Conditionals

A conditional statement allows us to change program control flow based on the truth value of a condition. They are a form of decision-based logic that some people use in their daily lives. For example, if the temperature is above 4°C, then wear sandals.

temperature = 22
if temperature > 4:
    footwear = "sandals"
footwear # What's my footwear for today?

Booleans¶

To represent a concept like whether the temperature is above freezing, Python provides the bool data type that only has two possible values: True and False.

22 > 4
22 < 4
22 == 4
22 >= 22
(22 > 4) and (5 < 6)
(22 > 4) and (5 > 6)
not (5 <= 6)
not True

Booleans are values like numbers and text, so we can also assign them to variables.

x = 22 < 4
x

Practice: Sequential conditionals¶

Conditional statements can follow one after the other. How does Python evaluate this arithmetic program?

x = 10
y = 5
if x > y:
    x = x - 4
    y = y + 4
if x < y:
    x = x + 1
    y = y - 1
x, y # What is the value of x and y?

if-else statement¶

What happens if the temperature is right above freezing? It’s no longer sandals weather!

temperature = 1
if temperature > 4:
    footwear = "sandals"
footwear

To specify what to do when the condition is false, we can add an else case.

temperature = 1
if temperature > 4:
    footwear = "sandals"
else:
    footwear = "sandals with socks"
footwear

Practice: Nested conditionals¶

Conditional statements can be nested within each other to produce more complex logic. How does Python evaluate this program to report the absolute value?

val = 22
if val < 0:
    result = -val
else:
    if val == 0:
        result = val
    else:
        if val > 0:
            result = val
        else:
            result = val
result # What's the absolute value of val?

How would you rewrite this program to simplify the control flow?

if-elif-else statement¶

What if it’s below freezing? We probably need close-toed shoes.

temperature = -5
if temperature > 4:
    footwear = "sandals"
elif temperature >= 0:
    footwear = "sandals with socks"
else:
    footwear = "sneakers"
footwear

We can introduce as many elif cases as we like. Python will check cases from top to bottom and only run the first successful condition, or the else case if all other conditions failed. When does this code produce a different result from the previous code snippet?

temperature = 0
if temperature >= 0:
    footwear = "sandals with socks"
elif temperature > 4:
    footwear = "sandals"
else:
    footwear = "sneakers"
footwear

print function¶

While the Python console allows us to directly evaluate expressions like footwear and see the answer ("sandals"), Python Tutor doesn’t work the same way. To report a value, we can call the print function, which displays values in human-readable form. What do you notice is different about the output when we print a value?

print(footwear)

print is particularly helpful for showing multiple lines of text on the screen at once!

print("You: How is the weather today?")
print("Me: It's", footwear, "weather!")
print("You: Are you sure it's " + footwear + " weather today?")

Calling print() (with empty parentheses) produces a blank line.

We’ll learn more about functions in a future lesson: for now, we just need to be familiar with these different ways of calling print to show certain outputs.

Practice: Control flow¶

Which statements will be printed when we run the following program?

x = 5
y = 5
if x > 0:
    print("Statement 1")
elif y > 0:
    print("Statement 2")
else:
    print("Statement 3")