Lab 1: Math expressions and input/output | CMSC 105 Elementary Programming - Fall 2024

Lab 1: Math expressions and input/output

Points Possible: 100

Due: Thursday, September 5th

In this lab, we will practice several Python statements from the IDLE (Integrated Development and Learning Environment). Then we will write a program that will input data do a small calculation and then output (print) the result.

Objectives

The purpose of this assignment is to give an experience with:

Part 1: The Python Prompt

In this lab, I have you type expressions into Python. First, you must start IDLE. When I show you an expression that starts with a prompt like this:

>>> 4

Then, that means you should type the expression into the Python Shell window in IDLE.
For example, this expression:

>>> 1 + 5

means that you should type the part that follows the prompt, meaning the expression 1 + 5, into Python IDLE prompt, but not the >>> part.

float and int

As we discussed in the lecture yesterday, there are three main data types that we will be using: int, float, string. There is a built-in function called type in Python that will tell you which of the two types of number you’re looking at. Give it a try:

>>> type(5)

<class ‘int’>

That means that the number 5 is treated as an int.

>>> type(2.5)

<class ‘float’>

That means that the number 2.5 is treated as an float.



You can convert easily between the numeric types:

>>> float(2)

2.0

>>> int(2.5)

2

>>> int('100')

100

>>> float('100')

100.0

But this doesn’t work:

>>> int('200.5')

Traceback (most recent call last):
File “", line 1, in ValueError: invalid literal for int() with base 10: '200.5'


It’s ok to convert a string that looks like an int into an int, but it won’t convert a string that looks like a float into an int.

The input function

The input function in Python is a convenient way to let the user enter information that your program can use.

>>> input()
Hello

‘Hello’

Notice that the string was shown with quotation marks around it. That’s because Python showed me that string as the result of evaluating an expression, not as the output from a print statement.

It’s more useful to use an assignment statement to save the result of the input() function call so that you can use it later.

>>> x = input()
Hello

That statement stores the result of the input() function call into a variable called x.

>>> x

‘Hello’

>>> x = input()
100

>>> x

‘100’


It displays all the input values with quotation marks, meaning it displays them as a string.
This is where we use eval function that converts the key strokes entered by a user into a value.

You can also supply a string as an argument to the input function and it will display the string as a prompt to the user:

>>> x = input('Please enter a number: ')
Please enter a number: 100
>>> x + 1

Traceback (most recent call last):
File “", line 1, in TypeError: Can't convert 'int' object to str implicitly


It’s important to see error messages like this so that you can start to learn what they mean.
Here it’s trying to convert an int (integer) into a str (string). Python does not let you do math using a string and a number, even if the string looks like a number.

Try this,

>>> x = eval(input('Please enter a number: '))
Please enter a number: 100
>>> x + 1

Do you observe any error now? That’s how eval works!

Converting input to a float

You can call the float function directly on the result of the input function:

>>> x = float(input('Please enter a number: '))
123.5

Be mindful of the parentheses: They must match up.

Now have a look at what’s in the variable x:

>>> x

123.5

>>> type(x)

<class ‘float’>

You can call the float function on the variable after the input function:

>>> x = input('Please enter a number: ')
123.5
>>> x = float(x) + 1

Here, we are using the variable x twice, the original value of x entered by the user will be updated when converting to float(x).

>>> x

124.5

>>> type(x)

<class ‘float’>

You can also use another variable y so that the original value of x does not change. For example,

>>> x = input('Please enter a number: ')
123.5
>>> y = float(x) + 1

Now try this,

>>> x

‘123.5’

>>> y

124.5

Part 2: Program Assignment

Sample Code (for reference, please do NOT include this in your submission):

The code below calculates the sum of two numbers. It reads (takes as input) two numbers and displays the sum.

''' 
Author name: Dr. David Balash
This program calculates the sum of two numbers
'''

number1 = eval(input("Enter number 1"))  # Asks users to input number 1
number2 = eval(input("Enter number 2"))  # Asks users to input number 2

# Calculate the sum of the two numbers
add_result = number1 + number2

# Print the results
print("Sum of two numbers is", add_result)

Notes:

The basic structure of any program will have the following components:

You can choose any meaningful names for the variables (except any keywords/reserved words in Python like if, break, for). For example, in this program, number1, number2, add_result are variables that store values.

You can read/practice more based on Lecture 2 slides.

Write a program

Write a program that takes as input the radius of a circle and computes the circumference of the circle using the formula:

\[circumference = 2 \pi r\]

Please display the circumference using a print statement.

Save the program is circle.py and attach it to the Blackboard lab assignment.

Grading Rubric for the program:

Grading Points Possible
Appropriate header and comments 5
Input 5
Computation 10
Print output 5

TOTAL 25 points