Variables

Subsection 1.6.1 Identifying Variables in Apps

Essential Knowledge 1.6.1 1.6.A.1.

A variable is a named storage location used to hold a value that can change while the program is running. It is an abstraction that lets programmers use a name to represent a value.

Essential Knowledge 1.6.2 1.6.A.2.

Apps use variables to keep track of changing information by updating their values as the user interacts with the app. Programs can then use a variable’s current value to decide what to do next and what feedback to provide, such as updating the screen, playing a sound, or triggering a vibration.

Try the following pizza web app by typing in your name and choosing different options. Think about the inputs and outputs to the app. When you type in your name and other inputs, your information gets saved in variables in the program. These values can then be used in the outputs of the app.

Activity 1.6.1.

Interact with the pizza app above. Which of the following are variables that the app above might use to store information about the user.

name
The app needs to store the user’s name to display it in the output.
favoriteColor
The app needs the favorite color to change the background to that color.
address
The app does not need the user’s address to function, so it would not use a variable to store this information.
age
The app needs to store the user’s age to display it in the output.
temperature
The app does not need to know the temperature to function, so it would not use a variable to store this information.
likesPizza
The app needs to store whether the user likes pizza to display it and pizza images in the output.

Essential Knowledge 1.6.3 1.6.A.5.

Variables are often used to store information about the user or a program’s state, such as a username, a selected theme, a score, progress toward a goal, a shopping cart total, or whether notifications are on. The program can use the stored value to adjust its behavior or display the value to the user.

Essential Knowledge 1.6.4 1.6.A.6.

Program state is the set of values and information a program is storing at a specific moment, including variable values and any inputs it is using. As the program runs, the state changes when inputs are received and variables are updated.

Some video games keep a list of players with the highest scores in the game. Each entry in the list a snapshot of the program state at a certain time. For example, the player with the name or initials "AKI" had a score of 12,000 at a certain point.

Essential Knowledge 1.6.5 1.6.A.7.

Users can infer the use of variables in an app by observing information that changes over time, such as counters, totals, settings, progress, or status, and reasoning about what values the app must store and update to produce those changes.

Activity 1.6.2.

Subsection 1.6.2 Variables in Python

Variables are names for memory locations that store values used in a program. In Python, a variable is created when a value is assigned to it. For example, the following code creates a variable named name and assigns the value "Dani" to it and a variable named age with the value 16. Note that text values like "Dani" are placed in quotes (""), but numbers like 16 are not.

# variable = value
name = "Dani"
age = 16

In Python, multiple values can be printed out separated by commas. The variables, name and age, are never put inside quotes because we do not want to literally print the variable names; we want to print the values stored in the variables.

print("Hello", name, "you are", age, "years old")

Activity 1.6.3.

Run the following code. Change the variable values and run again.

Activity 1.6.4.

Drag or click on the blocks you need to move them from the top section into the yellow area to create a print statement with the variable name. For example, if name = "Alex", it will print "Hello Alex!". There are extra blocks that you don’t need.

Subsection 1.6.3 Data Types

The pizza app above stores lots of different types of data: the user’s name, their age, their favorite color, and whether they like pizza. The user’s name is text (which is called a string in coding, but the age is a number. Whether they like pizza or not could be stored as a Boolean value of true or false (the type Boolean is named after the mathematician George Boole who invented symbolic logic). These are all different data types.

Essential Knowledge 1.6.6 1.6.A.3.

Every value in a program has a data type, such as a number, Boolean, or string. Some programming languages require programmers to specify the type, while others determine it automatically by how the data is used.

Essential Knowledge 1.6.7 1.6.A.4.

The choice of data type depends on what a value represents. Values used to count or measure are best represented as numbers, values that record a true or false condition are best represented as Booleans, and values that store text are best represented as strings. The data type affects what operations can be performed on the variable and how accurately the output is represented in an app.

In some languages, you have to declare the data type of a variable, but in Python, the data type is assigned automatically depending on its value during run time. Python has several built-in data types, including:

int - for integers (whole numbers)
float - for floating-point numbers (decimals)
str - for strings (text)
bool - for boolean values (True or False)

name = "Dani"   # type string
age = 16        # type int

You can check the data type of a variable using the type() function. For example:

print(type(name)) # This will print str, for a string or text data type
print(type(age)) # This will print int for an integer whole number

Activity 1.6.5.

