lesson 3.1.3 the problem with numbers

Ooo - an adventure game!

Introduction for the learner

Learning Outcomes

The Building Blocks (Factual Knowledge)

The Connections and Theories (Conceptual Knowledge)

Digital Skill Focus:

The Skills and Methods (Procedural Outcomes)

The Problem with Numbers

Let's try to write a program that asks for a user's age and tells them how old they will be in 5 years.

age = input("How old are you? ")
future_age = age + 5
print("In 5 years, you will be", future_age)

🤔 Code Explainer

Line 1: Ask for user's age and store in the variable

age

.
Line 2: Calculate the user's age in 5 years time.
Line 3: Print out the user's future age.

When we run this, the program crashes! It gives a

TypeError

, saying you can't add a string and an integer.

TypeError: cannot concatenate 'str' and 'int' objects on line 2

Why? Because the

input()

function always gives us a string, even if the user types in numbers. So, Python sees "13" (text) not 13 (a number). Trying to do "14" + 5 is like trying to do "hello" + 5 - it doesn't make sense!

Typecasting: Changing a Variable's Type

To fix this, we need to convert the data type from a stringA sequence of characters delimited with quotation marks. to an integerA whole, positive or negative number.. This process is called typecastingI have no idea what this means. We can do this using the

int()

function. It takes a string containing numbers and turns it into a proper integer that we can do maths with.

Here's the corrected code:

age = input("How old are you? ")
future_age = int(age) + 5
print("In 5 years, you will be", future_age)

🤔 Code Explainer

Line 2: Notice, we've changed

age

int(age)

to convert the string to an integer.

This now works perfectly! Understanding data types and knowing when to typecast is a super important skill for any Software Developer.

Task 3 A Simple Calculator

Let's build a program that can add two numbers together. Follow the PRIMM steps carefully!

🤔 Code Explainer

Line 1: Print a nice title.
Line 2-3: Ask user for two numbers and store them in variables.
Line 4: Add the numbers together (or does it?).
Line 5: Print out the answer.

Predict

Look at the starter code in the widget.

If the user enters 5 and then 3, what do you think the program will print? Why?

Run

Now run the code.

Enter

and

What was the result?

Was your prediction correct?

Investigate

The program printed "53", not 8! This is because

input()

gives us strings. The

symbol concatenated (joined) the strings "5" and "3" instead of adding them as numbers. How can we fix this?

Modify

Modify the code to correctly convert both num1 and num2 into integers before you add them together.

Run the program again. Does it now correctly calculate 5 + 3 = 8?

Make

In the empty Python Widget below, create a "Rectangle Area Calculator".

It should ask the user for the

width

and

height

of a rectangle.

It should then calculate the

area

(

area = int(width) * int(height)

) using typecastingI have no idea what this means.

Finally, it should print the answer in a clear sentence, like "The area of the rectangle is 30."

Use the Print to PDF button to print out your work and hand it to your teacher.

Outcome: I have written a Python program that takes number inputs from a user, correctly converts them from strings to integers using typecasting, and performs a mathematical calculation with them.