Day 5#

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Chapter 5: Branching — Programs That Make Decisions#

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5.1 What Is Branching?#

So far, all programs we've written execute line by line, top to bottom. Every line runs every time. But real programs need to make decisions: do one thing if a condition is true, a different thing if it's false.

This is branching — or conditional execution. It is the first of the three fundamental control structures in programming (the others are loops and functions).

The Real-World Analogy#

Think of a recipe that says: "If the dough is too sticky, add more flour; otherwise, continue to kneading." The chef evaluates a condition (stickiness) and follows a different path depending on the result. Programs do the same.

5.2 Boolean Values and Conditions#

A condition is an expression that evaluates to either True or False. These are the two possible values of the bool type.

Comparison Operators#

>>> 10 == 10
True
>>> 10 == 10.0
True     # int and float can be equal
>>> "hello" == "hello"
True
>>> "Hello" == "hello"
False    # strings are case-sensitive!
>>> 5 > 3
True
>>> 5 < 3
False

Boolean Operators#

You can combine conditions using boolean operators:

x = 7
print(x > 0 and x < 10)    # True: x is between 0 and 10
print(x < 0 or x > 100)    # False: neither condition is true
print(not (x == 7))         # False: x IS 7, so not True = False

Truth table for and: | A | B | A and B | |---|---|---| | True | True | True | | True | False | False | | False | True | False | | False | False | False |

Truth table for or: | A | B | A or B | |---|---|---| | True | True | True | | True | False | True | | False | True | True | | False | False | False |

Short-Circuit Evaluation#

Python evaluates and and or lazily:

• A and B: if A is False, Python doesn't evaluate B (the result is already False)
• A or B: if A is True, Python doesn't evaluate B (the result is already True)

This matters when B has side effects (like calling a function). For now, just know it exists.

5.3 The if Statement#

The if statement is the simplest form of branching:

if condition:
    # block of code that runs only if condition is True
    statement1
    statement2
    ...
# code here runs regardless

Syntax rules:

1. if followed by the condition
2. Condition followed by a colon :
3. The body is indented — typically 4 spaces
4. All indented lines form the "block"
5. The block ends when indentation returns to the if level

temperature = float(input("What is the temperature? "))
if temperature < 0:
    print("It's freezing!")
    print("Wear a heavy coat.")
print("Have a nice day.")    # always runs

Indentation Is Mandatory#

Python uses indentation to define code blocks. This is unusual — most languages use braces {}. In Python, wrong indentation is a SyntaxError:

if True:
print("hello")     # IndentationError! Body must be indented

By convention, use 4 spaces per indent level. Don't use tabs (some editors convert them; this causes confusing errors when mixed with spaces).

5.4 if–else#

Add an else clause to specify what happens when the condition is False:

if condition:
    # runs when condition is True
    ...
else:
    # runs when condition is False
    ...

One and only one branch always runs:

age = int(input("How old are you? "))
if age >= 18:
    print("You are an adult.")
else:
    print("You are a minor.")
print("Thanks!")    # always runs

5.5 if–elif–else#

For multiple mutually exclusive conditions, use elif (short for "else if"):

score = int(input("Enter your score: "))

if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
elif score >= 70:
    print("Grade: C")
elif score >= 60:
    print("Grade: D")
else:
    print("Grade: F")

Python checks each condition in order. As soon as one is True, it runs that block and skips all remaining elif/else branches. If none of the if or elif conditions are True, the else block runs.

Important: Order Matters with elif#

score = 95

# WRONG ORDER — "B" condition fires before "A" is checked
if score >= 80:
    print("B or higher")    # This prints for score=95!
elif score >= 90:
    print("A")              # This never runs

# CORRECT ORDER — more restrictive conditions first
if score >= 90:
    print("A")
elif score >= 80:
    print("B")

5.6 Nested Conditionals#

You can place an if statement inside another if statement. This is called nesting:

x = float(input("Enter a number: "))
y = float(input("Enter another number: "))

if x == y:
    print("x and y are equal")
    if y != 0:
        print("Their ratio is", x / y)
elif x < y:
    print("x is smaller than y")
else:
    print("y is smaller than x")

A Warning About Nesting Depth#

Deep nesting (more than 2–3 levels) makes code hard to read:

# Hard to follow:
if a:
    if b:
        if c:
            if d:
                print("deep!")

If you find yourself nesting deeply, there's often a cleaner way using and to combine conditions, or by breaking code into functions (Week 3).

5.7 Tracing Branching Programs#

One of the most important skills in programming is tracing — stepping through code mentally (or on paper) and tracking what each variable holds and which branches execute.

Example: Trace This Program#

answer = ''
x = 11
y = 2
if x == y:
    answer = answer + 'M'
if x <= y:
    answer = answer + 'i'
else:
    answer = answer + 'T'
print(answer)

Trace:

1. answer = '' → answer is ''
2. x = 11, y = 2
3. if x == y: → 11 == 2 → False → skip the answer + 'M' line
4. answer is still ''
5. if x <= y: → 11 <= 2 → False → go to else
6. answer = '' + 'T' → answer is 'T'
7. print('T')

Now trace it with y = 11: 1–2. Same.

1. if x == y: → 11 == 11 → True → answer = '' + 'M' → answer is 'M'
2. if x <= y: → 11 <= 11 → True → answer = 'M' + 'i' → answer is 'Mi'
3. print('Mi')

Practice habit: Whenever you read a branching program, trace it with at least 3 different inputs before running it. This builds the skill of reading code, not just writing it.

5.8 Common Mistakes in Branching#

Mistake 1: Assignment Instead of Comparison#

x = 5
if x = 5:        # SyntaxError! Should be ==
    print("five")

if x == 5:       # Correct
    print("five")

Python's = is assignment. == is equality comparison. Don't confuse them.

Mistake 2: Indentation Errors#

if x > 0:
    print("positive")
  print("still positive?")    # IndentationError — inconsistent indent

if x > 0:
    print("positive")
print("always prints")         # This is NOT in the if block — correct!

Mistake 3: Missing elif (Using if When You Mean elif)#

score = 85

# WRONG: multiple if's are all evaluated independently
if score >= 90:
    grade = 'A'
if score >= 80:      # This ALSO runs! Now grade is 'B', not 'A'
    grade = 'B'
if score >= 70:
    grade = 'C'
print(grade)    # prints C for score=95!

# CORRECT: elif ensures only one branch runs
if score >= 90:
    grade = 'A'
elif score >= 80:
    grade = 'B'
elif score >= 70:
    grade = 'C'
print(grade)    # prints A for score=95

Mistake 4: Redundant else After a return or print#

This isn't a bug, just poor style:

# Redundant (works but unnecessary):
if x > 0:
    print("positive")
else:
    if x == 0:
        print("zero")
    else:
        print("negative")

# Cleaner:
if x > 0:
    print("positive")
elif x == 0:
    print("zero")
else:
    print("negative")