> For the complete documentation index, see [llms.txt](https://codingnotes.gitbook.io/coding_notes/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://codingnotes.gitbook.io/coding_notes/coding/python/python-list.md). # Python List ## 1. INTRODUCTION TO PYTHON’S LIST ### 1.1. What Is a List? * A list is a collection of items **in a particular order.** * In Python, square brackets `[]` indicate a list, and individual elements in the list are separated by commas. * List can contain heterogeneous data, means, your list can contain integer, strings, floats * **Naming Tip:** Because a list usually contains more than one element, it’s a good idea to make the name of your list plural, such as letters, digits, or names. Here is an example of list ```python wrestlers = ['John Cena','Rock','Rick Flair','Goldberg'] print(wrestlers) ``` ``` ['John Cena', 'Rock', 'Rick Flair', 'Goldberg'] ``` ### 1.2. Indexing (Accessing Elements in a List) To access an element in a list, provide the name of the list followed by the index of the item enclosed in square brackets. For example, we want to access the first element in the list `wrestlers` **Index starts from zero (0)**, therefore to access the first item in the list, provide `wrestlers[0]` ```python wrestlers = ['John Cena','Rock','Rick Flair','Goldberg'] print(wrestlers[0]) ``` ``` John Cena ``` We can also apply any method while printing the element. For example, as we have covered in part 1, let’s convert the string to upper case using `upper()` method ```python wrestlers = ['John Cena','Rock','Rick Flair','Goldberg'] print(wrestlers[0].upper()) ``` ``` JOHN CENA ``` ### 1.3. Position of Elements in the List For a list of `n` elements, the index level starts from `0` and ends at `n-1` In addition, Python has a special syntax for accessing the last element in a list. — by providing index value of `-1`. In this way, the index `-2` returns the second last item , the index `-3` returns the third last item, and so forth. Let’s fetch the last item from the list `wrestlers` by using `wrestlers[-1]` ```python wrestlers = ['John Cena','Rock','Rick Flair','Goldberg'] print(wrestlers[-1]) ``` ``` Goldberg ``` Therefore, the index values from left to right would be: ``` [0,1,2,3,4,5 ...] ``` And Index values from right to left would be: ``` [... -3,-2,-1] ``` ### 1.4. Using Individual Values from a List Let’s apply our knowledge up-to this point by pulling (my favorite) wrestler from the `wrestlers` list and compose a message using that value: ```python wrestlers = ['John Cena','Rock','Rick Flair','Goldberg'] message = f"My favorite wrestler is {wrestlers[1]}" print(message) ``` ``` My favorite wrestler is Rock ``` ## 2. CHANGING, ADDING, AND REMOVING ELEMENTS In this section, our focus is to learn technique to modify the elements of the list. Suppose we are storing favorite cars in the list `cars` Let’s print the list and also its data type: ```python cars = ['Honda','Toyota','BMW','Cadillac','Tesla'] print(cars) print(type(cars)) ``` ``` ['Honda', 'Toyota', 'BMW', 'Cadillac', 'Tesla'] ``` Now lets modify the element of this list, here are the possible cases: ### 2.1. Modifying Elements in a List I would like *to modify* the element `BMW` with `Mercedes` in my list. To do this, we will use the index position of the element `BMW` and then declare new value `Mercedes` at that position: ```python cars = ['Honda','Toyota','BMW','Cadillac','Tesla'] print(f"Before Modification: {cars}") cars[2] = 'Mercedes' print(f"After Modification: {cars}") ``` ``` Before Modification: ['Honda', 'Toyota', 'BMW', 'Cadillac', 'Tesla'] After Modification: ['Honda', 'Toyota', 'Mercedes', 'Cadillac', 'Tesla'] ``` ### 2.2. Adding Elements to a List Now, we would like *to add* a new car to list of `cars` #### a. Appending Elements *to the End of a List* When you append an item to a list using `.append()` method, the new element is added to the end of the list. ```python cars = ['Honda', 'Toyota', 'Mercedes', 'Cadillac', 'Tesla'] cars.append('Ford') print(cars) ``` ``` ['Honda', 'Toyota', 'Mercedes', 'Cadillac', 'Tesla', 'Ford'] ``` #### b. Inserting Elements *into a List* You can add a new element *at any position* in your list by using the `insert()` method.\*\* \*\* We can do this by specifying the index position of the new element and the value of the new item, inside the `( )` ```python cars = ['Honda', 'Toyota', 'Mercedes', 'Cadillac', 'Tesla'] cars.insert(0,'Ford') print(cars) ``` ``` ['Ford', 'Honda', 'Toyota', 'Mercedes', 'Cadillac', 'Tesla'] ``` ### 2.3. Removing Elements from a List We can *remove* element from the list based on its *position* or *value* #### a. Removing an Item Using the del Statement If you know the position of the item you want to remove from a list, you can use the `del` statement. Note, that `del` is a *statement* and not a *method* ```python cars = ['Honda', 'Toyota', 'Mercedes', 'Cadillac', 'Tesla'] del cars[-1] print(cars) ``` ``` ['Honda', 'Toyota', 'Mercedes', 'Cadillac'] ``` **💡Remember:** You can no longer access the value that was removed from the list after the `del` statement is used. #### b. Removing an Item Using the pop() Method Sometimes you want to use the value of an item after you remove it from a list. In such cases, we should use `pop()` method. To pop the last item, we can use `pop()` with empty `()` Here is a quick demonstration of this concept: ```python cars = ['Honda', 'Toyota', 'Mercedes', 'Cadillac', 'Tesla'] print(cars) popped_car = cars.pop() print(cars) print(popped_car) ``` ``` ['Honda', 'Toyota', 'Mercedes', 'Cadillac', 'Tesla'] ['Honda', 'Toyota', 'Mercedes', 'Cadillac'] Tesla ``` #### c. Popping Items from any Position in a List We can actually use pop() to remove an item in a list at any position by including the index position of the item we want to remove in parentheses `()` Let suppose, we would like to pop the first element in the list: ```python cars = ['Honda', 'Toyota', 'Mercedes', 'Cadillac', 'Tesla'] first_car = cars.pop(0) print(f"First car in the list is {first_car}.") ``` ``` First car in the list is Honda. ``` 💡 **del or pop() ?