Lesson 10: Conditionals
Real programs don't just do one thing; they adapt to the situation. For example, your web browser doesn't just show you the page by slowly and automatically scrolling through it. It listens to your input. When you scroll up, it moves the page up. When you scroll down, it moves the page down. So it has the ability to make decisions based on your input. We'd like to start doing that in our programs as well. Conditional statements are essential for it, as we'll see.
if statements
An if statement is one of the most commonly used tools in a computer
programmer's toolbox. It's important to understand how if statements work.
Let's start with an example:
n = 5
if n == 5:
print('n is 5. Sweet!')
if n == 6:
print('this will not get printed :(')
As you can see, the second print statement did not get executed. You may have
gathered by now that an if statement is able to decide whether certain
statements get executed based on the value of a boolean expression. In the first
case, the n == 5 boolean expression evaluates to True, causing the print
statement to execute. But the second boolean expression evaluates to False and
that's why the second print statement never executes.
The diagram below shows the basic structure of an if statement.
As you can see, the statements are only executed if the if statement allows
them to.
It's important to note that the "contents" of an if statement are have spaces
before them. The if statement can have multiple python statements inside of it.
They are together (if statement + contents) called the if block.
Take a look at this example:
Hold shift + scroll to zoom
n = 5
if n == 5:
print('n is 5 and this statement is getting printed')
print('and this statement is getting printed too')
if n == 6:
print('n is not 6 and this is not getting printed')
print('and this line is not getting printed either')
print('This line is getting printed. It\'s not part of the if block above')
if-else statement
What if we wanted to execute some statements if a condition was True and a
separate set of statements if it was False? Well, we could always do something
like:
Hold shift + scroll to zoom
n = 5
if n == 5:
print('n is 5')
if n != 5:
print('n is not 5')
Unfortunately, as your boolean expression grows in complexity, coming up with
the negative version of it can be tedious and error-prone (and not to mention,
unreadable). Fortunately, most programming languages (including Python) provide
an easier way of expressing this intent using an if-else statement:
Hold shift + scroll to zoom
n = 5
if n == 5:
print('n is 5')
else:
print('n is not 5')
The two examples above are identical, but one should prefer to use the second
form (the one involving else). Try changing the value of n in both of
the examples above to trigger the 2 print statements.
The diagram below shows the basic structure of an if-else statement:
Chained conditionals (elif)
What if you wanted to use multiple boolean expressions to decide what to run?
Specifically, what if you wanted to evaluate a second boolean expression if a
first boolean expression evaluates to False?
You could get away with something like:
Hold shift + scroll to zoom
n = 5
color = 'red'
if n == 5:
print('n is 5')
else:
if color == 'red':
print('n is not 5, but the color is red. All is not lost.')
else:
print('n is not 5 and the color is not red. What am I to do?')
Note that we "nested" a second if-else statement inside the else clause of
the first. This is called "Nested conditionals". See the diagram below to help
you visualize how nested conditionals work:
While nested conditionals are useful, in this case we can simplify the logic by
using an elif (short for "else-if") clause:
Hold shift + scroll to zoom
n = 5
color = 'red'
if n == 5:
print('n is 5')
elif color == 'red':
print('n is not 5, but the color is red. All is not lost.')
else:
print('n is not 5 and the color is not red. What am I to do?')
That's much easier to read and in programming, readability counts for quite a
lot. The great thing about elif is that you can have as many of them as you
want:
Hold shift + scroll to zoom
from ctb import place, Color
color = input('Choose a color from {red, green, blue}')
if color == 'red':
place(0, 0, 0, Color.RED)
elif color == 'green':
place(0, 0, 0, Color.GREEN)
elif color == 'blue':
place(0, 0, 0, Color.BLUE)
else:
print('You don\'t like to follow instructions do you?')
place(0, 0, 0, Color.BLACK)
The example above combines multiple concepts that we've learnt so far into a single program. Mess around with it and have fun! Try adding a few colors of your own to the choices.
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