Lesson 7: Data Types¶

Authors:	Paul Xu, Marcus Penny
Date:	Feb 2, 2018
Time:	2 hours

Lesson Information¶

In previous lessons, the students learned to use Python as a calculator, to output messages to SenseHAT, and to use the input() function to get user input from the Python Shell. In this lesson we will write a program that mashes all these together. We formulate the concept of data types and explain in detail why the number 123 is different from the string "123" in Python. Unlike other sessions, this one takes about 2 hours, with ample time for students to complete their project on their own.

The purpose of the lesson is to:¶

Deepen the understanding of data types
Explain how computers handle different data types differently
Review all students have learned so far

Driving Questions:¶

What are data types?
How do computers handle different data types?
What are binary numbers?

Computer Science Concepts:¶

data types, binaries, string concatenation

Materials Needed:¶

Raspberry Pi 3 with SenseHAT, coins (or other chips),

Target Skills:¶

Students will be able to

SWBAT perform simple binary conversion (under 10)
SWBAT understand the differences between data types
SWBAT perform data type conversion

Instructional Plan and Structure¶

This lesson features a task that applies all the skills that the students have learned previously and reintroduces the concept of data types. It is a conceptual review of all of the topics covered in the previous seven lessons.

Overview of the lesson (5 minutes)¶

Explain to the students that on our first day with Python, we learned how to use Python as a sophisticated calculator (Lesson 3: A Sophisticated Calculator). However, at that time only we could use Python as a calculator. This time we are going to write a calculator that asks someone else for numbers and then do the calculation, so that someone else can use the program as well.

Demonstrate to the students this code without showing them the content::

# Initialization
from sense_hat import SenseHat
sense = SenseHat()

# Input
number1 = input("Give me number 1")
number2 = input("Give me number 2")

number1 = int(number1)
number2 = int(number2)

# Processing
result = number1 + number2
msg = "{0} + {1} is {2}".format(number1, number2, result)

# Output
sense.show_message(msg, text_colour = [255, 0, 0], back_colour = [0, 0, 255])

Figuring it out together (10 minutes)¶

Ask the students to discuss in groups what they would write to achieve this result. They will likely come up with the input() function, the initialization, and the sense.show_message() function. As they discuss, write this program::

# Initialization
from sense_hat import SenseHat
sense = SenseHat()

# Input
number1 = input("Give me number 1")
number2 = input("Give me number 2")

# Processing

# Output
sense.show_message(number1 + number2, text_colour = [255, 0, 0], back_colour = [0, 0, 255])

Show the students what this program outputs.

How do computers “see” numbers? (20 minutes)¶

So, rather than adding the numbers together, we only saw on SenseHAT that the computer joined the numbers together. This is weird. Let’s figure out what is going on. Remember in the last class (Lesson 6), when we used the input() function, what is the type of the value that we got from it?

>>> input("What is your name?\n")
What is your name?
Captain America
'Captain America'

Turns out that we got a string from this function. When we added strings together, Python will join, or concatenate, them for us, instead of adding them together.

>>> 123 + 456
579
>>> "123" + "456"
'123456'

Why? It’s because computers see things very differently from us. Computers are only capable of seeing 0 and 1. Those are called binary numbers. For example, computers see 2 as 10, and 3 as 11. How about 7?

We can use this worksheet. Use 7 coins, fill the columns with coins. However, remember, when you are finished, the columns have to be either empty or full. After you are finished, write a 0 under the columns that are empty, and a 1 under those that are full. You should get 111 after this. How about 9?

We can also use the bin() function in Python to get the binary versions of numbers::

>>> bin(7)
'0b111'
>>> bin(123)
'0b1111011'
>>> bin(255)
'0b11111111'

Note

The 0b prefix tells us that this is a binary representation. Actual values come after 0b. How many ones are in there in the binary version of 255? How about 1023?

How do computers “see” strings? (10 minutes)¶

How the computers see strings is a completely different story. First, for each character, Python uses a number to represent them. We can use ord() function to get them.:

>>> ord("a")
97
>>> ord("A")
65
>>> ord("1")
49

Then we can use the bin() function to convert them to binary numbers. The last step is to fill the number with preceeding 0s if there are fewer than 0 digits.

>>> bin(49) # "1" is actually 00110001
'0b110001'

We can use the following code to see how computers see 123 and "123":

binary.py¶

def int_to_bin(num):
    return bin(num)[2:]

def str_to_bin(string):
    return [bin(ord(ch))[2:].zfill(8) for ch in string]

if __name__ == "__main__":
    print(int_to_bin(123))
    print(str_to_bin("123"))

This is what we get when we run the code::

1111011
['00110001', '00110010', '00110011']

Because computers see data differently like this, they perform different operations on different data types. What type takes more space to store?

Coding using `int()` and `str()` functions (60 minutes)¶

Now that they have deeper understanding of the differences between data types, the students can complete their coding tasks using type conversions. They can begin with the code we wrote earlier::

# Initialization
from sense_hat import SenseHat
sense = SenseHat()

# Input
number1 = input("Give me number 1") # What is the type of number1?
number2 = input("Give me number 2") # What is the type of number2?

# Processing

# Output
sense.show_message(number1 + number2, text_colour = [255, 0, 0], back_colour = [0, 0, 255])
# What is the type of number1 + number2 now?

Challenges:¶

Change the program so that instead of doing addition, it does multiplication.
Unfortunately, if we provide the computer with decimal numbers, the calculations will be off. This is because the int() function only converts strings to integers (hence the name int). When we are using decimals, we can convert them using float(). Try to use this function in your program and test what happens.

Review and Assessment¶

How do computers see data types differently?
How do we convert between different data types?