** * If you want to delete an item from a list and do not want to use that item later, use the `del` statement, * if you want to use an item after you remove it from the list, use the `pop()` method ### 2.4. Removing an Item by Value If you only know the value of the item you want to remove, you can use the `remove()` method. ```python cars = ['Honda', 'Toyota', 'Mercedes', 'Cadillac', 'Tesla'] print(cars) cars.remove('Tesla') print(cars) ``` ``` ['Honda', 'Toyota', 'Mercedes', 'Cadillac', 'Tesla'] ['Honda', 'Toyota', 'Mercedes', 'Cadillac'] ``` If you want to work with the value being removed, then you can first store it inside a variable ```python cars = ['Honda', 'Toyota', 'Mercedes', 'Cadillac', 'Tesla'] print(cars) electric_car = 'Tesla' cars.remove(electric_car) print(cars) print(f"Electric cars are not available in the country so I removed {electric_car} from your list") ``` ``` ['Honda', 'Toyota', 'Mercedes', 'Cadillac', 'Tesla'] ['Honda', 'Toyota', 'Mercedes', 'Cadillac'] Electric cars are not available in the country so I removed Tesla from your list ``` **💡 Remember:** The `remove()` method deletes only the first occurrence of the value you specify. If there’s a possibility that the value appears more than once in the list, you’ll need to use a `for` loop to determine if all occurrences of the value have been removed. We will study `for` loops later in part 3. ## 3. ORGANIZING A LIST In this section we will study few methods to organize the content of the list. ### 3.1. Sorting a List *Permanently* with the *sort()* Method Python’s `sort()` method makes it relatively easy to sort a list. In this example, ee will work with the list of `cars` and change the order of the elements in the list, to store them alphabetically. ```python cars = ['honda', 'toyota', 'mercedes', 'cadillac', 'tesla'] print(f"Before sort: {cars}") cars.sort() print(f"After sort: {cars}") ``` ``` Before sort: ['honda', 'toyota', 'mercedes', 'cadillac', 'tesla'] After sort: ['cadillac', 'honda', 'mercedes', 'tesla', 'toyota'] ``` We can also sort this list in reverse alphabetical order by passing the argument `reverse=True` to the `sort()` method. ```python cars = ['honda', 'toyota', 'mercedes', 'cadillac', 'tesla'] print(f"Before sort: {cars}") cars.sort(reverse=True) print(f"After reverse sort: {cars}") ``` ``` Before sort: ['honda', 'toyota', 'mercedes', 'cadillac', 'tesla'] After reverse sort: ['toyota', 'tesla', 'mercedes', 'honda', 'cadillac'] ``` **💡Remember** : The `sort()` method changes the order of the list permanently. ### 3.2. Sorting a List *Temporarily* with the *sorted()* Function The `sorted()` function lets you display your list in a particular order but *doesn’t affect the actual order of the list* ```python cars = ['honda', 'toyota', 'mercedes', 'cadillac', 'tesla'] print(f"Original list:\n {cars}") print(f"\nAfter sorted function is applied to the list:\n {sorted(cars)}") print(f"\nHere is the original list again:\n {cars}") ``` ``` Original list: ['honda', 'toyota', 'mercedes', 'cadillac', 'tesla'] After sorted function is applied to the list: ['cadillac', 'honda', 'mercedes', 'tesla', 'toyota'] Here is the original list again: ['honda', 'toyota', 'mercedes', 'cadillac', 'tesla'] ``` The `sorted()` function can also accept `reverse=True` keyword argument, if you want to display a list in reverse alphabetical order. ### 3.3. Printing a List in Reverse Order To reverse the original order of a list, you can use the `reverse()` method ```python cars = ['honda', 'toyota', 'mercedes', 'cadillac', 'tesla'] print(f"Before reverse: {cars}") cars.reverse() print(f"After reverse: {cars}") ``` ``` Before reverse: ['honda', 'toyota', 'mercedes', 'cadillac', 'tesla'] After reverse: ['tesla', 'cadillac', 'mercedes', 'toyota', 'honda'] ``` 💡`reverse()` method doesn’t reverse based on alphabetic order, it just reverse the order of items in the original list. \* The reverse() method changes the order of a list permanently\*, but you can revert to the original order by applying `reverse()` to the same list a second time. ### 3.4. Finding the Length of a List You can find the length of a list (that it, number of elements in the list) by using the `len()` function. ```python cars = ['honda', 'toyota', 'mercedes', 'cadillac', 'tesla'] print(f"There are total {len(cars)} elements in the list") ``` ``` There are total 5 elements in the list ``` ### 3.5. IndexError An `IndexError` means Python can’t figure out the index we have asked for. Let’s say you have a list with five items, and you ask for the sixth item: ```python cars = ['honda', 'toyota', 'mercedes', 'cadillac', 'tesla'] print(cars[5]) ``` ``` IndexError: list index out of range ``` 💡If an `IndexError` occurs and you can’t figure out how to resolve it, try printing your list or just printing the length of your list.