Python Programming: Conditionals

Learning Objectives

After this lesson, you will be able to:

Use comparison and equality operators to evaluate and compare statements.
Use if/elif/else conditionals to control program flow.

Unit 2 Kickoff

In Unit 1, we ended by printing the rating for a movie: print('The rating for', movie_title, 'is', movie_rating).

In Unit 2, we’re going to learn to add logic and make this much more complex. By the end of this:

We’ll have a variable that’s set to either 1 or 2. If the variable is a 1, we’ll print the movie title, and if the variable is a 2, we’ll print the rating.
We’ll have many movies in a list and print them all out with just one print statement using a loop.
We’ll make pieces of our program easy to reuse using functions.

Ready? Let’s go!

Discussion: What Do You Notice?

Consider the following pseudocode for “French toast à la GA.”

1) Dip the bread in eggs.
2) Cook the bread for 3 minutes on each side.

Now, consider this:

1) Dip the bread in eggs.
2) If the bread is thicker, dip the bread again until it's soaked through.
3) Cook the bread for 3 minutes.
4) Check if the bread is brown on the bottom. If not, keep cooking the bread.
5) Flip the bread, and repeat steps 3 and 4.

What do you notice?

Saying “Yes” or “No”

- **If** the bread is thicker…
- **If** the bread is brown…

Goal: Programs need to make choices.

To do that, programs need to be able to say, “Is this bread thick? Yes or no?”

Question: How does a computer say “yes” or “no”?

Boolean Values: The Foundation of Programming

“Yes” in computer is True. “No” in computer is False.

This is the case in every programming language — it’s specific to computers themselves.

These are called Boolean values.

Is the bread sliced?
- True.
Is the bread brown?
- False.
Is 2 larger than 6?
- False.
Is 6 larger than 2?
- True.

Comparison and Logic in Programming

Now we can say “yes” or “no,” but how do we ask the question?

The first way is with comparison operators.

How does a computer decide True or False?

Comparison Types Practice

Check out these comparison operators. Why do you think the last one is False?

Talking Points:

Why is that last one false? Because d occurs after a in the character set.
To a computer, characters go a, b, c, d… Because d is after a in a computer’s order, a < d. Therefore, this string comparison will evaluate to False.

Repl.it note: This repl.it contains the following code:

print("3 < 5 is...", (3 < 5))

print("13 >= 13 is....", (13 >= 13))

print("50 > 100 is...", (50 > 100))

print("'d' < 'a' is...", ("d" < "a"))

Equality Operators: Equality (`==`)

Accept any two types of data as inputs.
Will only evaluate to True if both sides are completely identical in data type and value.

Teaching Tips:

Spend just a minute on this slide! The repl.it is there for student to run and see, not as the basis for any exercises. It’s especially important to point out strings versus numbers.
Make sure students understand types — strings versus numbers, for example. "7" compared to 7 can be tough.

Talking Points:

Now, let’s take a look at equality operators.
Equality operators check to see whether or not two values are the same as, or equal to, each other.
This operator will accept any two types of data as inputs and evaluate to a Boolean value (True or False).
It will only evaluate to True if both sides are completely identical in data type and value (i.e., a string and a number will never be equal because they are different data types.)
For example, 5 == 5 will evaluate to True, while 5 == "5" will evaluate to False, as, while the values are the same, 5 is a number and "5" is a string. (Strings always have quotes.)

Repl.it note: This repl.it contains the following code:

print("5 == 5 is..", 5 == 5)

print("6 == 3 is...", 6 == 3)

print("'5' == 5 is..", "5" == 5)

Equality Operators: Inequality (`!=`):

Will accept any two types of data as inputs.
The reverse of the equality operator.

Comparison Operators: Knowledge Check

What do you think the following will equal?

8 > 8
8 >= 8
8 <= 15
7 != "7"
6 == 7
6 != 7

“Truthy” and “Falsey”

Something that’s True is always true… right?

Yes, I totally cleaned my room. Just don't look under the bed…

Sometimes, we need “truthy” and “falsey.” They’re not explicitly True or False, but implicitly behave in the same way.

Sometimes, True and False really mean, “Is there anything there?”

"Hello, World!"  # A non-empty string: Truthy / True.
13               # A non-zero number: Truthy / True.
""               # An empty string: Falsey / False.
0                # The number 0: Falsey / False.

Teaching Tips:

Give a variety of real-world examples to contextualize these concepts, especially for truthy and falsey.
It can be difficult to remember what gets returned in an and or or statement. Continuously recap both if something is True or False, but also what value would get returned.

Talking Points:

True and False are the standard Boolean values that we’ll be using with our comparisons. However, in Python there are also other values that will evaluate to True or False if they are used in a comparison. These are called “truthy” and “falsey” values because they are not explicitly True or False but they implicitly behave in the same way.
Any string (or other collection, like a list, which we’ll learn about soon) that is empty is considered “falsey,” so it evaluates to False. Similarly, any number with a value of zero is considered False. In these situations, True and False results basically indicate whether or not the variable you are comparing contains a value.

