Part 03 - Flow Control - Conditionals

If Statement

A control statement that contains one or more Boolean expressions whose results determine whether to execute other statements within the If statement.

An if statemnt allows you to travel down multiple logical paths, depending on a condition given. If the condition given is true, the block of code associated with it will be run.

i = 5
if i == 5:
    print "i is equal to 5."
i is equal to 5.

notice the difference between i = 5 and i == 5.
i = 5 is an assignment
i == 5 is a comparison

If you try an assignment while running a conditional, Boo will emit a warning.

You may have noticed that unlike other languages, there is no then-endif or do-end or braces { }. Blocks of code are determined in Boo by its indentation. By this, your code blocks will always be noticeable and readable.

Always use tabs for indentation.
In your editor, set the tab-size to view as 4 spaces.

You can have multiple code blocks within eachother as well.

i = 5
if i > 0:
    print "i is greater than 0."
    if i < 10:
        print "i is less than 10."
        if i > 5:
            print "i is greater than 5."
i is greater than 0.
i is less than 10.

If-Else Statement

With the if statement comes the else statement. It is called when your if statement's condition is false.

i = 5
if i > 5:
    print "i is greater than 5."
    print "i is less than or equal to 5."
i is less than or equal to 5.

Quite simple.

If-Elif-Else Statement

Now if you want to check for a condition after your if is false, that is easy as well. This is done through the elif statement.

i = 5
if i > 5:
    print "i is greater than 5."
elif i == 5:
    print "i is equal to 5."
elif i < 5:
    print "i is less than 5."
i is equal to 5.

You can have one if, any number of elif s, and an optional else .

Unless Statement

The unless statement is handy if you want a readable way of checking if a condition is not true.

i = 5
unless i == 5:
    print "i is not equal to 5."

It didn't output because i was equal to 5 in that case.

Statement with Modifier

Like in Ruby and Perl, you can follow a statement with a modifier.

i = 0
print i
i = 5 if true
print i
i = 10 unless true
print i

Don't use Statement with Modifier on a long line. In that case, you should just create a code block.
A good rule of thumb is to not use it if the statement is more than 3 words long.
This will keep your code readable and beautiful.

Some common conditionals:






5 == 5


not equal

0 != 5


greater than

4 > 2


less than

2 < 4


greater than or equal to

7 >= 7 and 7 >= 4


less than or equal to

4 <= 8 and 6 <= 6

Not Condition

To check if a condition is not true, you would use not.

i = 0
if not i > 5:
    print 'i is not greater than 5'
i is not greater than 5

Combining Conditions

To check more than one condition, you would use and or or. Use parentheses ( ) to change the order of operations.

i = 5
if i > 0 and i < 10:
    print "i is between 0 and 10."
if i < 3 or i > 7:
    print "i is not between 3 and 7."
if (i > 0 and i < 3) or (i > 7 and i < 10):
    print "i is either between 0 and 3 or between 7 and 10."

Note that and requires that both comparisons are true, while or requires that only one is true or both are true.

i is between 0 and 10.


  1. Given the numbers x = 4, y = 8, and z = 6, compare them and print the middle one.

Go on to Part 04 - Flow Control - Loops