3.1 Variables and Assignments

Variables

What is a variable?

  • An abstraction inside a program that can hold value/store information
  • Note: a variable should be concise but not too short (i.e. one letter), and it shouldn’t have spaces or any special characters
    • Ex: “highScore” is more concise than “highestscoreinthegame”
    • Ex: “name” is more specific than “n”
    • Ex: “isitraining” doesn’t have spaces in between the words
    • Ex: “phonenumber” doesn’t have a hyphen like “555-number”

Examples

numStudents = 26
print(numStudents)

car = "Tesla"
print(car)

26
Tesla

Popcorn Hack 1

Define your own variable and print it in a separate code cell using Python. Make it relate to one of your hobbies or interests.

Data Types

A data type is a type of variable, and these are 3 of the 4 main types:

  • integer: a mathematical number
  • boolean: a yes/no or true/false statement
  • text (or string): for any text-based variable (including phone numbers)

Examples

  • Note that for Python you need to classify anything other than a string as a string (or “str”) in order to print
  • the following examples represent the storing of data within a variable
highScore = 9 
isRaining = False
firstName = "Nikki"
print(str(highScore) + " is an integer")
print(str(isRaining) + " is a boolean")
print(firstName + " is a string")

9 is an integer
False is a boolean
Nikki is a string

Popcorn Hack 2

Define a string, boolean, and integer. Print all 3 together like so.

Changing Values

A variable itself isn’t static, so you can change it to give different or more updated information. An example of this is having a variable that calculates changing age.

Examples

The following examples shows how you can replace one variable with another. See if you can figure out what data will be printed out before it runs!

currentScore = 10
highScore = currentScore
currentScore = 7
print(str(highScore))

10

num1 = 25
num2 = 15
num3 = 30
num2 = num3 
num3 = num1
num1 = num2
print(str(num1))
print(str(num2))
print(str(num3))

30
30
25

Popcorn Hack 3

Write a program to equate the value of a first variable to a second variable. Print the second variable, and you should see the same content as the first variable.

3.2 Data Abstraction

New Types of Variables

  • Strings
    • an ordered sequence of characters
    • may contain letters, numbers, and all other special characters
    • Ex: words, phrases, sentences, ID numbers
  • Lists
    • an ordered sequence of elements
    • each element is a variable
    • Ex: playlist of songs, names of students in a class, contacts in your phone

List Index

  • Each element of a string is referenced by an index
  • For the AP test an index will start at 1 (example image below), but in actuality it starts at 0 (the following examples)

listindex.png

Examples

groupNames = ["Nikki", "Ankit", "Varun", "Monika"]
print(groupNames[0])
print(groupNames[1])
print(groupNames[2])
print(groupNames[3])

Nikki
Ankit
Varun
Monika

Popcorn Hack 4

Create a list with your current schedule of classes. Print only the 2nd class in your schedule.

Data Abstraction - Lists

  • lists allow for data abstraction
    • bundle variables together
      • strings, numbers, characters, etc.
    • give one name to a set of memory cells
      • do not need to know how many variables will be needed
    • do not need to know how the elements are stored together

Examples

The following example showcases the concept of changing values but with lists instead.

scores1 = [89, 72, 34, 56, 83]
scores2 = [23, 25, 96, 55]
scores1 = scores2
print(str(scores1))

[23, 25, 96, 55]

How Lists Manage Complexity of a Program

  • May not need as many variables (improves readability)
    • Ex: variable for each student is more tedious than one variable that holds all students
  • Change the number of variables
    • Ex: if a student transfers in/out of school, you won’t need to add/delete an entire variable (you’re just deleting one element from a list)
  • Consistent computations
    • Ex: a list of test scores can be curved with the same calculation for all scores (you can do it to everything all at once)

Examples

The following example showcases the convenience a list offers. The first series of scores would require more time to print out versus the second version (in list form). We would be able to curve the scores of a class all at once more easily using the second version.

score1 = 23
score2 = 35
score3 = 25
score4 = 44
score5 = 56
score6 = 78
score7 = 62
score8 = 92
score9 = 94
score10 = 47

scores = [23, 35, 25, 44, 56, 78, 62, 92, 94, 47]
print(str(scores))

[23, 35, 25, 44, 56, 78, 62, 92, 94, 47]

Converting Data Types to and from JSON

This is useful for converting Python lists containing integers to a string. It can be used when connecting backend code (Python) to frontend code (JavaScript)

The following shows us assigning a variable “a” to a list of integers that has been converted to a string using json.dumps

import json
lst = [1,2,3,4]
print(type(lst))
a = json.dumps(lst)
print(a)
print(type(a))

<class 'list'>
[1, 2, 3, 4]
<class 'str'>

Popcorn Hack 5

Convert a list of integers in Python to a string using JSON. Assign a new variable to the newly converted string and print it. Also print the type of the new variable to ensure it is a string.

Hack 1

Sort the variables in the code cell below into these categories and write the adjoining numbers for each type:
  • Integer:
  • List:
  • Boolean:
  • String: 
# Variable 1

numStudents = 26
print(numStudents)

# Variable 2

car = "Tesla"
print(car)

# Variable 3

groupMates = ["Nikki", "Monika", "Ankit", "Varun"]
print(groupMates)

# Variable 4

dogsbeatcats = True
print(dogsbeatcats)

26
Tesla
['Nikki', 'Monika', 'Ankit', 'Varun']
True

Hack 2

Now in a separate code cell, define one variable for each type above! Make them unique to your interests and be creative. For your list, convert it to a string using JSON and print it out.