16 Control flow

This chapter is about how you make your program do different things under different circumstances. Making functionality dependent on data is what makes programs useful.

The `if` statement

So far, the small programs you have written run the same sequence of statements (lines). Imagine if you could control which statements were run depending on the circumstances. Then, you would be able to write more flexible and useful programs. Cue the music - and let me introduce the “if-statement”.

Read the following code carefully in the cell below. It is a small program that monitors bus passenger status. Notice the colon ending the if-statements. Also, note that the lines below each if-statement are indented with precisely four spaces. While reading the program, figure out what the if-statement does. Then, run the code and see what happens.

import sandbox_widget
import steps_widget

%%sandbox
bus_seats = 32
passengers = 20
bags = 20

print(passengers, "people ride the bus")

if bus_seats >= passengers + bags:
    print("Smiles, everyone has room for bags")

if bus_seats >= passengers:
    print('Everyone gets to sit down, no complaints')

if bus_seats < passengers:
    print('Some passengers standing, annoyed')
    
if bus_seats < passengers / 3:
    print("General dissatisfaction, some swearing too")

Try changing the values of bus_seats, passengers, and bags and see how the program executes.

You have probably realized that the if-statements control which prints statements that are evaluated. A statement nested under an if-statement is only evaluated if the expression between the if keyword and the : reduces to a value Python considers as true. This does not happen if the expression between the if and : reduces to a value Python considers false.

When asked to evaluate something as true or false, Python will interpret zero and empty values (like 0 and '') as False and all other non-zero and non-empty values as True.

Exercise 16-1

SOLO

Which of the following letters are printed: A, B, C, D, E, F, G. Make up your mind before you run the code.

%%sandbox
if 0:
    print('A')

if "Banana":
    print('B')

if 3.14159265359:
    print('C')

if False:
    print('D')

if 9 > 5 and 4 < 7:
    print('E')

if '':
    print('F')

if False or "banana":
    print('G')

Exercise 16-2

AI: Explainer

What happens if you forget to write the : in the if-statement?

%%sandbox
if 4 > 2
    print('Hi!')

Ask the assistant what error this will produce, word for word, before you run it. Then run it, and see whether it got the wording right or only the general idea. Predicting the exact wording is much harder than predicting that something goes wrong, and the difference between the two is worth noticing.

Exercise 16-3

AI: Explainer

What happens if you do not indent the code under the if-statement?

%%sandbox
if 4 > 2:
print('Hi!')

Same again, but ask a sharper question: ask the assistant why Python cares about indentation here at all, when many other languages do not. Then run the code and check that the error it predicted is the error you got. The two errors in these last two exercises have different names, and you should be able to tell them apart.

By now, you probably know that your text editor is intelligent regarding indentation. If you hit Enter after a statement ending with :, it will indent the next line with four spaces. Also, if you use the tab in Python code, it will produce four spaces for you.

FAQ

Q: Isn’t “If” a poem by Rudyard Kipling?
A: Yes.

Else-statement

Sometimes you not only want your program to do something if an expression reduces to True, you also want it to do something else if it is False. It is as simple as it looks:

%%sandbox
cookies = 3

if cookies > 0:
    print("Uh, I wonder if we have some milk too...")
else:
    print("Sigh!")

Remember to put a : after the else keyword. Run the code and change the value of cookies to 0.

Exercise 16-4

SOLO

Test your understanding about which expressions that reduce to a True or False value. Run the code below and then see how it responds to different values of x. Try to come up with other variations yourself.

%%sandbox
x = 0.0
# x = '0'
# x = '   '
# x = ''
# x = not 0
# x = 'zero'

if x:
    print('x is substituted with True in the if-statement')
else:
    print('x is substituted with False in the if-statement')

FAQ

Q: Isn’t “Else” a poem by Rudyard Kipling?
A: No.

Exercise 16-5

SOLO

What do you think this code prints? Notice how you can nest if and else-statements under other if and else-statements. This way, you can make your program include only some statements when certain combinations of conditions are met. Just remember that the code below each if or else is indented by four spaces. Try to change the True/False values of milk and cookies.

