# Collections and Data Structures > In computer programming, a collection is a grouping of some variable number of data items (possibly zero) that have some shared significance to the problem being solved and need to be operated upon together in some controlled fashion. \[[1](#references)] This page provides syntax for different types of collections and data structures in Python (arrays, sets, dictionaries, etc.). Each section includes an example to demonstrate the described methods ## Arrays Arrays are ordered collections of elements. In Python they are automatically indexed (consecutively numbered) by an integer starting with 0. | Action | Syntax | | ---------------------------- | --------------------- | | New array (empty) | `[]` | | Array with values (integers) | `[1, 2, 3, 4, 5]` | | Array with values (string) | `[“a1”, “ab2”, “c3”]` | | Array of numbers | `list(range(1, 11))` | ### Creating Array From String | Action | Syntax | | ------------------------------------------------------ | ---------------- | | Split string str by delimiter into words (e.g., space) | `str.split(“ “)` | ### Accessing Elements | Action | Syntax | | -------------------------------------------- | ----------------- | | Get length of array `my_array` | `len(my_array)` | | Get first element of array `my_array` | `my_array[0]` | | Get last element of array `my_array` | `my_array[-1]` | | Get nth element of array `my_array`(e.g., 2) | `my_array[1]` | | Check if element is in array | `str in my_array` | ### Adding and Removing Elements | Action | Syntax | | ----------------------------- | ------------------------- | | Add element to end | `my_array.append(str)` | | Remove element from end | `my_array.pop()` | | Remove element from beginning | `my_array.pop(0)` | | Add element to beginning | `my_array.insert(0, str)` | ### Sort and Unique | Action | Syntax | | ----------------------------------------- | --------------------- | | Sort array (will not change array itself) | `sorted(my_array)` | | Sort array in place (will change array) | `my_array.sort()` | | Get unique elements in array | `list(set(my_array))` | ### Compare Arrays
ActionSyntax
Intersectionset(my_array).intersection(your_array)
Unionset(my_array).union(your_array)
Input: ```python # Initialize the list and day variable day_array = ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday"] day = "Thursday" # Get the array length and specific days array_length = len(day_array) array_first_day = day_array[0] # Indexing in Python starts at 0 array_last_day = day_array[-1] # Python's negative indexing for last element # Print information about the array print(f"Length of array: {array_length}") print(f"First day of week: {array_first_day}") print(f"Third day of week: {day_array[2]}") print(f"Last day of week: {array_last_day}") # Check if the day is in the array print(f"{day} is in {day_array}: {day in day_array}") # Add "Sunday" to the beginning and "Saturday" to the end day_array.insert(0, "Sunday") day_array.append("Saturday") # Print each element in the array print("Day of week:") for day in day_array: print(f" {day}") # Join array elements with ";" and print print(f"Day of the week: {';'.join(day_array)}") # Sort the array and print again day_array.sort() print(f"Day of the week (sorted): {';'.join(day_array)}") ``` Output: ```python Length of array: 5 First day of week: Monday Third day of week: Wednesday Last day of week: Friday Thursday is in ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday']: True Day of week: Sunday Monday Tuesday Wednesday Thursday Friday Saturday Day of the week: Sunday;Monday;Tuesday;Wednesday;Thursday;Friday;Saturday Day of the week (sorted): Friday;Monday;Saturday;Sunday;Thursday;Tuesday;Wednesday ``` ## Sets Sets are an unordered collection of unique elements. ### Creating Sets | Action | Syntax | | --------------- | ----------------------------- | | New set (empty) | `[]` | | Set with values | `my_set = {1, 2, 3, 4, 5}` | | Set with values | `my_set = {"a1", "b2", "c3"}` | ### Interacting With Sets | Action | Syntax | | ------------------------- | ------------------- | | Get length of set my\_set | `len(my_set)` | | Check if value is in set | `"str" in my_set` | | Add value | `my_set.add("str")` | ### Comparing Sets | Action | Syntax | | ------------ | ------------------------------- | | Intersection | `my_set.intersection(your_set)` | | Union | `my_set.union(your_set)` | | Difference | `my_set.difference(your_set)` | Input: ```python color_set = {"red", "yellow", "blue"} color_set2 = {"red", "orange", "yellow"} print("Length of set:", len(color_set)) print("Color Set 1") for color in color_set: print(" ", color) print("Color Set 2:", "---".join(color_set2)) print("Intersection:", color_set.intersection(color_set2)) print("Union:", color_set.union(color_set2)) print("Difference:", color_set.difference(color_set2)) print("Difference:", color_set2.difference(color_set)) ``` Output: ```python Length of set: 3 Color Set 1 yellow blue red Color Set 2: yellow---orange---red Intersection: {'yellow', 'red'} Union: {'yellow', 'orange', 'blue', 'red'} Difference: {'blue'} Difference: {'orange'} ``` ## Dictionaries Dictionaries are unordered collection of key-value pairs where the key serves as the index (“associative collection”). Similar to elements of a set, keys are always unique. ### Creating Dictionaries
ActionSyntax
New Dictionary (empty){}
Dictionary with values{"one": 1, "two": 2, "three": 3, "four": 4}
### Accessing Dictionaries | Action | Syntax | | ----------------------------------------- | -------------------------------------------------------------------------------------------- | | Get value for key in dictionary `my_dict` | `my_dict["one"]` | | Check if dictionary has key | `"one" in my_dict` | | Check for key/value pair | `("one", 1) in my_dict.items()` | | Get value and set default |

