Booleans and Conditional Statements - Comprehensive Notes

Commenting shortcut

• You’ll see this used very often: you can select lines to temporarily disable them without deleting
• Mac: Command + /; Windows: Ctrl + /
• What it does: comments out every line in the selected block
• This is useful when you’re experimenting or changing code but want to keep the old version for reference
• Example in lecture: after highlighting lines, press the shortcut to comment, then write a new version to compare
• You can toggle comments with the same shortcut (comment/uncomment)
• This concept is simply called “comments” in code

Booleans: a new data type in Python

• Booleans (bool) are a new data type in Python
• Previously discussed data types: Integers, Floats, and Strings
• Booleans have only two possible values: true/false in everyday language, but in Python the literals are capitalized: True and False (keywords in Python)
• Lowercase true/false are not standard boolean literals in Python; they are treated as names if defined by the user, otherwise they’re not booleans
• Note on null: the transcript mentions null as a concept but we won’t cover it here; focus is on True/False
• Booleans can be assigned like other data types, e.g., flag = True or flag = False

Boolean expressions

• A Boolean expression is an expression that evaluates to a Boolean value: True or False
• Think of Boolean expressions as yes/no questions or statements that evaluate to True or False
• Examples:
  • It is raining outside → can be either True or False
  • The expression 10 > 5 → True, because 10 is greater than 5
  • A string comparison example: comparing two strings using relational operators will yield a Boolean result
• Boolean expressions are often built using relational operators (see below)

Relational operators and equality

• Relational operators (operators that compare values) include:
  • Greater than: ">"
  • Less than: "<"
  • Greater than or equal to: ">="
  • Less than or equal to: "<="
  • Equal to: "==" (note: this is the equality test in Python)
  • Not equal to: "!="
• In Python, there is an important distinction between the assignment operator and the equality operator:
  • Assignment operator: = (used to assign a value to a variable)
  • Equality operator: == (used to compare two values and produce a Boolean result)
  • The analogy the lecture uses: assignment is a statement (it assigns), while == is a question (are these two values equal?)
• Practical note:
  • When you write a condition, you’ll commonly use these relational operators to form a Boolean expression, which can then drive conditional statements

Strings and ASCII-based comparisons

• Python can compare strings using relational operators because each character has an ASCII value
• How it works: compare strings left to right, character by character, using their ASCII codes until a difference is found
• Example: compare 'code' and 'coat' (all lowercase)
  • Characters:
  • 'c' vs 'c' → both 99, so equal
  • 'o' vs 'o' → both 111, so equal
  • 'd' vs 'a' → 100 vs 97; since 100 > 97, 'code' > 'coat'
  • ASCII values to recall:
  • 'c' = 99, 'o' = 111, 'd' = 100, 'a' = 97
• Important caveat: string comparisons depend on case; all-lowercase or all-uppercase strings compare lexicographically within the same case
• Lexicographic order (alphabetization) can be used to sort strings, assuming consistent case across all strings being compared
• This concept is often used in homework and programs to order words or lists of strings

Why Boolean expressions matter

• Boolean expressions are used to control the flow of a program via conditional statements
• They enable programs to perform actions only when certain conditions are met
• The next topic shows how to build sequences of conditions using three clause types: if, elif, and else

Conditional statements (if, elif, else)

• Conditional statements check whether a Boolean expression is true and execute code accordingly
• There are three kinds of clauses you can use, in combination:
  • if clause (required, and must be the first clause)
  • elif clause (optional, can have zero or more; used for additional conditions)
  • else clause (optional, handles the default case when none of the previous conditions are true)
• Indentation is critical in Python: the block of code that runs if a condition is true must be indented under the corresponding clause
• Conceptual intuition: an if clause is like asking a question; if True, do something; if False, do nothing (or wait for other branches)

If clause

• Structure of an if clause in Python:
  • if <Boolean_Expression>:
  • (newline, indented) code to execute if the expression is True
• Examples and explanations from the lecture:
  • Example 1: if 10 > 5: followed by an indented block, e.g., print("10 is greater than 5") → executes because the expression is True
  • Example 2: if my_age > your_age: with appropriate values; if True, prints a message; if False, nothing happens in that branch
  • Example 3: if my_num == your_num: then print("Great minds think alike") → True if numbers are equal
  • Example 4: if my_num != your_num: → executes only when the numbers are not equal
• Notes in the lecture about equality and inequality:
  • == checks for equality; != checks for inequality
  • The lecture also mentions a hypothetical about a not-equal case and “not equal to” logic

Elif clause

• Elif (else-if) clauses provide additional conditions to check after the initial if
• You can have zero or more elif clauses; they are optional
• Important rules:
  • An elif clause must come after at least one if clause
  • The syntax is similar to the if clause: elif <Boolean_Expression>: followed by an indented action block
• Conceptual example (traffic light):
  • If the light is red, stop
  • Elif the light is yellow, slow down
  • Elif the light is green, go
  • Else (if none of the above), handle a default case (e.g., broken light)
• In practice, you can chain multiple elifs, for example: if condition1: ... elif condition2: ... elif condition3: ...

Else clause

• Else clause provides a default action when none of the previous conditions are true
• Structure: else: followed by an indented block of code
• Use case: catch-all when no earlier condition matches
• Note: Else is optional and should come after all if/elif blocks

Indentation and syntax essentials in Python

• Indentation determines blocks of code (no braces like in some other languages)
• The code in the if/elif/else blocks must be indented consistently (e.g., one tab or a consistent number of spaces)
• Mis-indentation leads to syntax errors and can break program execution
• The lecture shows examples where lines within the condition’s block are indented one level; the condition line itself is at the outer level

Real-world analogy: traffic lights

• Approach to a traffic light illustrates a chain of conditional checks:
  • Is the light red? If True, stop
  • Else, is the light yellow? If True, slow down
  • Else, is the light green? If True, proceed
  • At most one condition is true at a time, guiding the action
• This maps directly to Python’s if/elif/else flow control

Practical notes and next steps

• You’ve seen how boolean values, boolean expressions, and relational operators come together to form conditional logic
• In upcoming work, you’ll apply these concepts to real programs and homework (including string ordering and conditional decision-making)
• Practically, this is foundational for control flow in almost all Python programs
• Ethical or philosophical implications are not discussed in this section; the focus is on programming syntax and logical reasoning

Quick reference: key symbols and terms

• Boolean values: True, False (capital T/F)
• Booleans in code: used to represent truth values
• Assignment vs equality:
  • Assignment: =
  • Equality: ==
• Relational operators: >, <, >=, <=, ==, !=
• String comparison relies on ASCII values and lexicographic order; example sequence involves character-by-character comparison
• Indentation is essential in Python blocks under if/elif/else
• Commenting shortcut: Mac: Command + /, Windows: Ctrl + /

Examples to try (summary you can reproduce in Python)

• If example 1:
  • if 10 > 5:
  • print("10 is greater than 5")
• If example 2 (age):
  • my_age = 28
  • your_age = 19
  • if myage > yourage:
  • print("I am older")
• If example 3 (equality):
  • a = 7; b = 7
  • if a == b:
  • print("Great minds think alike")
• If example 4 (not equal):
  • if a != b:
  • print("They are different")
• Elif example:
  • age = 20
  • if age < 13:
  • print("child")
  • elif age < 20:
  • print("teenager")
  • else:
  • print("adult")
• String comparison example:
  • s1 = 'code'
  • s2 = 'coat'
  • if s1 > s2:
  • print("'code' is greater than 'coat'")