CS1116/CS5018
Web Development 2
Dr Derek Bridge
School of Computer Science & Information Technology
University College Cork
Body Mass Index (BMI)
-
A controversial measure, defined as:
$$\mathit{bmi} = \frac{\mathit{mass\_kg}}{\mathit{height\_m}^2}$$
-
Even more controversially, for Irish adults:
| BMI range | Category |
|---|
| less than 18.5 | underweight |
| 18.5 to 25 | 'normal' |
| more than 25 | overweight |
bmi.html: a page with a form
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8" />
<title>BMI</title>
</head>
<body>
<form action="bmi.py" method="get">
<label for="mass_kg">Mass (kg): </label>
<input type="text" name="mass_kg" id="mass_kg" />
<label for="height_m">Height (m): </label>
<input type="text" name="height_m" id="height_m" />
<input type="submit" value="Calculate BMI" />
</form>
</body>
</html>
bmi.py: a program with bugs
#!/usr/local/bin/python3
from cgi import FiledStorage
print('Content-Type: text/html')
form_data = FieldStorage()
mass_kg = form_data.getfirst('mass_kg')
height_m = form_data.getfirst('height_m')
bmi = mass_kg / heiight_m * height_m
category = ''
if bmi < 18.5
category = 'underweight'
elif bmi > 25
category ='overweight
else
category = 'normal'
print("""
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8" />
<title>BMI</title>
</head>
<body>
<p>
Your mass in kg is %s. Your height in m is %s.
Your BMI is %s. You are %s.
</p>
</body>
</html>""" % (mass_kg, height_m, bmi, category))
Bugs
Form data always arrives as strings
-
Suppose the user enters 82.1 and 1.65 resp.
-
mass_kg contains '82.1' (str), not 82.1 (float)
-
height_m contains '1.65' (str), not 1.65 (float)
— even if you use HTML5's <input type="number">
-
Python is a strongly-typed language
-
It won't calculate with strings — they need to be converted
bmi.py: fixed!
#!/usr/local/bin/python3
from cgitb import enable
enable()
from cgi import FieldStorage
print('Content-Type: text/html')
print()
form_data = FieldStorage()
mass_kg = float(form_data.getfirst('mass_kg'))
height_m = float(form_data.getfirst('height_m'))
bmi = mass_kg / (height_m * height_m)
category = ''
if bmi < 18.5:
category = 'underweight'
elif bmi > 25:
category ='overweight'
else:
category = 'normal'
print("""
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8" />
<title>BMI</title>
</head>
<body>
<p>
Your mass in kg is %.1f. Your height in m is %.1f.
Your BMI is %.2f. You are %s.
</p>
</body>
</html>""" % (mass_kg, height_m, bmi, category))
bmi.py: fixed!
#!/usr/local/bin/python3
from cgitb import enable
enable()
from cgi import FieldStorage
print('Content-Type: text/html')
print()
form_data = FieldStorage()
mass_kg = float(form_data.getfirst('mass_kg'))
height_m = float(form_data.getfirst('height_m'))
bmi = mass_kg / (height_m * height_m)
category = ''
if bmi < 18.5:
category = 'underweight'
elif bmi > 25:
category ='overweight'
else:
category = 'normal'
print("""
<!DOCTYPE html>
<html lang="en">
<head>
<title>BMI</title>
</head>
<body>
<p>
Your mass in kg is %.1f. Your height in m is %.1f.
Your BMI is %.2f. You are %s.
</p>
</body>
</html>""" % (mass_kg, height_m, bmi, category))