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Boolean Operators and Compound Expressions

Boolean Operators and Compound Expressions
Boolean Practice
Conditional Statements (if-statements)
[at home] More on Strings

Lecture 14, Data 6, Fall 2025

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[Warmup] Comparison expressions result in a bool

Let x = 3 and y = 2. In words, what is each expression asking?

Comparison Expression	Value
x == y	False
x == y + 1	True
x != y	True
2 < x < 5	True
y >= x - 1	True
y > x - 1	False

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Boolean operators

Boolean operators allow us to combine the result of multiple booleans.

and and or operate on two (or more) operands; not operates on just one.

Operator	Usage	Value
and	a and b	True if both a and b are True, False otherwise
or	a or b	True if at least one of a and b are True, False otherwise
not	not a	True if a is False False if a is True

(demo)

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Analogy, from number lines

# 2 < x < 5

(x > 2) and (x < 5)

(x < 2) or (x > 5)

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Boolean Practice

These are summary slides; see the notebook for demo

Boolean Operators and Compound Expressions
Boolean Practice
Conditional Statements (if-statements)
[at home] More on Strings

Lecture 14, Data 6, Fall 2025

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Custom Boolean Predicates and Filters

We can write more filters for tables with custom functions.

“Apply-Filter-Drop”:

Define a function that returns a boolean value based on a row's values.
apply this function to create a new (but temporary) boolean column of Trues and Falses.
Filter (use where) rows based on this new Boolean column.
drop the temporary boolean column so that our resulting table has the original table's columns.

(demo)

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Truth Tables

To understand boolean expressions, it is useful to create truth tables to consider how the expression will evaluate with all possible boolean value inputs:

x	y	(boolean expression involving x and y)
False	False	???
False	True	???
True	True	???
False	False	???

(demo)

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Also a boolean expression

NO FOOD

DRINK

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Boolean quirks

Short-circuiting (see next slide)
True counts as 1, False counts as 0

When casting booleans to ints

Truthy values (see last time)

(demo)

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Boolean Short Circuiting

Recall that Python evaluates expressions left to right, after prioritizing parentheses.

When evaluating a boolean expression, Python will “short circuit” (or stop early) when the final result of the boolean expression is known.

1 / 0 # ZeroDivisionError

(1 < 2) or (1 / 0) # True, short circuits

(2 < 2) and (1 / 0) # False, short circuits

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Conditional Statements (if-statements)

Boolean Operators and Compound Expressions
Boolean Practice
Conditional Statements (if-statements)
[at home] More on Strings

Lecture 14, Data 6, Fall 2025

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Control your computations with an if statement

x = 2

if x > 0:

print('Positive')

elif x < 0:

print('Negative')

elif x == 0:

print('Neither positive nor negative')

Positive

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If you have many conditions, put else at the end!

city = 'St. Petersburg'

if city == 'San Francisco':

print('The Bay!')

elif city == 'Anaheim':

print('Disneyland!')

else:

print('I don’t know where that is...')

'I don’t know where that is...'

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We often use if statements inside of functions

if x > 0:

print('Positive')

elif x < 0:

print('Negative')

elif x == 0:

print('Neither positive nor negative')

def sign(x):

(demo)

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If Statements: An Example

If statements allow our code to make decisions.

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Practice, Explained. Let x = 7

if x > 5:

print('somewhat big!')

if x % 2 == 0:

print('and even too!')

else:

print('tiny.')

if x > 5:

print('somewhat big!')

if x % 2 == 0:

print('and even too!')

else:

print('tiny.')

somewhat big!

tiny.

(left-hand-side)

(right-hand-side)

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Practice, Explained

x > 5?

tiny.

somewhat big!

x % 2 == 0?

and even too!

x > 5?

tiny.

somewhat big!

x % 2 == 0?

and even too!

(left-hand-side)

(right-hand-side)

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The right-hand one is two conditional statements

# clearer, with whitespace

if x > 5:

print('somewhat big!')

if x % 2 == 0:

print('and even too!')

else:

print('tiny.')

(right-hand-side)

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Here’s a recap on if statements!

Watch your syntax!

colons
indentation

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Here’s a recap on if statements!

Begin with an if expression
Add elif (else-if) expressions
else is optional

use if there are too many cases to count

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Announcements

Project 1 due date tomorrow evening (because of Amazon AWS server outage)

Quiz 2 grades out later today

Soon: a gradebook, so you can see your progress in the course

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[at home] More on Strings

Boolean Operators and Compound Expressions
Boolean Practice
Conditional Statements (if-statements)
[at home] More on Strings

Lecture 14, Data 6, Fall 2025

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String Containment

The in keyword allows us to check if one string is a substring of another, or if an element is present in our array/list.

'berkeley' in 'uc berkeley' # True

'stanford' in 'uc berkeley' # False

'berkeley' in 'UC BERKELEY' # False

1 in [1, 2, 3] # True

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String Methods

Just like tables have table methods, strings have string methods.

s = 'JuNiOR12' # already run

Method	Example Call	Example Return Value
upper	s.upper()	'JUNIOR12'
lower	s.lower()	'junior12'
replace	s.replace('i', 'iii')	'JuNiiiOR12'
split *	s.split('iO')	['JuN', 'R12']
join *	' '.join(['hello',� 'world', '!'])	'hello world !'

*square brackets denote the list datatype.�For Data 6, lists share some characteristics with arrays.

(demo)

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Square bracket notation

some_list[index]

some_list[start:end] # slice

some_list[start:end:step] # slice, skipping elements

Get elements of a list at a particular index

Also used for strings
Lists and strings are zero-indexed, just like arrays

Many more details; see homework