Flow Control

Fundamentals: Flow Control in Python

Normally, Python executes instructions from top to bottom. Flow control allows a program to choose which instructions to execute.

For example, an app could warn you if you have spent too much time on your phone:

time_on_phone = 20

if time_on_phone > 15:
    print("Warning: you have been on your phone for too long!")

Warning: you have been on your phone for too long!

time_on_phone = 10

if time_on_phone > 15:
    print("Warning: you have been on your phone for too long!")

# Nothing is printed because the condition is False

To make decisions, Python needs questions whose answers are either True or False.

Comparison Operators

Comparison operators allow us to compare values, variables, or expressions.

Operator	Meaning	Example
==	Equal to	x == 3
!=	Not equal to	x != 3
<	Less than	x < 3
>	Greater than	x > 3
<=	Less than or equal to	x <= 3
>=	Greater than or equal to	x >= 3

The double equals sign == compares two values. The single equals sign = assigns a value to a variable.

Open a new Python notebook and run the following snippets. Remember that you can copy code using the button at the top-right of each cell and run it using the button at the top-left.

x = 3
y = 2

# Are x and y equal?
print(x == y)

False

# Are x and y different?
print(x != y)

True

Comparisons return either True or False. These two values are called Booleans.

# Is x greater than y?
print(x > y)

True

Comparison operators do not distinguish between equivalent integers and floats.

x = 3
y = 3.0

print(x == y)

True

We can store the result of a comparison in a variable:

temperature = 38
fever = temperature >= 38

print(fever)

True

Check Your Understanding

Predict whether each expression is True or False. Check your answers in Python afterward.

7 > 4
7 == 7.0
5 != 5
3 <= 3
10 < 2

Solution

True
True
False
True
False

Strings and Comparison Operators

Comparison operators care about the types of the values being compared. A string and a number are different values.

x = 3
y = "3"

print(x == y)

False

We can compare two strings too.

x = "hi"
y = "hello"

print(x == y)

False

String comparison is case-sensitive. Two strings are equal only when their contents and capitalization match exactly.

x = "hi"
y = "HI"

print(x == y)

False

Python can also order strings. It compares them lexicographically, one character at a time. Capitalization affects this ordering.

x = "hello"
y = "hola"

print(x > y)

False

Exercise: Security Check

A secret code is stored as the string "Python". Create a variable called attempt and compare it with the secret code.

Try the values "Python", "python", and "Python ". Explain why only one of them matches.

Solution

secret_code = "Python"
attempt = "Python"

print(attempt == secret_code)

True

The comparison is case-sensitive, and spaces are characters too. Therefore, "python" and "Python " do not equal "Python".

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The if Statement

The if statement executes a block of code only when a condition is True. It lets a program make a decision.

if <condition>:
    <code to run if the condition is True>

Pay special attention to the colon and indentation. The indentation determines which instructions belong to the if statement.

<this instruction always runs>

if <condition>:
    <this instruction runs only if the condition is True>

<this instruction always runs>

For instance, let us write code that changes any number below 5 into 5.

x = 2

# If x is lower than 5, change it to 5
if x < 5:
    x = 5

print(x)

5

Exercise

Create a numeric variable called years. Print I am X years old, where X is the value stored in years, but only if years is positive.

Solution

years = 5

if years > 0:
    print(f"I am {years} years old")

I am 5 years old

Try changing years to a negative number. The program should print nothing.

What happens if years contains a string? The comparison raises an error because Python cannot order a string and an integer.

Exercise: Battery Warning

A robot has a variable called battery. If the battery is at or below 20, print Low battery: return to base. Regardless of the battery level, print Status check complete afterward.

Try your program with battery = 15 and battery = 80.

Solution

battery = 15

if battery <= 20:
    print("Low battery: return to base")

print("Status check complete")

Low battery: return to base
Status check complete

Debugging Exercise

Fix the following code blocks.

# Print the square root of a number only if it is positive
x = 4

if x >= 0:
print(f"The square root of {x} is {x ** 0.5}")

# Replace a number by its square if the original is even
x = 4

if x % 2 = 0
    x = x ** 2

print(x)

# Print "Hello" if a name has more than three letters
name = "John"

if name > 3:
    print("Hello")

Solution

The first block needs indentation. The second uses = instead of == and is missing a colon. The third compares a string with a number instead of checking the length of the string.

x = 4

if x >= 0:
    print(f"The square root of {x} is {x ** 0.5}")

The square root of 4 is 2.0

x = 4

if x % 2 == 0:
    x = x ** 2

print(x)

16

name = "John"

if len(name) > 3:
    print("Hello")

Hello

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The else Statement

The else statement provides an alternative. Its block executes when the if condition is False.

if <condition>:
    <code to run if the condition is True>
else:
    <code to run if the condition is False>

Only one of the two branches executes.

