Sometimes we want our program to do different things depending on different situations.
For example, let’s write a program that checks whether someone is tall enough to ride a rollercoaster or not. Let’s say they need to be at lest 165cms tall to ride the rollercoaster.
Add the following to conditionals.py:
# conditionals.py
height = 185
if height > 165:
print('Hop on!')
Change the height variable to a number less than 165, what happens?
We get no output.
This isn’t very meaningful to the user, let’s add an else statment to fix that.
Add the following to conditionals.py:
# conditionals.py
height = 185
if height > 165:
print('Hop on!')
+else:
+ print('Sorry, not today :(')
But what if someone is exactly 165cms? Change height to 165.
Our program still says they are not tall enough.
Let’s modify our if statement to allow 165 and taller, rather than only taller than 165.
Add the following to conditionals.py:
# conditionals.py
height = 185
-if height > 165:
+if height >= 165:
print('Hop on!')
else:
print('Sorry, not today :(')
Try running the program with the height set to 155, then 165, then 175 (i.e. less than, the same as, and greater than 165) and make sure you get the correct output each time.
So how does this actually work? We are writing what we call if statements. We are saying if something is true, do this, otherwise, do this. In our case, we are telling the user they can ride the rollercoaster if checking their height is greater than or equal to 165cm is true, otherwise it is false and they cannot ride the rollercoaster.
What if we want to check that something is exactly the same as something else?
For example, maybe we are checking someone has entered the correct pincode to unlock their phone.
Add the following to conditionals.py:
# conditionals.py
correct_code = 1234
entered_code = 1234
if entered_code == correct_code:
print('Unlocked!')
Modify your code to print Access denied! if the entered_code is incorrect.
Hint: an else statement like we used in our last example would be useful here.
Quite often we have situations that can only be in two states, for example, it is either raining or it isn’t.
Let’s write a program that tells you whether or not to take an umbrella based on if it is raining or not:
# conditionals.py
is_raining = True
if is_raining:
print('Take an umbrella.')
Try changing is_raining to False and adding an else to tell you do not need an umbrella.
Up until now, our if statements have been doing comparisons, i.e. they compare two things (height vs a number, correct_code and entered_code.
This time we just use the variable name.
That is because comparisons either result in a True or False, but our is_raining umbrella is already defined as either True or False so we don’t need to compare it against anything.
Write a program that defines a variable for temperature and set its value to a number.
Tell the user to take a jumper if the temperature is less than or equal to 18, and not to worry if it is greater than 18.
These next one is a little tougher, it’s an extra challenge for those of you who have programmed before!
Modify your program to define another variable for wind_chill, and set its value to a number.
Calculate the real feel temperature (the temperature minus the wind chill).
Tell the user to take a jumper if the real feel temperature is less than or equal to 18, and not to worry if it is greater than 18.
If you’ve used nested if statements before, give this a go too…
This time we are going to also factor in if it is raining or not. If the real feel temperature is less than or equal to 16 and it is raining, tell the user to just stay home. Otherwise if it is raining tell them to take an unbrella, or if the real feel temperature is less than or equal to 16 tel lthem to take a jumper.
How did you get here?