that class.

 

When you copy a list variable into another, both variables refer to the same list. 

The second variable is an alias for the first because they reference the same list. 

 

scores = [10, 9, 8, 7]

values = scores # Copying list reference

 

Thus, if you change the element of alias, the original variable also gets affected.

 

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List Operations

Appending Elements

Use .append method

ex) friends = [ ]

friends.append(“Harry”)

friends.append(“Bob”)

…

 

Inserting Elements

Use .insert method

ex) friends = [“Harry”, “Emily”, “Bob”, “Cari”]

friends.insert(1, “Cindy”)

# “Cindy” is inserted at index 1, and all of the elements following position 1

  are moved down by one position.

print(friends) # [“Harry”, “Cindy”, “Emily”, “Bob”, “Cari”] 

 

Finding Elements

If you simply want to know whether an element is present in a list, use in operator

ex) if “Cindy” in friends:

print(“She’s a friend”)

 

If you want to know the index, use .index method

ex) friends = [“Harry”, “Cindy”, “Emily”, “Mike”, “Cindy”]

n = friends.index(“Cindy”) # n = 1

 

There might be more than one value, then you can specify the starting index

ex) n2 = friends.index(“Cindy”, n + 1) # n = 4, searched from index 2

 

application: 

if “Cindy” in friends:

n = friends.index(“Cindy”)

else:

n = -1

 

Removing Elements

The .pop method removes the element at a given position.

It also returns the removed value.

If used without an argument, it removes the last element.

ex) friends = [“Cindy”, “Micah”, “Ricky”]

print(friends.pop(1))

# Prints “Micah” and removes it from the list friends

 

The .remove method removes an element by value instead of position.

ex) if “Ricky” in friends:

friends.remove(“Ricky”)

 

Concatenation and Replication

The concatenation can be done by plus (+) operator.

The replication can be done by replication (*) operator.

ex) myFriends = [“Mike”, “Julie”]

yourFriends = [“Sylas”, “Amy”]

ourFriends = myFriends + yourFriends # [“Mike”, “Julie”, “Sylas”, “Amy”]

 

monthlyScores = [0] * 12 # [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

 

Sum, Min, Max, and Sorting

You can use sum, min, max, and .sort( ) to a list.

ex) myList = [1, 9, 4, 16]

print(sum(myList)) # 30

print(max(myList)) # 16

print(min(myList)) # 1

myList.sort() # myList = [1, 4, 9, 16]

 

Copying List 

If you copy the reference, you get another reference to the same list.

ex) prices = values

 

If you want to make a copy of a list; that is, a new list that has the same elements in the same order as a given list.

Use the list function:

ex) values = [10, 20, 30]

prices = list(values)

# Now, values and prices refer to different lists

# You can modify either without affecting the other.

 

When list function is used to a string:

ex) characters = list(“Hello”) # characters = [“H”, “e”, “l”, “l”, “o”]

 

Slicing Lists

If you are interested in specific index of values, you can use Python’s slice operator (:) to obtain them. 

 

ex) temperatures = [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

print(temperatures[2: 5]) # [8, 7, 6]

print(temperatures[5:]) # [5, 4, 3, 2, 1]

print(temperatures[:3]) # [10, 9, 8]