16. Python Lists (cont.3)
16.1 List Comprehensions
List comprehension offers a shorter syntax when you want to create a new list based on the values of an existing list.
Example:
Based on a list of fruits, you want a new list, containing only the fruits with the letter "a" in the name.
fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
newlist = []
for x in fruits:
if "a" in x:
newlist.append(x)
print(newlist)
With list comprehension you can do all that with only one line of code:
fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
newlist = [x for x in fruits if "a" in x]
print(newlist)
The Syntax
newlist = [expression for item in iterable if condition == True]
The return value is a new list, leaving the old list unchanged.
Condition
The condition is like a filter that only accepts the items that evaluate to True.
Example
Only accept items that are not "apple":
newlist = [x for x in fruits if x != "apple"]
The condition if x != "apple" will return True for all elements other than "apple", making the new list contain all fruits except "apple".
The condition is optional and can be omitted.
Example
With no if statement:
newlist = [x for x in fruits]
Iterable
The iterable can be any iterable object, like a list, tuple, set etc.
Example
You can use the range() function to create an iterable:
newlist = [x for x in range(10)]
Same example, but with a condition.
Example
Accept only numbers lower than 5:
newlist = [x for x in range(10) if x < 5]
Expression
The expression is the current item in the iteration, but it is also the outcome, which you can manipulate before it ends up like a list item in the new list.
Example
Set the values in the new list to upper case:
newlist = [x.upper() for x in fruits]
You can set the outcome to whatever you like.
Example
Set all values in the new list to 'hello':
newlist = ['hello' for x in fruits]
The expression can also contain conditions, not like a filter, but as a way to manipulate the outcome.
Example
Return "orange" instead of "banana":
newlist = [x if x != "banana" else "orange" for x in fruits]
The expression in the example above says:
"Return the item if it is not banana, if it is banana return orange".
16.2 Sort Lists
Sort List Alphanumerically
List objects have a sort() method that will sort the list alphanumerically, ascending, by default.
Example
Sort the list alphabetically:
thislist = ["orange", "mango", "kiwi", "pineapple", "banana"]
thislist.sort()
print(thislist)
Example
Sort the list numerically:
thislist = [100, 50, 65, 82, 23]
thislist.sort()
print(thislist)
Sort Descending
To sort descending, use the keyword argument reverse = True.
Example
Sort the list descending:
thislist = ["orange", "mango", "kiwi", "pineapple", "banana"]
thislist.sort(reverse = True)
print(thislist)
Another example:
thislist = [100, 50, 65, 82, 23]
thislist.sort(reverse = True)
print(thislist)
Customize Sort Function
You can also customize your own function by using the keyword argument key = function.
The function will return a number that will be used to sort the list (the lowest number first).
Example
Sort the list based on how close the number is to 50:
def myfunc(n):
return abs(n - 50)
thislist = [100, 50, 65, 82, 23]
thislist.sort(key = myfunc)
print(thislist)
Case Insensitive Sort
By default the sort() method is case sensitive, resulting in all capital letters being sorted before lower case letters.
Example
Case sensitive sorting can give an unexpected result:
thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.sort()
print(thislist)
Luckily we can use built-in functions as key functions when sorting a list.
So if you want a case-insensitive sort function, use str.lower as a key function.
Example
Perform a case-insensitive sort of the list:
thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.sort(key = str.lower)
print(thislist)
Reverse Order
What if you want to reverse the order of a list, regardless of the alphabet?
The reverse() method reverses the current sorting order of the elements.
Example
Reverse the order of the list items:
thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.reverse()
print(thislist)
16.3 Copy and Join Lists
Copy a List
You cannot copy a list simply by typing list2 = list1, because: list2 will only be a reference to list1, and changes made in list1 will automatically also be made in list2.
You can use the built-in List method copy() to copy a list.
Example
thislist = ["apple", "banana", "cherry"]
mylist = thislist.copy()
print(mylist)
Use the list() method
Another way to make a copy is to use the built-in method list().
