More with lists!
- Press O or Escape for overview mode.
- Visit this link for a nice printable version
- Press the copy icon on the upper right of code blocks to copy the code
Class outline:
- Review
- More list functionality
- Nested lists
- Getting data from CSV files
Review
Data structures
A data structure is a particular way of organizing data in a computer so that it can be used effectively.
Data structures in programming are a bit like lists and tables in text docs/presentations - we use them to organize information and make it more meaningful/useful.
Lists
A list is a container that holds a sequence of related pieces of information.
The shortest list is an empty list, just 2 square brackets:
members = []
Lists can hold any Python values, separated by commas:
# all strings
members = ["Liz", "Tinu", "Brenda", "Kaya"]
# all integers
ages_of_kids = [1, 2, 7]
# all floats
prices = [79.99, 49.99, 89.99]
# mixed types
mixed_types = ["Liz", 7, 79.99]
Accessing items (Bracket notation)
Lists are ordered. Each list item has an index, starting from 0.
temps = [48.0, 30.5, 20.2, 99.0, 52.0]
# Index: 0 1 2 3 4
Access each item by putting the index in brackets:
temps[0] # 48.0
temps[1] # 30.5
temps[2] # 20.2
temps[3] # 99.0
temps[4] # 52.0
temps[5] # 🚫 Error!
List length
Use the global len() function
to find the length of a list.
attendees = ["Tammy", "Shonda", "Tina"]
print(len(attendees)) # 3
num_of_attendees = len(attendees)
print(num_of_attendees)
Modifying list items
Python lists are mutable, which means you can change the values stored in them.
Use bracket notation to modify single items in the list:
temps = [48.0, 30.5, 20.2, 99.0, 52.0]
temps[2] = 22.22
temps[-1] = 55.55
Modifying lists with methods
| Method | Description |
|---|---|
append(item)
| Adds item to the end of the list. This increases list length by one.
|
insert(index, item)
| Inserts item at the specified index. This increases the list length by one and
shifts all items after the specified index forward by one index.
|
remove(item)
| Remove the first item from the list whose value is equal to item.
It raises a ValueError if there is no such item.
|
index(item)
| Returns the index of the first occurrence of an item whose value is equal to item.
It raises a ValueError if there is no such item.
|
pop([index])
| If no index is specified, it removes the last item from the list and returns it.
Otherwise, it removes the item at the specified index and returns it.
|
See more in Python.org documentation
List method return values
Most of the list mutation methods perform an action and don't return a value.
An exception is .pop(), which removes a list item and returns its value, so you can assign it to a variable.
groceries = ["apples", "bananas"]
bought_food = groceries.pop()
For loop
The other type of loop supported in Python is a for loop
The for loop syntax:
for <value> in <sequence>:
<statement>
<statement>
For loops provide a clean way of repeating code for each item in a data structure (like a list).
Looping through a list
😍 For loops love lists!
scores = [80, 95, 78, 92]
total = 0
for score in scores:
total += score
This does the same thing as a while loop with a counter, but with less code.
scores = [80, 95, 78, 92]
total = 0
i = 0
while i < len(scores):
score = scores[i]
total += score
i += 1
Homework review!
More list functionality
Checking if value is in a list
The in keyword checks if
a value is in a list (or any sequence).
if "bananas" in groceries:
print("Watch your step!")
if "bleach" in groceries:
print("Watch what you eat!")
When used inside if conditions, in expressions return a Boolean (True/False).
Helpful list methods
| Method | Description |
|---|---|
count(item)
| Returns the number of occurrences of item in the list. If item is not found, returns 0.
|
reverse()
| Reverses the elements of the list in place. |
sort([key=function])
| Sorts the items of the list in place. The optional key argument specifies
a function to compute a comparison key for each item in the list. Otherwise, items are compared based on their values.
|
See more in Python.org documentation
Examples with reverse
If we start with...
nums = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
spelling = ["B", "U", "M", "B", "L", "E"]
Then call the reverse method...
nums.reverse() # [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
spelling.reverse() # ["E", "L", "B", "M", "U", "B"]
What do you think of this?
nums = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
reversed_nums = nums.reverse()
It's a bit strange, since reverse()
does an in-place reversal, so now the program has the same list in two variables!
Examples with sort
foods = ["orange", "apple", "Pear", "banana"]
The simplest sort:
foods.sort()
# ["Pear", "apple", "banana", "orange"]
Specifying a key argument of an existing function:
foods.sort(key=str.lower)
# ["apple", "banana", "orange", "Pear"]
.sort() modifies a list in place. To sort a list into a new list, use the global sorted() function.
