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❮ Previous Next ❯Python Collections (Arrays)
There are four collection data types in the Python programming language:
- List is a collection which is ordered and changeable. Allows duplicate members.
- Tuple is a collection which is ordered and unchangeable. Allows duplicate members.
- Set is a collection which is unordered and unindexed. No duplicate members.
- Dictionary is a collection which is unordered, changeable and indexed. No duplicate members.
When choosing a collection type, it is useful to understand the properties of that type. Choosing the right type for a particular data set could mean retention of meaning, and, it could mean an increase in efficiency or security.
List
A list is a collection which is ordered and changeable. In Python lists are written with square brackets.
Example
Create a List:
thislist = ["apple", "banana", "cherry"] print(thislist) Run example »Access Items
You access the list items by referring to the index number:
Example
Print the second item of the list:
thislist = ["apple", "banana", "cherry"] print(thislist[1]) Run example »Negative Indexing
Negative indexing means beginning from the end, -1 refers to the last item, -2 refers to the second last item etc.
Example
Print the last item of the list:
thislist = ["apple", "banana", "cherry"] print(thislist[-1]) Run example »Range of Indexes
You can specify a range of indexes by specifying where to start and where to end the range.
When specifying a range, the return value will be a new list with the specified items.
Example
Return the third, fourth, and fifth item:
thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"] print(thislist[2:5]) Run example »Note: The search will start at index 2 (included) and end at index 5 (not included).
Remember that the first item has index 0.
Range of Negative Indexes
Specify negative indexes if you want to start the search from the end of the list:
Example
This example returns the items from index -4 (included) to index -1 (excluded)
thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"] print(thislist[-4:-1]) Run example »Change Item Value
To change the value of a specific item, refer to the index number:Example
Change the second item:
thislist = ["apple", "banana", "cherry"] thislist[1] = "blackcurrant" print(thislist) Run example »Loop Through a List
You can loop through the list items by using a for loop:
Example
Print all items in the list, one by one:
thislist = ["apple", "banana", "cherry"] for x in thislist: print(x) Run example »You will learn more about for loops in our Python For Loops Chapter.
Check if Item Exists
To determine if a specified item is present in a list use the in keyword:
Example
Check if "apple" is present in the list:
thislist = ["apple", "banana", "cherry"] if "apple" in thislist: print("Yes, 'apple' is in the fruits list") Run example »List Length
To determine how many items a list has, use the len() method:
Example
Print the number of items in the list:
thislist = ["apple", "banana", "cherry"] print(len(thislist)) Run example »Add Items
To add an item to the end of the list, use the append() method:
Example
Using the append() method to append an item:
thislist = ["apple", "banana", "cherry"] thislist.append("orange") print(thislist) Run example »To add an item at the specified index, use the insert() method:
Example
Insert an item as the second position:
thislist = ["apple", "banana", "cherry"] thislist.insert(1, "orange") print(thislist) Run example »Remove Item
There are several methods to remove items from a list:
Example
The remove() method removes the specified item:
thislist = ["apple", "banana", "cherry"] thislist.remove("banana") print(thislist) Run example »Example
The pop() method removes the specified index, (or the last item if index is not specified):
thislist = ["apple", "banana", "cherry"] thislist.pop() print(thislist) Run example »Example
The del keyword removes the specified index:
thislist = ["apple", "banana", "cherry"] del thislist[0] print(thislist) Run example »Example
The del keyword can also delete the list completely:
thislist = ["apple", "banana", "cherry"] del thislist Run example »Example
The clear() method empties the list:
thislist = ["apple", "banana", "cherry"] thislist.clear() print(thislist) Run example »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.
There are ways to make a copy, one way is to use the built-in List method copy().
Example
Make a copy of a list with the copy() method:
thislist = ["apple", "banana", "cherry"] mylist = thislist.copy() print(mylist) Run example »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) Run example »Join Two Lists
There are several ways to join, or concatenate, two or more lists in Python.
One of the easiest ways are by using the + operator.
Example
Join two list:
list1 = ["a", "b" , "c"] list2 = [1, 2, 3] list3 = list1 + list2 print(list3) Run example »Another way to join two lists are 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) Run example »Or you can use the extend() method, which 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) Run example »The list() Constructor
It is also possible to use the list() constructor to make a new list.
Example
Using the list() constructor to make a List:
thislist = list(("apple", "banana", "cherry")) # note the double round-brackets print(thislist) Run example »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 |
Test Yourself With Exercises
Exercise:
Print the second item in the fruits list.
fruits = ["apple", "banana", "cherry"] print()Submit Answer » ❮ Previous Next ❯
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