Python: Body Mass Index Calculator

Introduction

• This lesson will introduce the if statement in Python through a BMI Calculator application

• The if statement is used to assess information and choose one course of action or another for decision making

• This construct processes conditional execution of a statement or a group of statements according to the value of an expression

• Students will learn about conditional algorithms and develop a BMI Calculator to assess their weight status

Preparation

• Students should have a laptop or device with mBlock 5 installed (desktop version or cloud-based version) and mBlock Python editor (mLink) installed. Click here to download the Python editor (mLink).

• Worksheet: Lesson4_BMICalculator_Worksheet

Warm Up

1. Let's begin by familiarizing ourselves with the key points of this lesson

• How to convert and calculate the input data

• The if statements and indentation

• Logical comparison operators in Python

• The concept and standards of the BMI

2. Ask: What is BMI and what is it used for?

• Answer: The Body Mass Index (BMI) is a measure that uses your height and weight to work out if your weight is healthy. To calculate the BMI, you can divide your weight in kilograms by the square of your height in meters.

• A normal BMI is between 18.5 and 25; a person with a BMI between 25 and 30 is considered overweight, and a person with a BMI over 30 is considered obese. A person is considered underweight if the BMI is less than 18.5.

3. Have students fill out the ‘What I Know’ column of the K-W-L chart before the class

Tour of mBlock 5

1. When you open the editor for the first time you will see these explanations mapping the interface. Encourage students to thoroughly read through it. Below are the screen captures if you ever need to reference them again.

Figure 1: To connect your CyperPi

Figure 2: Creating Projects and Project files (see How to Create, Open, Import, or Export Project Files)

Figure 3: Editing Area

Figure 4: Console Commands to run code and more

Figure 5: Terminal (Console) to display output from Editing Area

Figure 6: Libraries containing example program. Feel free to explore these at your leisure

Figure 7: Click "Tutorials" to view these helpful instructions anytime

Hands On

1. Start of by reviewing the concept of BMI

• Say: The Body Mass Index is a measure that uses your height and weight to work out if your weight is healthy. To calculate the BMI, you can divide your weight in kilograms by the square of your height in meters.

2. Demonstrate how to calculate the BMI:

• Suppose a person’s height is 1.72m and his weight is 60kg, his BMI would be

• 60 ÷ 1.722 ≈ 20.3

• Evaluate a BMI of 20.3 using the table below, also found in the student worksheet

3. Explain to the class that in this lesson they will create a BMI calculator through Python, which can calculate the BMI automatically and indicate the level of weight status.

Activity 1:

1. Have students read "Activity 1" in the example program

2. Ask students to "comment" the program just as they did in the last lesson. This should become a common practice whenever they are given or writing their own programs.

3. Instruct them to input different values for the height and weight and record their findings in the chart provided

4. Instruct students to think about the questions below while running the program:

• Did the program work in as you expected?

• Why are we using float data typed?

• How are we dividing the BMI into ranges?

5. Explain the use of the if statements

• Say: The BMI Calculator program uses selection statements (if statements) to divide the BMI into four ranges

• In Python, a selection statement (or selection construct) is used to test a condition and make a decision

6. Our program works as follows:

• Ask the height and weight

• Calculate the BMI by using the equation

• IF the result (20.3) is equal to or greater than 18.5 AND equal to or less than 24.9, THEN the program reports ‘healthy weight’

Try It

Activity 2:

1. Instruct students to draw a flowchart on their worksheets. Show an example of the selection construct flow for reference:

• Note: Conditional statements are put inside diamond shapes and outputs are put inside rectangles.

• Use the appropriate shapes given on your worksheet to complete the flow chart

2. Have students work in pairs to complete the tasks as follow:

• Task 1: Comment the code on the worksheet. Briefly explain the function.

• Task 2: Investigate the conditions of the statements. Why does the program use the ‘≥’ and ‘≤’ operators to define the lowest and/or highest values of a range?

• Task 3: Investigate the indentation. Remove the leading whitespace before each of the print() statements. Does the program still run? Is the output ?

• Task 4: Calculate Zoe’s BMI and output her weight status. Could the program decide weight status? Why?

3. Solution to Task 4: The program cannot decide whether the BMI of ‘24.913494809688583’ should belong to ‘healthy weight’ or ‘overweight’ because the number does not fall into either range

• The round() function can solve this problem by rounding off a number to the specified number of digits:

• round(parameter 1, parameter 2)

• The ‘parameter 1’ is the number (or variable) to be rounded, while the ‘parameter 2’ represents the number up to which the given number is rounded.

• In this case it can be used as follows: BMI = round(BMI, 1)

Activity 3:

1. Instruct students to modify the example program to create a BMI Calculator for children and teenagers

2. Say: The example program of BMI Calculator is for adults. Let’s make a new one for teenagers

3. Instruct students to refer to the article About Child & Teen BMI available from here

• Note: The above table and chart show how to rescale the BMI for teenagers.

4. Take a 13-year-old boy as an example to demonstrate how to rescale the ranges of his BMI weight status categories:

• Find the ’13 years old’ from the axis ‘Age (years)’

• Read the maximum and/or minimum values of each category, for example:

Reflection

• Have students fill out the K-W-L chart

• Remind students to always comment their code, regardless of the simplicity

Extension Activities

1. For students who finish early, encourage them to think about how to add another selection construct that can divide the BMI calculation into two paths according to the gender.

2. Add some health advice responses in the BMI Calculator project, so that the user can know their weight status and some simple suggestions on health management. Here is a link with a few suggestions

Follow-Up Questions/Discussion

Summary

1. What are "if statements" and how are they used?

2. What are different logic operators used to make comparisons inside "if statements"?

3. The purpose of indentation when using "if statements"?

Example Program

Worksheet

Grade 9

Mathematics

Algebra

• C2.1 use coding to demonstrate an understanding of algebraic concepts including variables, parameters, equations, and inequalities

• C2.2 create code by decomposing situations into computational steps in order to represent mathematical concepts and relationships, and to solve problems

• C2.3 read code to predict its outcome, and alter code to adjust constraints, parameters, and outcomes to represent a similar or new mathematical situation

Grade 10

Computer Science

B1: ICS20 Arithmetic Operations 

• B1.1 use correct terminology to describe programming concepts; 

• B1.2 describe the types of data that computers can process and store 

• B1.4 determine the expressions and instructions to use in a programming statement, taking into account the order of operations

• B1.5 identify situations in which decision and looping structures are required;

• B1.6 describe the function of Boolean operators 

B2: Writing Programs

• B2.1 use a visual problem-solving model to plan the content of a program;

• B2.2 use variables, expressions, and assignment statements to store and manipulate numbers and text in a program 

• B2.3 write keyboard input and screen output statements that conform to program specifications;

• B2.4 write a program that includes a decision structure for two or more choices

B3: Code Maintenance

• B3.1 write clear and maintainable code using proper programming standards 

• B3.3 use a tracing technique to understand program flow and to identify and correct logic and run-time errors in a computer program;

• B3.4 demonstrate the ability to validate a computer program using test cases.