Boolean Expressions and Comparisons in Python
If Statements and Boolean
Expressions
CSE 1310 – Introduction to Computers and Programming
Vassilis Athitsos
University of Texas at Arlington
Credits: a significant part of this material has been created by Dr. Darin Brezeale
and Dr. Gian Luca Mariottini
1
The Boolean Type
•
Expressions of type boolean can only have two
values: True, or False.
–
True and False are reserved keywords in Python.
2
>>> 3 > 2
True
>>> type(True)
<type 'bool'>
>>> a = 3 == 2
>>> a
False
>>> a = (3 == 2)
>>> a
False
Preferred style
(parenthesize)
>>> b = 4;
>>> a = (b != 5)
>>> a
True
Comparisons Generating Booleans
•
The following operators compare numerical
values, or strings, and generate boolean
results:
== equality
!= not equal
< less than
> greater than
<= less than or equal to
>= greater than or equal to
3
Doing Numerical Comparisons
4
>>> a = 3
>>> b = 5
>>> c = 3
>>> a == b
False
>>> a == c
True
>>> a >= c
True
>>> a != c
False
Doing Comparisons on Strings
5
>>> a = "hello"
>>> b = "goodbye"
>>> c = "hello"
>>> a == b
False
>>> a == c
True
>>> a < b
False
>>> a >= "world"
False
>>> a != "hello"
False
>>> a == "hello"
True
>>> a != b
True
>>> a < "sky"
True
>>> "apple" < "car"
True
>>> "apple" < "Car"
False
Doing Comparisons on Strings
•
Note how "greater than" and "less than" behave
on strings:
–
Not exactly based on alphabetical order.
–
Capital letters come before (are "less than") lower
case letters.
>>> "apple" < "car"
True
>>> "apple" < "Car"
False
6
Logical Operators
•
The following logical operators can be used to
produce boolean results:
not
and
or
7
Using Logical Operators
8
>>> a = 3
>>> b = 4
>>> (a == b) or (a+b == 7)
True
>>> (a == b) and (a+b == 7)
False
>>> not(a == b)
True
The
in
operator
•
The
in
operator checks if a value is included in a set
of values.
•
For now, we will use
in
to check if a letter appears in
a string.
–
We will see more uses when we do lists.
>>> var1 = 'a'
>>> vowels = 'aeiouAEIOU'
>>> var1 in vowels
True
>>> 'c' in vowels
False
9
Combining operators
•
What does this line do?
>>> (3 == 5) and (2 < 3) or (3 >= 0)
10
Combining operators
•
What does this line do?
>>> (3 == 5) and (2 < 3) or (3 >= 0)
•
I don't know, and I don't want to know.
–
Use parentheses to make the meaning of these
statements clear.
>>> ((3 == 5) and (2 < 3)) or (3 >= 0)
True
>>> (3 == 5) and ((2 < 3) or (3 >= 0))
False
11
Conditionals -
if
statements
•
An
if
statement is defined as follows:
if (
expression
):
line 1
line 2
…
line n
•
Line 1, line 2, …, line n are called the
body
of the
if
statement.
12
An example of an
if
statement
number_text = input("enter a number: ")number = float(number_text)
if (number > 2):
print(number, "is greater than 2")
print("good bye")13
Another example of an
if
statement
number_text = input("enter a number: ")number = float(number_text)
if (number > 2) and (number < 34):
print(number, "is greater than 2")
print(number, "is less than 34")
print("good bye")14
Conditionals -
if-else
•
An
if-else
statement is defined as follows:
if (
expression
):
one or more lines
else:
one or more lines
15
An example of an
if-else
statement
number_text = input("enter a number: ")number = float(number_text)
if (number > 2):
print(number, "is greater than 2")
else:
print(number, "is less than or equal to 2")
print("good bye")16
Another example of an
if-else
number_text = input("enter a number: ")number = float(number_text)
if (number > 2) and (number < 34):
print(number, "is greater than 2")
print(number, "is less than 34")
else:
print(number, "is less than or equal to 2")
print("or greater than or equal to 34")print("good bye")17
Conditionals -
elif
•
An
if-else
can include additional
conditions, using
elif
:
if (
expression1
):
one or more lines
elif (
expression2
):
one or more lines.
elif (
expression2
):
one or more lines
…
18
Conditionals -
elif
•
You can use
elif
zero, one, or more times.
if (
expression1
):
one or more lines
elif (
expression2
):
one or more lines.
