# List

Lists are used to store multiple items in a single variable.

Lists are one of 4 built-in data types in Python used to store collections of data, the other 3 are [Tuple](https://www.w3schools.com/python/python_tuples.asp), [Set](https://www.w3schools.com/python/python_sets.asp), and [Dictionary](https://www.w3schools.com/python/python_dictionaries.asp), all with different qualities and usage.

Lists are created using square brackets:

```
//Create a List:

thislist = ["apple", "banana", "cherry"]
print(thislist)
```

### List Items

List items are ordered, changeable, and allow duplicate values.

List items are indexed, the first item has index `[0]`, the second item has index `[1]` etc.

### Ordered

When we say that lists are ordered, it means that the items have a defined order, and that order will not change.

If you add new items to a list, the new items will be placed at the end of the list.

### Changeable

The list is changeable, meaning that we can change, add, and remove items in a list after it has been created.

***

### Allow Duplicates

Since lists are indexed, lists can have items with the same value:

```
// Example
// Lists allow duplicate values:

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

```

### List Length

To determine how many items a list has, use the `len()` function:

```
// Example
Print the number of items in the list:

thislist = ["apple", "banana", "cherry"]
print(len(thislist))

```

### List Items - Data Types

List items can be of any data type:

```
// Example
//String, int and boolean data types:

list1 = ["apple", "banana", "cherry"]
list2 = [1, 5, 7, 9, 3]
list3 = [True, False, False]
```

A list can contain different data types:

```
//Example
//A list with strings, integers and boolean values:

list1 = ["abc", 34, True, 40, "male"]
```

### type()

From Python's perspective, lists are defined as objects with the data type 'list':

```
<class 'list'>
```

#### Example

What is the data type of a list?

```
// code
mylist = ["apple", "banana", "cherry"]
print(type(mylist))
```

### The list() Constructor

It is also possible to use the list() constructor when creating a new list.

```
// Example
// Using the list() constructor to make a List:

thislist = list(("apple", "banana", "cherry")) # note the double round-brackets
print(thislist)
```

### 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**](https://www.w3schools.com/python/python_tuples.asp) is a collection which is ordered and unchangeable. Allows duplicate members.
* [**Set**](https://www.w3schools.com/python/python_sets.asp) is a collection which is unordered, unchangeable\*, and unindexed. No duplicate members.
* [**Dictionary**](https://www.w3schools.com/python/python_dictionaries.asp) is a collection which is ordered\*\* and changeable. No duplicate members.

\*Set *items* are unchangeable, but you can remove and/or add items whenever you like.

\*\*As of Python version 3.7, dictionaries are *ordered*. In Python 3.6 and earlier, dictionaries are *unordered*.

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.

### &#x20;Python - Access List Items

### Access Items

List items are indexed and you can access them by referring to the index number:

```
// Example
// Print the second item of the list:

thislist = ["apple", "banana", "cherry"]
print(thislist[1])
```

**Note:** The first item has index 0.

#### Negative Indexing

Negative indexing means start 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])
```

#### 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])
```

**Note:** The search will start at index 2 (included) and end at index 5 (not included).

Remember that the first item has index 0.

By leaving out the start value, the range will start at the first item:

```
// Example
// This example returns the items from the beginning to, but NOT including, "kiwi":

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
print(thislist[:4])
```

By leaving out the end value, the range will go on to the end of the list:

#### 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 "orange" (-4) to, but NOT including "mango" (-1):

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
print(thislist[-4:-1])
```

### 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")
```

### 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)

```

### Change a Range of Item Values

To change the value of items within a specific range, define a list with the new values, and refer to the range of index numbers where you want to insert the new values:

```
// Example
// Change the values "banana" and "cherry" with the values "blackcurrant" and "watermelon":

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "mango"]
thislist[1:3] = ["blackcurrant", "watermelon"]
print(thislist)
```

If you insert *more* items than you replace, the new items will be inserted where you specified, and the remaining items will move accordingly:

```
// Example
// Change the second value by replacing it with two new values:

thislist = ["apple", "banana", "cherry"]
thislist[1:2] = ["blackcurrant", "watermelon"]
print(thislist)
```

{% hint style="info" %}
**Note:** The length of the list will change when the number of items inserted does not match the number of items replaced.
{% endhint %}

If you insert *less* items than you replace, the new items will be inserted where you specified, and the remaining items will move accordingly:

```
// Example
// Change the second and third value by replacing it with one value:

thislist = ["apple", "banana", "cherry"]
thislist[1:3] = ["watermelon"]
print(thislist)
```

### Insert Items

To insert a new list item, without replacing any of the existing values, we can use the `insert()` method.

The `insert()` method inserts an item at the specified index:

```
// Example
// Insert "watermelon" as the third item:

thislist = ["apple", "banana", "cherry"]
thislist.insert(2, "watermelon")
print(thislist)
```

{% hint style="info" %}
**Note:** As a result of the example above, the list will now contain 4 items.
{% endhint %}

### Append 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)
```

### Insert Items

To insert a list item at a specified index, use the `insert()` method.