<this instruction always runs>

if <condition>:
    <one branch runs>
else:
    <the other branch runs>

<this instruction always runs>

# Check whether a student passed an exam
# Assume that the mark is between 0 and 10
grade = 7

if grade >= 5:
    print("You passed")
else:
    print("You have not passed")

You passed

Exercise: Even or Odd

Write a program that reports whether a number is even or odd.

Solution

x = 7

if x % 2 == 0:
    print("x is even")
else:
    print("x is odd")

x is odd

Exercise: Locked Door

A player can open a door only when the Boolean variable has_key is True. Print The door opens if the player has the key. Otherwise, print The door is locked.

Test both possible values of has_key.

Solution

has_key = False

if has_key:
    print("The door opens")
else:
    print("The door is locked")

The door is locked

Predict or Debug

For each code cell, decide whether it prints a message or raises an error. If it prints something, write the exact output.

name = "Alberto"

if "a" in name:
    print("The name contains the letter a")
else:
    print("The name does not contain the letter a")

x = 9

if x % 3 == 0:
    print("x is divisible by 3")
else
    print("x is not divisible by 3")

x = 5
y = x < 0

if y:
    print("x is negative")
else:
    print("x is zero or positive")

Solution

The first block prints The name does not contain the letter a. The search is case-sensitive, and Alberto contains uppercase A, not lowercase a.

The second block raises a syntax error because else is missing a colon.

The third block prints x is zero or positive. The comparison 5 < 0 produces False, so the else branch runs.

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The elif Statement

Sometimes a program must choose between more than two possibilities. The elif keyword, short for “else if,” checks another condition when the preceding conditions were False.

if <condition1>:
    <code if condition1 is True>
elif <condition2>:
    <code if condition2 is True>
else:
    <code if no earlier condition is True>

Python checks the conditions from top to bottom and executes the first branch whose condition is True. After selecting a branch, it skips the rest of the chain.

x = -5

if x > 0:
    print("x is positive")
elif x < 0:
    print("x is negative")
else:
    print("x is zero")

x is negative

We can chain several elif clauses:

x = -10

if x > 10:
    print("x is greater than 10")
elif x > 0:
    print("x is positive")
elif x < -5:
    print("x is lower than -5")
elif x < 0:
    print("x is negative")
else:
    print("x is zero")

x is lower than -5

The order matters. An early condition can capture a value before Python reaches a later, more specific condition.

Exercise: An Impossible Branch

Which branch can never execute, regardless of the value of x?

if x > 10:
    print("greater than 10")
elif x > 0:
    print("positive")
elif x < 0:
    print("negative")
elif x < -5:
    print("lower than -5")
else:
    print("zero")

Solution

The branch elif x < -5 can never execute. Every number below -5 already satisfies the earlier condition x < 0.

Put the more specific condition first:

x = -10

if x > 10:
    print("greater than 10")
elif x > 0:
    print("positive")
elif x < -5:
    print("lower than -5")
elif x < 0:
    print("negative")
else:
    print("zero")

lower than -5

Exercise: Theme-Park Ticket

Calculate a ticket price from a visitor’s age:

• Under 4 years: free.
• From 4 to 12 years: €8.
• From 13 to 64 years: €15.
• 65 years or older: €10.

Store the result in price, then print it. Test at least one age from every category.

Solution

age = 20

if age < 4:
    price = 0
elif age <= 12:
    price = 8
elif age <= 64:
    price = 15
else:
    price = 10

print(f"Ticket price: €{price}")

Ticket price: €15

After the first condition fails, Python already knows that age >= 4. We do not need to repeat that comparison in the second condition.

Exercise: A Piecewise Function

Write a program that calculates the following function:

\[ f(x) = \begin{cases} x^2, & x < 0, \\ 2x + 1, & 0 \leq x < 3, \\ 7, & x \geq 3. \end{cases} \]

Store the result in fx and print it. Test x = -2, x = 1, and x = 5.

Solution

x = 1

if x < 0:
    fx = x ** 2
elif x < 3:
    fx = 2 * x + 1
else:
    fx = 7

print(fx)

3

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Logical Operators

Logical operators combine or modify Boolean conditions.

and

The and operator returns True only when both conditions are True.

if <condition1> and <condition2>:
    <code to run if both conditions are True>

# A student passes only with a sufficient grade and attendance
grade = 8
attendance = 89

if grade >= 5 and attendance >= 80:
    print("You passed")
else:
    print("You have not passed")

You passed

or

The or operator returns True when at least one condition is True.

if <condition1> or <condition2>:
    <code to run if either condition is True>

grade = 8
attendance = 89

if grade < 5 or attendance < 80:
    print("You have not passed")
else:
    print("You passed")

You passed

not

The not operator reverses a Boolean value. not True is False, and not False is True.

if not <condition>:
    <code to run if the condition is False>

# Add a period if the string does not already end with one
s = "Hello world"

if not s.endswith("."):
    s = s + "."

print(s)

Hello world.

