Conditional Statements

Table of Contents

If Statements

An if-statement is a way to change what happens depending on the circumstances.

If-statements are compound statements, made up of the one or more conditions, followed by the group of statements that belong to it.

An if-statement can have three types of clauses:

• an if clause is always present, followed by a condition

• an optional elif clause may appear multiple times and is also followed by a condition

• and an optional else clause, which is like a default, and has no condition.

Let’s take a look at some examples.

Listing 48 one if clause

if status == "failed":
  print("Sorry, you don't have access.")
  exit()

Listing 49 an if and else clause

if winner == player:
  print("Congratulations, you win!")
else:
  print("Better luck next time.")

Listing 50 an if clause, multipe elif clauses, and an else clause

if choice == "left":
  print("You decide to take the path to the left.")
elif choice == "right":
  print("You decide to take the path to the right.")
elif choice == "middle":
  print("You decide to take the middle path.")
else:
  print("Invalid choice.")

Exercises and questions

1. What is the value of options if level is "moderator".

options = ["view", "list", "exit"]

if level == "admin":
  options.append("remove")
elif level == "moderator":
  options.append("flag")
elif level == "owner":
  options.append("edit")

2. Fix the following.

if choice > highest:
  print("Sorry, your pick choice is too high.")
else choice < lowest:
  print("Sorry, your pick choice is too low.")

  Cell In[1], line 3
    else choice < lowest:
         ^
SyntaxError: invalid syntax

Conditions

The expressions following the if and elif keyword in an if-statement are called conditions. This is what tells Python whether to execute the suite of statements in this clause.

Comparison Operators

Conditional statements often use Comparison operators which compare two values and evaluate to either True or False.

Some examples of expressions using comparison operators:

• 5.5 < 3

• "fox" in "The quick brown fox jumps over the lazy dog"

• 5*3 == 30

Table 1 Comparison operators

operator	meaning	examples
==	equivalent values	choice == "a"	5.0 == 5
!=	not equivalent values	a != b
<	less than	"a" < "c"	balance < price
<=	less than or equal to	a <= b
>	greater than	"z" > "d"	price > balance
>=	greater than or equal to	a >= b
in	is member of	"h" in "hello"	5 in [1, 3, 5, 7, 9]
not in	is not a member of	"a" not in "hello  "	2 not in [1, 3, 5, 7, 9]

Exercises and questions

Open up a Python shell and use comparison operators to answer each of the following.

1. Is F (capital F) greater than z (lower case z)?

2. Is 0.0 equal to 0

3. Is [1, 2, 3, 4] greater than [5, 6, 7]?

4. Is the letter "z" in the string "zebra"?

5. Is the number 5 in the list [1, 2, 4]?

Logical Operators

When you have more than one expression in a condition, you need a logical operator which evaluates to either True or False depending on the boolean value of both the left and right hand values.

Some examples of expressions using logical operators:

• 5 < 3 or 5 > 2

• "h" in "hello" and "H" in "hello"

• not 1 == 2

Table 2 Logical operators

operator	meaning	example
and	both conditions are true	choice > lowest and choice < highest
or	either condition is true	choice < lowest or choice > highest
not	the following is not true	not something

The not operator is special in that it does not include a left-hand value. You can think of it like the opposite of the value that follows.

Exercises and questions

Tip: Check if a number is divisible by another

To test if a number is divisible by another number use the modulo operator % which will give you the division remainder.

For example, 5/2 is 2.5, or 2 with a remainder of 1.
So 5%2 is 1.

And so to check if a number is divisible by another, just check if the remainder is zero.

num % 2 == 0

For each of the following get a random number using random.randint().

1. Get a random number between 1 and 100 then check to see if it is greater than 50 or if it’s an even number.

2. Get a random number between 1 and 100 then check to see if it is an even number and divisible by ten.

3. Use the input() function to ask for a number and save it to a variable called num. Check num.isnumeric() and print an error message if it is not.

Truthy and Falsy

Python evaluates conditional expressions in a boolean context which determines if the resulting value is truthy or falsy.

A truthy value is one that Python considers to be the equivalent of True, while a falsy value is one that Python considers to be the equivalent of False. It determines this by first converting it to a boolean value, which can be done using the bool() function.

Some examples of falsy values are

• 0 zero

• "" an empty string

• [] an empty list

Some examples of truthy values are:

• 5 (a non-zero number)

• "hello" (a non-blank string)

• [35, 32, 89] (a non-empty list)

Here are details for each data type.

Table 3 truthy examples and falsy values for each data type

Type	Name	Falsy	Truthy
str	string	""	"a"
int	integer	0	1
float	float	0.0	0.5
dict	dictionary	{}	{"a": 1}
list	list	[]	[1]
tuple	tuple	()	(1)
bool	boolean	False	True
None	none	None

Since conditions are evaluated in a boolean context (meaning the result of the expression is converted to a bool) you can use a value as condition for truthiness, or add the not operator for falsiness.

In this example we check the truthiness of an integer which we know will be either 1 or 0.

Listing 51 the truthiness of an int

is_winner = random.randint(0, 1)
if is_winner:
  print("Contgratulations, you win!")

This example checks to make sure that a user response is not blank.

Listing 52 the falsyness of a string

response = input("What's your name? ")
if not response:
  print("Didn't get that. Try again.")

This example function expects a list of items. It checks to make sure the list is not empty first.

Listing 53 the falsyness of a list

def buy_items(items):
  if not items:
    print("Oops, the list of items is empty.")
    return

  print("You are buying", len(items), "items.")
  for item in items:
    buy(item)

Exercises and questions

Open up a Python shell and use the bool() function to find out if each of the following is truthy or falsy.

1. -1 negative one

2. " " a space

3. {} an empty dictionary

4. [0] a list containing the value 0

Glossary

Conditional Statements

comparison operator

An operator that compares the left-hand value to the right-hand value then evaluates to either True or False.

condition

conditional

A valid piece of code that, when evaluated, results in boolean value.

falsy

A value that is False when evaluated in a boolean context, or when converted to a boolean using the bool() function.

if statement

a compound statement that changes what code is executed depending on its conditions

logical operator

An operator that considers both left-hand value and right-hand value then evaluates to either True or False.

truthy

A value that is True when evaluated in a boolean context, or when converted to a boolean using the bool() function.

boolean context

Where the result of an expression is converted to a bool.

Exercises

Exercise 3 (Flip a coin)

1. Pick a random number between 0 and 1 and assign it to a variable coin.

2. Print “You tossed”, and the value of coin.

3. If the value of coin is truthy, print “You win the coin toss!”

Exercise 4 (Computer guessing game)

1. Pick a number between 1 and 100 and assign it to the variable pick. (Note: not random.)

2. Get a random number between between 1 and 100 and assign it to the variable guess.

3. Print "The computer guessed” and the value of guess.

4. If guess is the same as pick print "The computer got it right!"

5. If guess is within 30 of pick print "The computer was close." (Hint: You’ll need the and operator.)

6. Otherwise print "The computer got it wrong."

Exercise 5 (Heads or Tails)

In this exercise use the and operator and determine the truthiness of a value.

1. Ask the user "heads or tails?"

2. If the answer is blank, tell them they have to enter something.

3. If the answer is not "heads" or "tails", tell them they need to pick "heads" or "tails". (Bonus: Make this case-insensitive.)

4. Randomly decide if they won the coin toss.