Making Choices
Overview
Teaching: 20 min
Exercises: 20 minQuestions
How can my programs do different things based on data values?
Objectives
Write conditional statements including
if
,elif
, andelse
branches.Correctly evaluate expressions containing
and
andor
.
A key part of programming is making the computer do different things depending on the outcome
of a test. This functionality is provided by the conditional statements if
, elif
, and else
.
Conditionals
We can ask Python to take different actions, depending on a condition, with an if
statement:
num = 37
if num > 100:
print('greater')
else:
print('not greater')
print('done')
not greater
done
The second line of this code uses the keyword if
to tell Python that we want to make a choice.
If the test that follows the if
statement is true,
the body of the if
(i.e., the lines indented underneath it) are executed.
If the test is false,
the body of the else
is executed instead.
Only one or the other is ever executed:
Conditional statements don’t have to include an else
.
If there isn’t one,
Python simply does nothing if the test is false:
num = 53
print('before conditional...')
if num > 100:
print(num,' is greater than 100')
print('...after conditional')
before conditional...
...after conditional
We can also chain several tests together using elif
,
which is short for “else if”.
The following Python code uses elif
to print the sign of a number.
num = -3
if num > 0:
print(num, 'is positive')
elif num == 0:
print(num, 'is zero')
else:
print(num, 'is negative')
"-3 is negative"
Note that to test for equality we use a double equals sign ==
rather than a single equals sign =
which is used to assign values.
We can also combine tests using and
and or
.
and
is only true if both parts are true:
if (1 > 0) and (-1 > 0):
print('both parts are true')
else:
print('at least one part is false')
at least one part is false
while or
is true if at least one part is true:
if (1 < 0) or (-1 < 0):
print('at least one test is true')
at least one test is true
True
andFalse
True
andFalse
are special words in Python calledbooleans
, which represent truth values. A statement such as1 < 0
returns the valueFalse
, while-1 < 0
returns the valueTrue
.
You can also perform special tests against lists and dictionaries to see if an object is contained
in them by using in
. For lists, this is simply if the object is in the list. For dictionaries,
it checks if the value is in the list of keys, not values.
my_list = ["apple", "orange", "pear"]
if "pear" in my_list:
print("I've found a pear!")
I've found a pear!
How Many Paths?
Consider this code:
if 4 > 5: print('A') elif 4 == 5: print('B') elif 4 < 5: print('C')
Which of the following would be printed if you were to run this code? Why did you pick this answer?
- A
- B
- C
- B and C
Solution
C gets printed because the first two conditions,
4 > 5
and4 == 5
, are not true, but4 < 5
is true.
What Is Truth?
True
andFalse
booleans are not the only values in Python that are true and false. In fact, any value can be used in anif
orelif
. After reading and running the code below, explain what the rule is for which values are considered true and which are considered false.if '': print('empty string is true') if 'word': print('word is true') if []: print('empty list is true') if [1, 2, 3]: print('non-empty list is true') if 0: print('zero is true') if 1: print('one is true')
That’s Not Not What I Meant
Sometimes it is useful to check whether some condition is not true. The Boolean operator
not
can do this explicitly. After reading and running the code below, write someif
statements that usenot
to test the rule that you formulated in the previous challenge.if not '': print('empty string is not true') if not 'word': print('word is not true') if not not True: print('not not True is true')
Close Enough
Write some conditions that print
True
if the variablea
is within 10% of the variableb
andFalse
otherwise. Hint: You can make the condition easier to understand if you use theabs
function —help
will tell you what it does!Solution 1
a = 5 b = 5.1 if abs(a - b) < 0.1 * abs(b): print('True') else: print('False')
Solution 2
print(abs(a - b) < 0.1 * abs(b))
This works because the Booleans
True
andFalse
have string representations which can be printed.
Key Points
Use
if condition
to start a conditional statement,elif condition
to provide additional tests, andelse
to provide a default.The bodies of the branches of conditional statements must be indented.
Use
==
to test for equality.
X and Y
is only true if bothX
andY
are true.
X or Y
is true if eitherX
orY
, or both, are true.Zero, the empty string, and the empty list are considered false; all other numbers, strings, and lists are considered true.
True
andFalse
represent truth values.