Exercise: Cat or Dog?

Write a sentence containing the word cat and store it in sentence. Replace cat with dog, but only if both love and cat appear in the sentence.

Solution

sentence = "I love cats"

if "love" in sentence and "cat" in sentence:
    sentence = sentence.replace("cat", "dog")

print(sentence)

I love dogs

Exercise: Spacecraft Launch

A spacecraft may launch only when all three conditions are met:

• fuel is at least 80.
• weather equals "clear".
• crew_ready is True.

Print Launch approved when all conditions are met. Otherwise, print Launch delayed.

Solution

fuel = 95
weather = "clear"
crew_ready = True

if fuel >= 80 and weather == "clear" and crew_ready:
    print("Launch approved")
else:
    print("Launch delayed")

Launch approved

Exercise: Coordinate Quadrants

Given coordinates x and y, report the point’s quadrant:

• Quadrant I: x > 0 and y > 0
• Quadrant II: x < 0 and y > 0
• Quadrant III: x < 0 and y < 0
• Quadrant IV: x > 0 and y < 0

If either coordinate equals zero, print The point is on an axis.

Solution

x = -2
y = 5

if x > 0 and y > 0:
    print("Quadrant I")
elif x < 0 and y > 0:
    print("Quadrant II")
elif x < 0 and y < 0:
    print("Quadrant III")
elif x > 0 and y < 0:
    print("Quadrant IV")
else:
    print("The point is on an axis")

Quadrant II

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Nested Conditions

We can place one if statement inside another. This is useful when the second decision matters only after the first condition is satisfied.

age = 20
has_ticket = True

if age >= 18:
    if has_ticket:
        print("You may enter")
    else:
        print("You need a ticket")
else:
    print("You must be at least 18")

You may enter

The inner condition is checked only when age >= 18 is True.

Exercise: Treasure Chest

A player finds a treasure chest. The chest opens only if has_key is True. If it opens, the player receives the treasure only if inventory_space > 0.

Print one of these messages:

• The chest is locked
• Your inventory is full
• Treasure collected!

Use one if statement inside another.

Solution

has_key = True
inventory_space = 2

if has_key:
    if inventory_space > 0:
        print("Treasure collected!")
    else:
        print("Your inventory is full")
else:
    print("The chest is locked")

Treasure collected!

Nested conditions are valid, but they are not always necessary. When two requirements lead to the same result, a logical operator may be clearer:

age = 20
has_ticket = True

if age >= 18 and has_ticket:
    print("You may enter")
else:
    print("Entry denied")

You may enter

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Optional: Concise Conditional Syntax

The regular indented form should be your default. Python also supports shorter forms that you may encounter in other people’s code.

One-Line if Statements

An entire if statement can appear on one line:

if <condition>: <code>

# Change negative numbers to 0
num = -3

if num < 0: num = 0

print(num)

0

Python also permits several instructions separated by semicolons, but this usually makes code harder to read. Prefer the indented form in this course.

Conditional Expressions

A conditional expression selects one of two values:

<expression1> if <condition> else <expression2>

# Format values below 100 with two decimals
# Format values of 100 or more with no decimals
num = 99
out = f"{num:.2f}" if num < 100 else f"{num:.0f}"

print(out)

99.00

Conditional expressions can be readable for short assignments. Use a normal if and else when the decision requires several instructions or is difficult to understand at a glance.

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Final Practice

Challenge: Delivery Decision System

A delivery drone uses three variables:

distance = 8
battery = 65
weather = "clear"

Apply these rules in order:

1. If the weather is "storm", the drone cannot fly.
2. Otherwise, if the battery is below 20, it must recharge.
3. Otherwise, if the distance is greater than 10, it needs at least 60 battery units.
4. For distances of 10 or less, it needs at least 30 battery units.

Print exactly one of these messages:

• Flight cancelled: storm
• Recharge required
• Delivery approved
• Insufficient battery for this distance

Test your program with several combinations. Try to make every possible message appear.

Example Solution

distance = 8
battery = 65
weather = "clear"

if weather == "storm":
    print("Flight cancelled: storm")
elif battery < 20:
    print("Recharge required")
elif distance > 10 and battery >= 60:
    print("Delivery approved")
elif distance <= 10 and battery >= 30:
    print("Delivery approved")
else:
    print("Insufficient battery for this distance")

Delivery approved

Reflection

For each concept, explain its purpose in one sentence:

1. A comparison operator
2. An if statement
3. An else branch
4. An elif branch
5. The and, or, and not operators

Then answer: why can an if statement make a decision but not repeat an action?

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Homework and Review

When you feel ready, test your knowledge using the review exercises.

Work through the “Flow Control” homework exercises available here. To earn participation credit, complete the exercises highlighted in red.

Homework

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In the Next Session…

We will learn about loops. They allow programs to repeat instructions and process several values without writing the same code again and again.

See you!