Useful functions for lists
These are functions, not methods.
| Module | Function | Description |
|---|---|---|
| Global | min(list, [key=function])
| Returns the minimum value in list. The optional key argument specifies a custom comparator function.
|
| Global | max(list, [key=function])
| Returns the maximum value in list. he optional key argument specifies a custom comparator function.
|
random
| choice(list)
| Returns a randomly selected item from list.
|
random
| shuffle(list)
| Shuffles the elements of the list in place. |
Nested Lists
Nested Lists
Since Python lists can contain any values, an item can itself be a list.
gymnasts = [
["Brittany", 9.15, 9.4, 9.3, 9.2], # gymnasts[0]
["Lea", 9, 8.8, 9.1, 9.5], # gymnasts[1]
["Maya", 9.2, 8.7, 9.2, 8.8] # gymnasts[2]
]
You can think of nested lists like rows in a table:
Nested Lists
gymnasts = [
["Brittany", 9.15, 9.4, 9.3, 9.2], # gymnasts[0]
["Lea", 9, 8.8, 9.1, 9.5], # gymnasts[1]
["Maya", 9.2, 8.7, 9.2, 8.8] # gymnasts[2]
]
- What's the length of
gymnasts? 3 - What's the length of
gymnasts[0]? 5
Accessing nested list items
gymnasts = [
["Brittany", 9.15, 9.4, 9.3, 9.2],
["Lea", 9, 8.8, 9.1, 9.5],
["Maya", 9.2, 8.7, 9.2, 8.8]
]
Access using bracket notation, with more brackets as needed:
gymnasts[0] # ["Brittany", 9.15, 9.4, 9.3, 9.2]
gymnasts[0][0] # "Brittany"
gymnasts[1][0] # "Lea"
gymnasts[1][4] # 9.5
gymnasts[1][5] # 🚫 IndexError!
gymnasts[3][0] # 🚫 IndexError!
Modifying nested lists: Bracket notation
gymnasts = [
["Brittany", 9.15, 9.4, 9.3, 9.2],
["Lea", 9, 8.8, 9.1, 9.5],
["Maya", 9.2, 8.7, 9.2, 8.8]
]
Modify using bracket notation as well, just like before:
gymnasts[0] = ["Olivia", 8.75, 9.1, 9.6, 9.8]
gymnasts[1][0] = "Leah"
gymnasts[2][4] = 9.8
Modifying nested lists: List methods
If we start with...
gymnasts = [
["Brittany", 9.15, 9.4, 9.3, 9.2],
["Lea", 9, 8.8, 9.1, 9.5]
]
We can append another list a few ways:
gymnasts.append(["Maya", 9.2, 8.7, 9.2, 8.8])
gymnasts += ["Maya", 9.2, 8.7, 9.2, 8.8]
gymnast = []
gymnast.append("Maya")
gymnast.append(9.2)
gymnast.append(8.7)
gymnast.append(9.2)
gymnast.append(8.8)
gymnasts.append(gymnast)
Looping through nested lists
gymnasts = [
["Brittany", 9.15, 9.4, 9.3, 9.2],
["Lea", 9, 8.8, 9.1, 9.5],
["Maya", 9.2, 8.7, 9.2, 8.8]
]
Use a nested for-in loop:
for gymnast in gymnasts:
print(gymnast)
for data in gymnast:
print(data)
Remember what type of data is being stored in the loop variable!
Exercise: Nested for loops
Reading data from CSV files
CSV files
Comma Separated Value (CSV) is a plain text format used to store "tabular" data (aka rows and columns), like from a spreadsheet or database table.
Because CSV is simple and non-proprietary, it's commonly used to exchange data between different applications.
Brittany, 9.15, 9.4, 9.3, 9.2
Lea, 9, 8.8, 9.1, 9.5
Maya, 9.2, 8.7, 9.2, 8.8
Opening files
We use the Python built in open() method to open files for reading or writing.
The keyword with tells Python to automatically close the file when we're finished.
with open('filename') as my_file:
# do something
Reading files with CSV module
Python has a built-in CSV module that includes many features used for reading/writing CSV files.
Open a CSV and read its contents
# import the csv module
import csv
# open the file
with open('fun_spreadsheet.csv') as my_file:
# load the contents of the CSV file into a variable
csv_reader = csv.reader(my_file, delimiter=',')
# CSV is a nested list!
# now we can loop through the rows in the file
for row in csv_reader:
print(row)
Reading files with CSV module
A CSV is a list of rows, which are lists of values.
We can use nested for loops to get values from rows and cells.
import csv
with open('fun_spreadsheet.csv') as my_file:
csv_reader = csv.reader(my_file, delimiter=',')
for row in csv_reader:
print(row)
for cell in row:
print(cell)