The Logical Operators: `or` and `and`

What if we need to check multiple things that must all be True?

To make a peanut butter and jelly sandwich, we need peanut butter, and jelly, and bread.

Or check multiple things and only one needs to be True?

To make a fruit salad, we only need oranges, or apples, or strawberries.

The Logical Operators: `or`

"or checks if either comparison is True.

Teaching Tips:

The repl.it is for an example, not an exercise. Run the code and walk students through it.

Talking Points:

"or checks if either comparison is True and returns the first True value it finds. If neither side is True, then or returns False and the last False value.

Repl.it note: The repl.it contains:

red_score = 7
blue_score = 5
green_score = 0
yellow_score = 0

# or prints the first truthy statement.
print(red_score or blue_score)
# 0 is considered False
print(green_score or blue_score)
# If all are false, or prints the last False statement.
print(green_score or yellow_score)

The Logical Operators: `or` Truth Table

The or truth table:

True or True
# => True
True or False
# => True
False or True
# => True
False or False
# => False

The Logical Operators: `and`

and checks if both comparisons are True.

Teaching Tips:

The repl.it is for an example, not an exercise. Run the code and walk students through it.

Talking Points:

“and checks if both comparisons are True. If both sides are True, then and will give back the last True value. If either side is False, and will return the first False value it finds.”

Repl.it note: The repl.it contains:

red_score = 7
blue_score = 5
green_score = 0
yellow_score = 0

# and returns the last True statement.
print(red_score and blue_score)
# and returns the first False statement.
print(green_score and blue_score)
print(green_score and yellow_score)

The Logical Operators: `and` Truth Table

The and truth table:

True and True
# => True
True and False
# => False
False and True
# => False
False and False
# => False

Quick Review: Comparing Variables Using Operators

When comparing, a computer always returns a Boolean value: True or False.
We compare with operators like <, <=, >, >=, ==, and !=.
We can also use the logical operators and and or.

Pro tip: Using only one equal (=) always assigns the variable!

Up next: Conditionals.

Conditionals: `if`

Do you remember this?

- **If** the bread is thicker…
- **If** the bread is brown…

How can we put that in a program?

if the bread is thick
    # print("Dunk the bread longer!")

# No matter what:
print("Finished dunking the bread")

`if` Syntax

if condition:
  # Run these lines if condition is True.
  # These lines are indented.

# Unindented things always happen.

We Do: It’s Too Hot In Here

Remember, in a We Do, you follow along!

Our goal: A temperature program that lets us know when it is too hot.

On your computer, open Atom and create a new file; save it as control_flow.md.
Set up a temperature variable.
Type this; don’t just copy! The more practice you have typing it, the easier it will be to remember.

temperature = 55
print("It's too hot!")

We Do: Add an `if` Statement

That’s not hot! Let’s add an if statement:

temperature = 55
if temperature > 80:
  print("It's too hot!")

What about a higher temperature? Like 95?

Teaching Tips:

Run this and have the students run this.
Encourage students to type, not copy.
Make sure they have the indent — they shouldn’t see anything print!
Then, change the temperature to 95 and run it.
This is an excellent time to mention space indents versus tab indents. Note that the Python style guide calls for four spaces, so that’s the students’ goal. Help them set up their text editor to use four spaces when they hit tab, for example. Show them that different numbers of spaces don’t work.

Talking Points:

OK, we have a program that sets the temperature to 55 degrees Fahrenheit and then immediately prints that it is too hot. But 55 degrees isn’t really hot, so our app is kind of useless. Let’s give it the ability to make a decision about whether or not it is too hot. Now our program will only complain about the heat if it is above 80 degrees.
At present, the program prints nothing. Let’s make sure our if statement works.

We Do: The `else` Statement

What about printing a message for when it isn’t too hot?

if condition:
  # Do something
else:
  # Do something else

The else block is executed only if the if condition evaluates to False.

Let’s try it:

temperature = 95
if temperature > 80:
  # If true, run this code block.
  print("It's too hot!")
else:
  # Otherwise, run this code block.
  print("It's just right!")

Teaching Tips:

Run this and have the students run this.
Encourage students to type, not copy.
Make sure they have the indents correct!
Then, change the temperature to 25 and run it again.

Talking Points:

Using the if statement like the one above gives us a situation where the program will do something if the condition is True but it will do nothing if the condition is False. What if we want it to do one thing if it’s True and a completely different thing if it is False? Python gives us the else statement. It has this basic structure. This works exactly the same as a simple if statement except that it adds else and another line that will be executed only if the condition evaluates to False.
Let’s use this to add some more messages to our temperature program so that it will say something for any temperature. Now for any temperature above 80, the program will print a complaint. Else, if the temperature is not above 80, the program will express its satisfaction. Change the temperature to 65 and run it again. Python chooses the other path now and executes the line saying that it is just right.

Discussion: Other Cases

What if it’s too cold? We need more conditions.

if temperature > 80:
  # If it is too hot, run this code block.
  print("It's too hot!")

