Lab 6 / Part 2: Some Simple, Stupid Python Programs
C O N T E N T S
- Recreating SayTwice.html but in Python
- Recreating (and Debugging) Temperature.html but in Python
- Calculating Body Mass Index (BMI) in Python
Setup for Python3
Let’s be sure your Terminal session will run Python3 (instead of the prepackaged Python 2.7.x) when calling python on the command line. We can crete an “alias” for python3 by running the following command:
$ alias python=/usr/local/bin/python3 Alternatively, you can simply run python3 in the Terminal instead of python, but this gets annoying after awhile.
Once you’ve done this, you’re all set.
1. Recreating SayTwice.html but in Python
In Lab 6 / Part 1, you made a program that asked for user input and echoed it back twice to the console. How would we do that in Python?
What if I told you… that you already know the two things in Python you’ll need: input() and print()? Remember those?!
I bet you can do the rest…
Save your new code to a file called saytwice.py. Make sure it’s in your /lab-06-review repository!
Now we just have to test our python script by running it on the command line.
Back in Terminal, simply type python, then a space, and then drag-and-drop your saytwice.py file onto the Terminal window (remember: this will quickly create a full path to your file on the command line!). Run the command by hitting enter, of course.
The output will look like this:
$ python /Users/louisgoldford/mtec1003/lab-06-review/saytwice.py
Give me a word to say twice: michaelCaine
michaelCaine! michaelCaine!
$ Make sure your file is included in your /lab-06-review folder, and don’t forget to add, commit, and push your changes!
Use your git cheatsheet (from lab 3) if you need it!
2. Recreating (and Debugging) Temperature.html but in Python
This one’s slightly more involved, so we’ll do it together. Firstly, we need a formula for our conversion from degrees in Celsius (°C) to degrees in Fahrenheit (°F):
\[{F} = {\frac{9}{5}}C + 32\]where:
- F will be our result; i.e. degrees in Fahrenheit (°F), and
- C is our “user input”; i.e. degrees in Celsius (°C)
Maybe you found this formula, or one like it, when you made your JavaScript version in Lab 6 / Part 1. But how should we reconstruct it in Python?
Initial “Translation” of our JavaScript Code
Consider your file temperature.html from Lab 6 / Part 1, and start your translation into Python by doing the obvious:
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tempincelsius = input("Please enter a temperature in celsius: ")
tempinfahrenheit = (tempincelsius * (9 / 5)) + 32.0
print("The temperature is " + tempinfahrenheit + "°F")
…Our Python code is looking pretty good…. I mean, right?………
……Yeah, life’s been pretty good to ya…. right?……
………We are aWeSoMe cOdErZ aren’t we… RIGHT?!!!!…
So, we save our code and go to our console to run it, and we are SO SURE it’s gonna work… right?!
Well, let’s see what happens…
$ python /Users/louisgoldford/mtec1003/lab-06-review/temperature.py
Traceback (most recent call last):
File "/Users/louisgoldford/mtec1003/lab-06-review/temperature.py", line 2, in <module>
tempinfahrenheit = (tempincelsius * (9 / 5)) + 32.0
TypeError: can't multiply sequence by non-int of type 'float'
$ WAIT, WHAT JUST HAPPENED ?!?!
Okay first, just breathe… Now let’s start to work the problem.
(“Incidentally,” RIP Fred Willard, who has absolutely nothing to do with Python. Whatsoever.)
We Need a Floating-Point Number!
The error we got mentions two crucial things: (1) it mentions that line 2 is a specific problem, and (2) it also mentions something about a float (non-int of type ‘float’). What does this mean?
It means that something on line 2 (which is reprinted in our error message, and that helps! tempinfahrenheit = tempincelsius * 1.8 + 32.0) is not the expected floating-point number (i.e. a number with a decimal, in computer science terms). Without a floating-point number, the equation we wrote on line 2 can’t be computed. We see that 1.8 and 32.0 are both numbers with decimals, so the only thing that might not be a floating-point number in line 2 must be our variable: ‘tempincelsius’.
So what is ‘tempincelsius’ then, anyway?
And how can we make it a floating-point number?
Debugging with the type() Function
Fortunately, in Python we can use a nifty function called type() to find out what ‘tempincelsius’ really is. Start by going back to your temperature.py file and commenting out lines 2 and 3 (since that’s where the error is occurring.)