The `insert()` method inserts an item at the specified index:

```
// Example
// Insert an item as the second position:

thislist = ["apple", "banana", "cherry"]
thislist.insert(1, "orange")
print(thislist)
```

> **Note:** As a result of the examples above, the lists will now contain 4 items.

### Extend List

To append elements from *another list* to the current list, use the `extend()` method.

```
// Example
// Add the elements of tropical to thislist:

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

The elements will be added to the *end* of the list.

### Add Any Iterable

The `extend()` method does not have to append *lists*, you can add any iterable object (tuples, sets, dictionaries etc.).

```
// Example
// Add elements of a tuple to a list:

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

### Remove Specified Item

The `remove()` method removes the specified item.

```
// Example
Remove "banana":

thislist = ["apple", "banana", "cherry"]
thislist.remove("banana")
print(thislist)

```

### Remove Specified Index

The `pop()` method removes the specified index.

```
// Example
// Remove the second item:

thislist = ["apple", "banana", "cherry"]
thislist.pop(1)
print(thislist)
```

If you do not specify the index, the `pop()` method removes the last item.

```
// Example
// Remove the last item:

thislist = ["apple", "banana", "cherry"]
thislist.pop()
print(thislist)
```

The `del` keyword also removes the specified index:

```
// Example
// Remove the first item:
thislist = ["apple", "banana", "cherry"]
del thislist[0]
print(thislist)
```

The `del` keyword can also delete the list completely.

```
// Example
// Delete the entire list:

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

### Clear the List

The `clear()` method empties the list.

The list still remains, but it has no content.

```
// Example
// Clear the list content:

thislist = ["apple", "banana", "cherry"]
thislist.clear()
print(thislist)

```

### 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)
```

### Loop Through the Index Numbers

You can also loop through the list items by referring to their index number.

Use the `range()` and `len()` functions to create a suitable iterable.

```
// Example
// Print all items by referring to their index number:
thislist = ["apple", "banana", "cherry"]
for i in range(len(thislist)):
  print(thislist[i])
```

The iterable created in the example above is `[0, 1, 2]`.

### Using a While Loop

You can loop through the list items by using a `while` loop.

Use the `len()` function to determine the length of the list, then start at 0 and loop your way through the list items by refering to their indexes.

Remember to increase the index by 1 after each iteration.

```
// Example
// Print all items, using a while loop to go through all the index numbers

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

### Looping Using List Comprehension

List Comprehension offers the shortest syntax for looping through lists:

```
// Example
// A short hand for loop that will print all items in a list:

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

```

### List Comprehension

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.

Without list comprehension you will have to write a `for` statement with a conditional test inside:

```
// Example
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:

```
// Example
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 valuate 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".*

## 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)
```

```
// Example
// Sort the list descending:

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

### 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)

```

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)
```

### 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)
```

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, 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)

```

### List Methods

Python has a set of built-in methods that you can use on lists.

| Method                                                             | Description                                                                  |
| ------------------------------------------------------------------ | ---------------------------------------------------------------------------- |
| [append()](https://www.w3schools.com/python/ref_list_append.asp)   | Adds an element at the end of the list                                       |
| [clear()](https://www.w3schools.com/python/ref_list_clear.asp)     | Removes all the elements from the list                                       |
| [copy()](https://www.w3schools.com/python/ref_list_copy.asp)       | Returns a copy of the list                                                   |
| [count()](https://www.w3schools.com/python/ref_list_count.asp)     | Returns the number of elements with the specified value                      |
| [extend()](https://www.w3schools.com/python/ref_list_extend.asp)   | Add the elements of a list (or any iterable), to the end of the current list |
| [index()](https://www.w3schools.com/python/ref_list_index.asp)     | Returns the index of the first element with the specified value              |
| [insert()](https://www.w3schools.com/python/ref_list_insert.asp)   | Adds an element at the specified position                                    |
| [pop()](https://www.w3schools.com/python/ref_list_pop.asp)         | Removes the element at the specified position                                |
| [remove()](https://www.w3schools.com/python/ref_list_remove.asp)   | Removes the item with the specified value                                    |
| [reverse()](https://www.w3schools.com/python/ref_list_reverse.asp) | Reverses the order of the list                                               |
| [sort()](https://www.w3schools.com/python/ref_list_sort.asp)       | Sorts the list                                                               |


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