# We want: Else if temperature < 40.
# We want: Print that it's too cold  .

else:
  # Otherwise, run this code block.
  print("It's just right!")

What do you think we need?

We Do: The `elif` Statement

That’s where the elif (“else if”) statement works its magic.

temperature = 60

if temperature > 80:
  print("It's too hot!")

elif temperature < 40:
  print("It's too cold!")

else:
  print("It's just right!")

Teaching Tips:

Run this and have the students run this.
Make sure they have the indents correct!
Then, change the temperature to different values and see it working.
Walk through this line by line.
Be clear that else always comes at the bottom — there is only one else! Any middle conditional is elif.

Talking Points:

That’s where the elif statement works its magic. elif is a portmanteau of “else if.” It is used when you need to have multiple branches of execution but each one needs to use a different comparison. Let’s use this to beef up our temperature program to give some nice feedback.
Let’s look at this line by line. We make our temperature variable and set it to 95. Then, we check to see if it is greater than 80. If it is, we print the “hot” message. If that condition is False, we then move to the next one, which checks to see if the temperature is between 60 and 80. Note the use of the and operator to make sure that both of those comparisons must be True for the whole conditional to be True. If the temperature is less than or equal to 80 and greater than 60, then we print the “just right” message. If that one is False, we proceed to the next elif, which checks for cold temperatures. Finally, we end with else. You will use else as the last statement in any block that uses elif statements."

We Do: Adding More `elif`

We can have as many elif as we’d like, but only one else.

Let’s change this up — remember, type this out for practice.

temperature = 95
if temperature > 80:
  print("It's too hot!")
elif temperature <= 80 and temperature > 60:
  print("It's just right!")
elif temperature <= 60 and temperature > 40:
  print("It's pretty cold!")
else:
  print("It's freezing!")

Thought Exercise

What do you think the following code will print? Why?

foo = 5
bar = 1
if foo > 13:
  print("Flip")
elif bar:
  print("Flop")
else:
  print("Fly")

Partner Exercise: Even or Odd

Pair with a new partner. Decide who will drive and who will navigate.

Open a new file in Atom; save it as check_even.py.

In it, write a program that prints whether a number is even or odd.

Do you remember how to determine that?

We can use the modulus operator (%) to check the remainder.

Here is some code to get you started:

number = 10
remainder = number % 2
# For an even number, print "It's even!"
# For an odd number, print "It's odd!"

Partner Exercise: `and` and `or`

Switch driver and navigator.

In a file (it can be the same one), write a program that compares two variables and prints out statements accordingly. Start here and follow this:

x = 8
y = 0
a = "Hello!"
b = ""

# Check if x and b are both True. If they are, print "Both of these are true."
# Check if y or a is False. If one is, print "One of these is false."
# Check if either x or y is False. If one is, print out "One is false."
# Then, only if either x or y is False, check if x is greater than y. If it is, print out "x is greater than y."

Summary: Boolean Values and Operators

We’ve started control flow — changing what our program does based on a decision. We used:

Boolean values

True and False.
The corresponding “truthy” and “falsey”.

Conditional operators

Comparison: <, >, <=, and >=.
Equality: == and !=.

Logical operators: all and or

or evaluates to True if any of the comparisons are True.
and evaluates to True only if all of the comparisons are True.

Summary and Q&A

Then, we went into if and else:

“If your toast is thick, dip the bread for longer, else do not.”

if: Use only as the first conditional operator.
elif: Adds multiple comparisons to your if blocks.
else: Use only at the end of your code block, for if the previous conditional tests are False.

Python Programming: Conditionals

Learning Objectives

Unit 2 Kickoff

Discussion: What Do You Notice?

Saying “Yes” or “No”

Boolean Values: The Foundation of Programming

Comparison and Logic in Programming

Comparison Types Practice

Equality Operators: Equality (==)

Equality Operators: Inequality (!=):

Comparison Operators: Knowledge Check

“Truthy” and “Falsey”

The Logical Operators: or and and

The Logical Operators: or

The Logical Operators: or Truth Table

The Logical Operators: and

The Logical Operators: and Truth Table

Quick Review: Comparing Variables Using Operators

Conditionals: if

if Syntax

We Do: It’s Too Hot In Here

We Do: Add an if Statement

We Do: The else Statement

Discussion: Other Cases

We Do: The elif Statement

We Do: Adding More elif

Thought Exercise

Partner Exercise: Even or Odd

Partner Exercise: and and or

Summary: Boolean Values and Operators

Summary and Q&A

Equality Operators: Equality (`==`)

Equality Operators: Inequality (`!=`):

The Logical Operators: `or` and `and`

The Logical Operators: `or`

The Logical Operators: `or` Truth Table

The Logical Operators: `and`

The Logical Operators: `and` Truth Table

Conditionals: `if`

`if` Syntax

We Do: Add an `if` Statement

We Do: The `else` Statement

We Do: The `elif` Statement

We Do: Adding More `elif`

Partner Exercise: `and` and `or`