Just after line 1, insert a line and type the following:
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tempincelsius = input("Please enter a temperature in celsius: ")
print(type(tempincelsius))
# tempinfahrenheit = (tempincelsius * (9 / 5)) + 32.0
# print("The temperature is " + tempinfahrenheit + "°F")
Now, on line 2 we’re printing out (print()) the result of the type() function.
Save your file, and re-run it in Terminal:
$ python /Users/louisgoldford/mtec1003/lab-06-review/temperature.py
Please enter a temperature in celsius: 45
<class 'str'>
$ AHA! The result of our
print(type(tempincelsius)) command tells us that the variable we’re testing, ‘tempincelsius’, is a STRING, NOT A DECIMAL (i.e. ‘str’ according to the result in our console). So, now we know we just need to convert the ‘tempincelsius’ variable into a float. For converting between strings and floats, python has two very handy functions: float() and str(). How nice!
Updating Our Code With the Right Conversions
Go back to your code, comment out the print function, and comment in lines 3 + 4. Revise them so that it reads:
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tempincelsius = input("Please enter a temperature in celsius: ")
# print(type(tempincelsius))
tempinfahrenheit = (float(tempincelsius) * (9 / 5)) + 32.0
print("The temperature is " + str(tempinfahrenheit) + "°F")
Here, we’ve updated our code so that we’re converting variable ‘tempincelsius’ into a float in order to compute the temperature. Then on line 4, we need to concatenate the result into a larger string and print it out to the console, so we have to convert the ‘tempinfahrenheit’ variable back into a string, because it’s a float. If we forget to convert it back into a string, we’ll get an error in the console similar to the one we got before we did our first conversion above.
Try it out in the Terminal… Now it should work!
$ python /Users/louisgoldford/mtec1003/lab-06-review/temperature.py
Please enter a temperature in celsius: 32
The temperature is 89.6°F
$ Save your code, make sure your temperature.py file is included in your /lab-06-review folder, and don’t forget to add, commit, and push your changes! Use your git cheatsheet (from lab 3) if you need it!
3. Calculating Body Mass Index (BMI) in Python
Let’s do something we haven’t already done in JavaScript…
Dunno about you, but the pandemic has led to me gaining some weight……… :$
How would you calculate the percentage of fat on a person’s body?
To do this we use a formula for Body Mass Index (BMI):
\[{BMI} = \frac{703 \times W}{H^2}\]where:
- W is a person’s weight (in pounds), and
- H is their height (in inches).
With just these two inputs to the equation, we can describe a person’s BMI; the percentage of their body weight that is fat.
How would we accomplish this with Python?
As you can imagine, you’ll need to start by asking the user for some input.
In this case, you’ll need to:
- ask the user for two things, and
- create separate variables and prompts for each. Then,
- you’ll convert the strings into floats (just like we did in temperature above),
- make your equation (using the formula for BMI above),
- convert the result back into a string, and finally
- print the final result to the console.
So, in your text editor, create a new file and call it bmi.py, and get started!
import sys
At the top of your new file on line 1, type the following:
import sys import is a statement in Python that allows you to “import” a “module.” In this case, sys is the module we’re importing. A Python module is basically a file containing more Python code; more functions, variables, classes, methods, etc. We’re using sys here to be sure you won’t get any errors resulting from weird characters you may type, or those that might throw errors on the command line. You might try safeguarding yourself with this particular module in some of your other Python files too!
Write your code below your import sys line, so that your output in the console resembles this:
$ python /Users/louisgoldford/mtec1003/lab-06-review/bmi.py
Enter your weight (in pounds): 153
Okay, also enter your height (in inches): 68
Your body mass index (BMI) is 23.2610294118%
$ Although you won’t need it, feel free to try out Python’s pow() method to aid in your calculation.
Save your code, make sure your bmi.py file is included in your /lab-06-review folder, and don’t forget to add, commit, and push your changes! Use your git cheatsheet (from lab 3) if you need it!
Your final /lab-06-review folder by the submission deadline should include the following files:
- saytwice.html
- saytwice.py
- shout.html
- temperature.html
- temperature.py
- miles.html
- bmi.py