Example
Make a copy of a list with the list() method:
thislist = ["apple", "banana", "cherry"]
mylist = list(thislist)
print(mylist)
Use the slice Operator
You can also make a copy of a list by using the : (slice) operator.
Example
thislist = ["apple", "banana", "cherry"]
mylist = thislist[:]
print(mylist)
Join Two Lists
There are several ways to join, or concatenate, two or more lists in Python.
One of the easiest ways is by using the + operator.
Example
Join two list:
list1 = ["a", "b", "c"]
list2 = [1, 2, 3]
list3 = list1 + list2
print(list3)
Another way to join two lists is by appending all the items from list2 into list1, one by one.
Example
Append list2 into list1:
list1 = ["a", "b" , "c"]
list2 = [1, 2, 3]
for x in list2:
list1.append(x)
print(list1)
Or you can use the extend() method, where the purpose is to add elements from one list to another list.
Example
Use the extend() method to add list2 at the end of list1:
list1 = ["a", "b" , "c"]
list2 = [1, 2, 3]
list1.extend(list2)
print(list1)
16.4 List Methods
Python
has a set of built-in methods that you can use on lists.
Method Description
append() Adds an element at the end of the list
clear() Removes all the elements from the list
copy() Returns a copy of the list
count() Returns the number of elements with the specified value
extend() Add the elements of a list (or any iterable), to the end of the current list
index() Returns the index of the first element with the specified value
insert() Adds an element at the specified position
pop() Removes the element at the specified position
remove() Removes the item with the specified value
reverse() Reverses the order of the list
sort() Sorts the list
16.5 Exercise
1. Consider the following code:
fruits = ['apple', 'banana', 'cherry']
newlist = [x for x in fruits if x == 'banana']
What will be the value of newlist?
A. ['apple', 'cherry']
B. ['banana']
C. True
D. False
2. What is the correct syntax for making a copy of a list?
A. list2 = list1
B. list2 = list1.copy()
C. list2.copy(list1)
D. list2 = ref(list1)
3. What is the correct syntax for joining list1 and list2 into
list3?
A. list3 = join(list1, list2)
B. list3 = list1 + list2
C. list3 = [list1, list2]
D. list3 = connect(list1, list2)
4. Write a program to input a string from the keyboard that contains a combination of the lower and upper case letters, then rearrange the characters of a string so that all lowercase letters should come first.
Input:
s1: hiBacTeo
Expected output:
hiaceoBT
5. Write a python program that allows the user to input a list of numbers from the keyboard. The user enters the character "q" to finish entering the list. Remove duplicate numbers from the list.
Input:
[1, 2, 3, 40, 2, 1, 5]
Expected output:
[1, 2, 3, 40, 5]
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The answer hints:
1(B), 2(B), 3(B), 4()
4. Write a program to input a string from the keyboard that contains a combination of the lower and upper case letters, then rearrange the characters of a string so that all lowercase letters should come first.
Input:
s1: hiBacTeo
Expected output:
hiaceoBT
[code16_4.py]
# user input string from the keyboard
s1 = input("Enter a string:")
# create 2 strings to store lower and upper characters
lowerString = []
upperString = []
for char in s1:
if char.islower():
lowerString.append(char)
elif char.isupper():
upperString.append(char)
# concatenate lowerString and upperString
resultString = lowerString + upperString
# convert a character list into string
resultString = ''.join(resultString)
print(resultString)
[code16_5.py]
#create a number list
numbers = []
# input a character list from the keyboard, enter "q" to quit
while True:
number = input("Enter a number (or
'q' to quit):")
if number == 'q':
break
else:
numbers.append(number)
print(numbers)
#create a unique number list
uniqueNumbers = []
#remove the duplicate item from the numbers
for i in numbers:
if i not in uniqueNumbers:
uniqueNumbers.append(i)
print(uniqueNumbers)
Cập nhật: 1/11/2024
Bài sau: Python căn bản (17): Python Tuples
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