%%sandbox
milk = False
cookies = True
if milk:
    if cookies:
        status = 'Good times!'
    else:
        status = 'Not thirsty, thanks or asking'
else:
    if cookies:
        status = 'How does something like this happen?'
    else:
        status = 'Whatever...'
        
print(status)

Blocks of code

In the examples above, some lines are indented more than others, and you probably already have some idea of how Python interprets this. Indentation defines blocks of code. The if and else statements control each code block’s evaluation when your code runs. The following three rules define individual blocks of code:

All statements in a code block have the same indentation. That is, they line up vertically.
A block of code begins at the first line of code at a line that is indented more than the one before it.
A block ends when it is followed by a less indented line or at the last line of code.

This way, a block can be nested inside another block by indenting it further to the right, as shown in Figure 16.1. Compare the example in Figure 16.1 to the code example above. Note how a colon at the end of a statement means “this applies the block of code below”. Make sure you understand which print statements are controlled by which if and else statements.

Figure 16.1: The amount of indentation defines blocks of code

Elif-statement

Say you need to test several mutually exclusive scenarios. For example, if a base is equal to A, T, C, or G. You can do that like in the example below, but it is very verbose and shifts your code further and further to the right.

%%sandbox
base = 'G'

if base == 'A':
    print('This is adenine')
else:
    if base == 'T':
        print('This is thymine')
    else:
        if base == 'C':
            print('This is cytosine')
        else:
            print('This is guanine')

This is where an elif statement can be helpful. It is basically short for “else if.” The correspondence is hopefully obvious if you compare it to the example below.

%%sandbox
base = 'G'

if base == 'A':
    print('This is adenine')
elif base == 'T':
    print('This is thymine')
elif base == 'C':
    print('This is cytosine')
else:
    print('This is guanine')

Here, we put an else-statement at the end to capture all cases not covered by the if-statement and the two elif-statements.

Exercise 16-6

SOLO

You can use logical operators (and, or, not) in the expressions tested in an if-statement. Can you change the program from Section 16.0.0.5 so that there are no nested if-statements - in a way that the program still does the same? You can use if, elif, and else and test if, e.g., milk and cookies are true using and.

Exercise 16-7

SOLO

The snippet of code below has three blocks with three statements. Which statements belong to which block? Which statements are executed?

%%sandbox
x = 5
if x > 4:
    y = 3
    if x < 1:
        x = 2
        y = 7
        z = 1
    x = 1
z = 4

Exercise 16-8

SOLO

Can you see four blocks of code? If not, read the three rules above again. Which statements are executed?

%%sandbox
x = 5
if x > 4:
    y = 3
    if x < 1:
        x = 2
        y = 7
    else:
        x = 1
        y = 9
z = 4

General exercises

Exercise 16-9

SOLO

Will this print You are a superstar!?

%%steps
if -4 and 0 or 'banana' and not False:
    print("You are a super star!")

Exercise 16-10

SOLO

Will this print You are a superstar!?

%%sandbox
if -1 + 16 % 5 == 0 :
    print("You are a super star!")

Exercise 16-11

SOLO

Assign values to two variables, x and y. Then, write code that prints OK if (and only if) x is smaller than five and y is larger than five. Do it using two if statements:

x = 3 # or something else
y = 7 # or something else

# rest of code here...

Now solve the same problem using only one if statement.

Exercise 16-12

SOLO

Assign values to two variables, x and y. Then, write some code that prints OK if and only if x is smaller than five or y is larger than five. Do it using two if statements:

x = 3 # or something else
y = 7 # or something else

# rest of code here...

Now solve the same problem using only one if statement and one elif statement.

Exercise 16-13

SOLO

Assign values to two variables, x and y. Then, write some code that prints OK if either x or y is zero but not if both are zero (this is tricky).

x = 3 # or something else
y = 7 # or something else

# rest of code here...

Exercise 16-14

SOLO

Which value of x makes the code below print Banana?

x = 
s = ''
if x**2 == 16:
    s = s + 'Ba'
if x + 6 == 2:
    s = s + 'na'
if 7 == x - 3:
    s = 'na' * 2
else:
    s = s + 'na'
print(s)

Exercise 16-15

SOLO

Make three exercises that require the knowledge of programming so far. Have your fellow students solve them.

The if statement

Exercise 16-1

Exercise 16-2

Exercise 16-3

FAQ

Else-statement

Exercise 16-4

FAQ

Exercise 16-5

Blocks of code

Elif-statement

Exercise 16-6

Exercise 16-7

Exercise 16-8

General exercises

Exercise 16-9

Exercise 16-10

Exercise 16-11

Exercise 16-12

Exercise 16-13

Exercise 16-14

Exercise 16-15

The `if` statement