Run the following code to see the data types of the variables. Add another print to see the type of the last variable which is a number inside quotes. You’ll see that anything in quotes is a string, even if it is a number inside.

Subsection 1.6.4 Naming variables

Although you can name a variable almost anything you want, there are some rules and conventions to follow. In Python, variable names must follow these rules:

It must start with a letter or an underscore _.
It can contain digits, like 1 or 9, but not as the first character.
It cannot have spaces, or special symbols other than _
It cannot be a Python keyword. Keywords are words that have special meaning in the language(see below for examples).
Case matters. A variable named result is not the same as one named Result.
Use meaningful variable names that describe what the variable holds, not letters like x,y,z.

Essential Knowledge 1.6.8 1.6.B.3.

Using meaningful variable names improves readability and helps programmers understand what the stored values represent.

Python has a few dozen keywords that you can’t use as variable names. Here is a list of the most common ones. If you ever have an error based on one of your variable names and do not know why, compare your name to this list to make sure you are not using a keyword as your variable name.

Table 1.6.9. Python keywords

and	as	assert	break	class	continue
def	del	elif	else	except	exec
finally	for	from	global	if	import
in	is	lambda	nonlocal	not	or
pass	raise	return	try	while	with
yield	True	False	None

Activity 1.6.6.

Which of the following is not a legal variable name in Python?

my name
Right, you can’t have a space in a variable name.
my_name
You can use an underscore between words in a variable name.
_a1
Although it is not very common, you can use an underscore as the first character in a variable name
amountOfStuff
You can use both uppercase and lowercase letters in a variable name.
1A
Correct, you can’t use a digit as the first letter in a variable name.

Subsection 1.6.5 Variables and Assignment

Essential Knowledge 1.6.10 1.6.B.1.

The assignment operator = allows a program to initialize or change the value stored in a variable. When the following code runs, expression is evaluated, and the resulting value is stored in the variable a. a = expression

Essential Knowledge 1.6.11 1.6.B.2.

Variables are initialized by assigning an initial value before they are used. After initialization, later statements can use the variable’s value and update it as the program runs.

An assignment statement assigns a value to a variable using the assignment operator =. The first assignment to a variable initializes the variable to its first value. It can then be changed with other assignment statements. For example, the variable score is initialized to 0 and then changes to 50 below. Note that the variable name must match exactly with the same capitalization and spelling to have it refer to the same variable.

score = 0
score = 50

Activity 1.6.7.

Python variables are case sensitive so gameScore and gamescore are not the same. Run and fix the code below to use the right variable name.

Essential Knowledge 1.6.12 1.6.B.4.

A variable stores its most recently assigned value.

Essential Knowledge 1.6.13 1.6.B.5.

The value stored in a variable after a sequence of assignment statements can be determined by tracing statements in order and updating the variable’s value after each assignment.

Activity 1.6.8.

Essential Knowledge 1.6.14 1.6.B.6.

Assignment statements are often used to update information an app remembers about the user or the program’s state, such as a score or setting, so the app responds appropriately as the user interacts with it.

Subsection 1.6.6 Strings

In coding, a string is a sequence of characters surrounded by quotes (""). Imagine strings with lined up letter beads like below.

Essential Knowledge 1.6.15 1.6.C.1.

A string is an ordered sequence of characters used to represent text in a program.

In the web app above, the text that the use types in like name was probably saved as strings.

Essential Knowledge 1.6.16 1.6.C.2.

Apps often use strings to store and display user-facing information, such as names, messages, labels, or search terms.

Essential Knowledge 1.6.17 1.6.C.3.

To initialize a string variable, programmers assign it an initial text value or an empty string before it is used so the program starts with a known value. An empty string has zero characters (""). It is not the same as a string that contains spaces (" ").

Variables can be assigned values or an empty string "" that will be filled in later. Python allows double quotes "" or single quotes ’ for string values.

name = "Sam"                 # double quotes
email = '[email protected]'      # single quotes
message = ""                 # empty string

Essential Knowledge 1.6.18 1.6.C.4.