my\_dict.get("one", 5)

my\_dict.setdefault("five", 5)

| | Add key/value pair | `my_dict["five"] = 5` | | Delete key/value pair | `my_dict.pop("four", None)` | | Get keys | `my_dict.keys()` | | Get values | `my_dict.values()` | ### Converting Dictionaries | Action | Syntax | | ----------------------- | ------------------------ | | Convert keys to array | `list(my_dict.keys())` | | Convert values to array | `list(my_dict.values())` | ### Sorting Dictionaries
ActionSyntax
Sorting keyssorted(my_dict.keys())
Sorting valuessorted(my_dict.values())
Sort by value (descending) with keyssorted(my_dict.items(), key=lambda x: x[1], reverse=True)
Sort by value (ascending) with keyssorted(my_dict.items(), key=lambda x: x[1])
Get top n by value (e.g., 3)sorted(my_dict.items(), key=lambda x: x[1], reverse=True)[:3]
Input: ```python day_dict = {} day_length_dict = {} day_dict["Mon"] = "Monday" day_dict["Tue"] = "Tuesday" day_dict["Wed"] = "Wednesday" day_dict["Thu"] = "Thursday" day_dict["Fri"] = "Friday" if "Wed" in day_dict: print(day_dict["Wed"]) if "Sat" not in day_dict: print('no key "Sat"') print("print key-value pairs") for day in day_dict.keys(): print(f" {day} = {day_dict[day]}") print("print values (sorted)") for day_value in sorted(day_dict.values()): print(f" {day_value}") # get length of each value and keep track of lengths for day_value in day_dict.values(): day_length = len(day_value) day_length_dict[day_value] = day_length print("print lengths") for day in day_length_dict.keys(): print(f" {day} = {day_length_dict[day]}") print("print lengths in descending order") for length, day in sorted(zip(day_length_dict.values(), day_length_dict.keys()), reverse=True): print(f" {day} = {length}") print("print lengths in ascending order") for length, day in sorted(zip(day_length_dict.values(), day_length_dict.keys()), reverse=False): print(f" {day} = {length}") ``` Output: ```python Wednesday no key "Sat" print key-value pairs Mon = Monday Tue = Tuesday Wed = Wednesday Thu = Thursday Fri = Friday print values (sorted) Friday Monday Thursday Tuesday Wednesday print lengths Monday = 6 Tuesday = 7 Wednesday = 9 Thursday = 8 Friday = 6 print lengths in descending order Wednesday = 9 Thursday = 8 Tuesday = 7 Monday = 6 Friday = 6 print lengths in ascending order Friday = 6 Monday = 6 Tuesday = 7 Thursday = 8 Wednesday = 9 ``` ## References 1. Wikipedia contributors (n.d.). Collection. In Wikipedia. Retrieved May 1, 2024, from ## Resources * W3 Schools: [Python Data Structures](https://www.geeksforgeeks.org/python-data-structures/) * Data Quest: [Python Data Structures](https://www.dataquest.io/blog/data-structures-in-python/)