elif (
expression2
):
one or more lines
…
19
An example of using
elif
number_text = input("enter a number: ")number = float(number_text)
if (number > 2) and (number < 34):
print(number, "is greater than 2")
print(number, "is less than 34")
elif (number <= 2):
print(number, "is less than or equal to 2")
elif (number >= 34):
print(number, "is greater than or equal to 34")
print("good bye")20
Another example of using
elif
number_text = input("enter a number: ")number = float(number_text)
if (number > 2) and (number < 34):
print(number, "is greater than 2")
print(number, "is less than 34")
elif (number <= 2):
print(number, "is less than or equal to 2")
else:
print(number, "is greater than or equal to 34")
print("good bye")21
Warning
•
Be careful of the difference between = and ==
22
The Importance of Indentation
number_text = input("enter a number: ")number = float(number_text)
if (number > 2) and (number < 34):
print(number, "is greater than 2")
print(number, "is less than 34")
print("good bye")23
number_text = input("enter a number: ")number = float(number_text)
if (number > 2) and (number < 34):
print(number, "is greater than 2")
print(number, "is less than 34")
print("good bye")These two programs
behave differently.
How, and why?
Successive ifs, vs. if-elif
number_text = input("enter a number: ")number = float(number_text)
if (number > 2)
print(number, "is greater than 2")
if (number > 10)
print(number, "is greater than 10")
if (number > 50)
print(number, "is greater than 50")
24
number_text = input("enter a number: ")number = float(number_text)
if (number > 2)
print(number, "is greater than 2")
elif (number > 10)
print(number, "is greater than 10")
elif (number > 50)
print(number, "is greater than 50")
These two programs
behave differently.
How, and why?
Conditionals with Strings: Example 1
m = input("Enter the name of a month: ")if (m == "January") or (m == "March") or (m == "May") or (m == "July"):
print(m, "has 31 days.")
elif (m == "August") or (m == "October") or (m == "December"):
print(m, "has 31 days.")
elif (m == "April") or (m == "June") or (m == "September") or (m == "November"):
print(m, "has 30 days.")
elif (m == "February"):
print(m, "has 28 or 29 days.")
else:
print(m, "is not a valid month")
25
Conditionals with Strings: Example 1
text = input("Enter a word: ")if ('a' in text) or ('A' in text):print(text, "contains at least one 'a'")
else:
print(text, "contains no 'a'")
if ('b' in text) or ('B' in text):print(text, "contains at least one 'b'")
else:
print(text, "contains no 'b'")
26
Boolean expressions and comparisons in Python involve using operators to evaluate conditions and generate boolean results. The language's reserved keywords, True and False, play a crucial role in determining the validity of expressions. Whether comparing numerical values, strings, or utilizing comparison operators like equality (==), not equal (!=), less than (), less than or equal to (=), understanding how boolean expressions work is essential for writing effective Python code.
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Presentation Transcript
If Statements and Boolean Expressions CSE 1310 Introduction to Computers and Programming Vassilis Athitsos University of Texas at Arlington Credits: a significant part of this material has been created by Dr. Darin Brezeale and Dr. Gian Luca Mariottini 1
The Boolean Type Expressions of type boolean can only have two values: True, or False. True and False are reserved keywords in Python. >>> b = 4; >>> a = (b != 5) >>> a True >>> a = 3 == 2 >>> a False >>> a = (3 == 2) >>> a False >>> 3 > 2 True >>> type(True) <type 'bool'> Preferred style (parenthesize) 2
Comparisons Generating Booleans The following operators compare numerical values, or strings, and generate boolean results: == equality != not equal < less than > greater than <= less than or equal to >= greater than or equal to 3
Doing Numerical Comparisons >>> a = 3 >>> b = 5 >>> c = 3 >>> a == b False >>> a == c True >>> a >= c True >>> a != c False 4
Doing Comparisons on Strings >>> a = "hello" >>> b = "goodbye" >>> c = "hello" >>> a != "hello" False >>> a == "hello" True >>> a != b True >>> a < "sky" True >>> "apple" < "car" True >>> "apple" < "Car" False >>> a == b False >>> a == c True >>> a < b False >>> a >= "world" False 5
Doing Comparisons on Strings Note how "greater than" and "less than" behave on strings: Not exactly based on alphabetical order. Capital letters come before (are "less than") lower case letters. >>> "apple" < "car" True >>> "apple" < "Car" False 6
Logical Operators The following logical operators can be used to produce boolean results: not and or 7
Using Logical Operators >>> a = 3 >>> b = 4 >>> (a == b) or (a+b == 7) True >>> (a == b) and (a+b == 7) False >>> not(a == b) True 8