String concatenation joins two or more strings to create a new string, such as combining a greeting with a user’s name.

We can concatenate or append or add strings in Python using the operator +, which can be used to add numbers or strings. Blank spaces are not automatically added when you use + (unlike ,), so if you want a blank space, you need to type it inside the quotes or add in a string with just a space in it " " as shown below.

name = "Sam"
message = "Hello " + name     # note the space after Hello
print(message)

Activity 1.6.9.

Run the following code to see the smushed output. Fix the error by putting in an extra space and concatenate an ! at the end.

Since the operator + is used to add numbers or concatenate strings in Python, the interpreter will get confused and show an error if you use it to concatenate a string with a number. To concatenate the number with a string we need to convert the number into a string first. The str(num) function will convert a number into a string. Another option is to use the , instead of + in print statements because that will print different data types with no problem, but it can only be used in print statements.

Activity 1.6.10.

Run the following code to see the error. Fix the error by using str to convert the number.

Essential Knowledge 1.6.19 1.6.C.5.

A substring is part of an existing string. Programs can create substrings to extract or use part of a text value, such as extracting the month from a date, reading a portion of a message, or checking for a word within a string.

In Python, every character in a string is associated with an index starting with 0. Square brackets and an index can pull out any character from a string.

Python has a special slicing operator to pull out a substring from a start index up to an end index (but not including the end index), message[start:end].

message = "Python is fun!"
letter = message[0]          # pull out 0th letter P
substring = message[0:6]     # substring "Python" from index 0 up to index 6

Activity 1.6.11.

Run the following code to see examples of slicing. Add to the code to print another substring of message that uses slicing to pull out the word "fun".

Essential Knowledge 1.6.20 1.6.C.6.

The output of code that uses string operations can be determined by tracing statements in order and keeping track of how string values change and what text is displayed.

Activity 1.6.12.

Essential Knowledge 1.6.21 1.6.C.7.

To communicate clearly with users, programmers use strings to store and generate text that apps display, such as prompts, feedback messages, and personalized responses.

Subsection 1.6.7 Input

Python has an input function that can be used to get input typed in by user. This input must be saved into a variable. This is another way to initialize or assign to a variable. The syntax for an input statement is:

variable_name = input("Input prompt or question? ")

The following code will ask the user for their name and then print out a greeting. Try it out by clicking the "Run" button. Run it twice and put in different names! Does it work? Yes, that is the power of abstraction! Our code is now more general or abstract and can work for any name! The input variable is an abstraction that can hold any name.

Activity 1.6.13.

Run the following code. Enter your name in the pop up input box and then scroll down to see the output.

Activity 1.6.14.

Drag or click on the blocks you need to move them from the top section into the yellow area to create an input statement that will ask for the user’s age. There are extra blocks that you don’t need.

The input() function in Python always returns a string, even if the user enters a number. For example, if the user enters "25", the variable will store the string "25", not the integer number 25. However, to do math with a number, as we will see in the next lesson, we need to store the value as a number rather than a string.

To convert a string to an integer whole number or a floating point decimal number, use the int() or float() functions. We will use these functions in the next lesson when performing calculations and making comparisons with user input. For example:

ageStr = input("Enter your age: ")
age = int(ageStr) # Convert the string to an integer

To convert a number back into a string, you can use the str() function. For example:

ageString = str(age) # Convert the integer back to a string

Activity 1.6.15.

Run the following code to see that there are some errors caused by the data types. Try using int or str functions to convert the variables to the correct data type.

Subsection 1.6.8 Coding Challenge: Story Project

Let’s make a poem or a story using input and variables in Python. Ask the user to input different nouns and verbs, and weave together a story.

Project 1.6.16 Story.

Finish the input statements below to ask the user for 2 colors and a food item. Run to see the silly poem. Then, ask the user for more input words and create your own poem or story using the variables in print statements.

Remember: Write the questions or prompts in your code, not the user’s answers. Enter the answers only when you run and test your program.

# Get user input
pluralnoun1 = input("Enter a plural noun: ")
pluralnoun2 = input("Enter another plural noun: ")
# Complete 3 input statements below with prompts or questions
color1 = input( )
color2 =
food =
# create 2 more variables and input statements

# Run to see the silly poem
print("Here's my silly poem!")
print("Roses are " + color1)
print(pluralnoun1 + " are " + color2)
print("I like " + food)
print("Do " + pluralnoun2 + " like them too?")
# Add at least 2 more lines to the poem
# using print and your last 2 variables
====
from unittest.gui import TestCaseGui

class myTests(TestCaseGui):
      def findLine(self, lines, target):
          for line in lines:
              if target.replace(" ", "") in line.replace(" ", ""):
                  #print(line)
                  return line
          return "None"
              
      def run_file(self, code, inputs):
          stars = "****************************************************************************" 
          print(stars)
          print("Testing code with inputs: " + inputs)
          print(stars)

lines = code.split("\n")
          ins = inputs.split(",")
          inN = 0

for i in range(len(lines)):
              if "input" in lines[i] and not "#" in lines[i]:
                  equals = lines[i].find("=") + 1
                  if inN < len(ins):
                      lines[i] = lines[i][0:equals] + "\"" + ins[inN].strip() + "\""
                      inN += 1

code = "\n".join(lines)

# Create an isolated scope for the code execution
          scope = {}
          exec(code, scope)
          output = self.getOutput()

ind = output.find(stars) + len(stars) + 1
          output = output[ind:]
          ind = output.find(stars) + len(stars) + 1
          output = output[ind:]

return str(scope), output.strip()
      
      def test1(self):
          code = self.getEditorText()
          lines = code.replace(" ", "").split("\n")

input_count = code.count("input(")
          self.assertLessEqual(7,input_count, "at least 7 input statements")

print_count = code.count("print(")
          self.assertLessEqual(7,print_count, "at least 7 print statements")

inputs = "cats,dogs,red,blue,pizza,added_input1,added_input2,added_input3,added_input4,added_input5,added_input6"
          scope, output = self.run_file(code, inputs)

num_lines = len(output.split("\n"))
          self.assertGreaterEqual(num_lines, 7, "at least 7 lines of output")

output_lines = output.split("\n")
          elephant_line = self.findLine(output_lines, "added_input1")
          self.assertIn("added_input1", elephant_line, "first input used in output")

funny_line = self.findLine(output_lines, "added_input2")
          self.assertIn("added_input2", funny_line, "second input used in output")

myTests().main()

Activity 1.6.17.

Is the variable color1 in line 5 of the code above used to store input or output?

input
Correct! The variable color1 is used to store user input.
output
Although the variable color1 is printed out, it is used to store user input, not output.

Activity 1.6.18.

What is the data type of the variable color1 in the code above?

int
Sorry, it is not an integer number.
float
Sorry, it is not a floating point decimal number.
string
Correct!
Boolean
Sorry, it is not a true or false value.

Activity 1.6.19 Project Reflection.

Identify an input and an output for your program above.

Subsection 1.6.9 Learning Objectives Review

After this lesson, you should be able to:

Learning Objective 1.6.22 1.6.A.

Identify the use of variables in an app while interacting with it.

Learning Objective 1.6.23 1.6.B.

Develop code for assignment statements using variables, and determine the values that are stored in the variables as a result.

Learning Objective 1.6.24 1.6.C.

Develop code to store and modify strings, and determine the output produced.

Learning Objective 1.6.25 Additional.

Write code using Python input and print statements.

Subsection 1.6.10 Vocabulary Review

Activity 1.6.20.

Review the vocabulary in this lesson. Drag the vocabulary term from the left and drop it on its correct definition on the right. Click the "Check Me" button to see if you are correct.

Find the words in the lesson above and try again.

variable
a named storage location used to hold a value that can change while the program is running
program state
the set of values and information a program is storing at a specific moment
data types
numbers, strings, or booleans
string
an ordered sequence of characters used to represent text in a program
concatenation
joins two or more strings to create a new string
substring
part of an existing string

Subsection 1.6.11 App Inventor Project (Optional)

If your class wants to create mobile apps using MIT’s App Inventor. do Paint Pot Tutorial Part 1.

Section 1.6 Variables