The in operator The in operator checks if a value is included in a set of values. For now, we will use in to check if a letter appears in a string. We will see more uses when we do lists. >>> var1 = 'a' >>> vowels = 'aeiouAEIOU' >>> var1 in vowels True >>> 'c' in vowels False 9
Combining operators What does this line do? >>> (3 == 5) and (2 < 3) or (3 >= 0) 10
Combining operators What does this line do? >>> (3 == 5) and (2 < 3) or (3 >= 0) I don't know, and I don't want to know. Use parentheses to make the meaning of these statements clear. >>> ((3 == 5) and (2 < 3)) or (3 >= 0) True >>> (3 == 5) and ((2 < 3) or (3 >= 0)) False 11
Conditionals - if statements An if statement is defined as follows: if (expression): line 1 line 2 line n Line 1, line 2, , line n are called the body of the if statement. 12
An example of an ifstatement number_text = input("enter a number: ") number = float(number_text) if (number > 2): print(number, "is greater than 2") print("good bye") 13
Another example of an ifstatement number_text = input("enter a number: ") number = float(number_text) if (number > 2) and (number < 34): print(number, "is greater than 2") print(number, "is less than 34") print("good bye") 14
Conditionals - if-else An if-else statement is defined as follows: if (expression): one or more lines else: one or more lines 15
An example of an if-elsestatement number_text = input("enter a number: ") number = float(number_text) if (number > 2): print(number, "is greater than 2") else: print(number, "is less than or equal to 2") print("good bye") 16
Another example of an if-else number_text = input("enter a number: ") number = float(number_text) if (number > 2) and (number < 34): print(number, "is greater than 2") print(number, "is less than 34") else: print(number, "is less than or equal to 2") print("or greater than or equal to 34") print("good bye") 17
Conditionals - elif An if-else can include additional conditions, using elif: if (expression1): one or more lines elif (expression2): one or more lines. elif (expression2): one or more lines 18
Conditionals - elif You can use elif zero, one, or more times. if (expression1): one or more lines elif (expression2): one or more lines. elif (expression2): one or more lines 19
An example of using elif number_text = input("enter a number: ") number = float(number_text) if (number > 2) and (number < 34): print(number, "is greater than 2") print(number, "is less than 34") elif (number <= 2): print(number, "is less than or equal to 2") elif (number >= 34): print(number, "is greater than or equal to 34") print("good bye") 20
Another example of using elif number_text = input("enter a number: ") number = float(number_text) if (number > 2) and (number < 34): print(number, "is greater than 2") print(number, "is less than 34") elif (number <= 2): print(number, "is less than or equal to 2") else: print(number, "is greater than or equal to 34") print("good bye") 21
Warning Be careful of the difference between = and == 22
The Importance of Indentation number_text = input("enter a number: ") number = float(number_text) if (number > 2) and (number < 34): print(number, "is greater than 2") print(number, "is less than 34") These two programs behave differently. print("good bye") How, and why? number_text = input("enter a number: ") number = float(number_text) if (number > 2) and (number < 34): print(number, "is greater than 2") print(number, "is less than 34") print("good bye") 23
Successive ifs, vs. if-elif number_text = input("enter a number: ") number = float(number_text) if (number > 2) print(number, "is greater than 2") if (number > 10) print(number, "is greater than 10") if (number > 50) print(number, "is greater than 50") These two programs behave differently. How, and why? number_text = input("enter a number: ") number = float(number_text) if (number > 2) print(number, "is greater than 2") elif (number > 10) print(number, "is greater than 10") elif (number > 50) print(number, "is greater than 50") 24
Conditionals with Strings: Example 1 m = input("Enter the name of a month: ") if (m == "January") or (m == "March") or (m == "May") or (m == "July"): print(m, "has 31 days.") elif (m == "August") or (m == "October") or (m == "December"): print(m, "has 31 days.") elif (m == "April") or (m == "June") or (m == "September") or (m == "November"): print(m, "has 30 days.") elif (m == "February"): print(m, "has 28 or 29 days.") else: print(m, "is not a valid month") 25
Conditionals with Strings: Example 1 text = input("Enter a word: ") if ('a' in text) or ('A' in text): print(text, "contains at least one 'a'") else: print(text, "contains no 'a'") if ('b' in text) or ('B' in text): print(text, "contains at least one 'b'") else: print(text, "contains no 'b'") 26
