String Comparison, Chained Conditionals, and Logical Operators¶

CS 65: Introduction to Computer Science I¶

Conditionals Review¶

So far, we've seen conditional statements

total_bill = 35.63
age = float(input("What is your age? "))

if age > 65:
    print("Applying 10% senior discount")
    total_bill = total_bill * 0.9
    #any indented code is only runs
    #when the condition is True
    
#un-indented code runs no matter what
print("Your total bill is", total_bill)

What is your age? 67
Applying 10% senior discount
Your total bill is 32.067

conditional statements with two alternatives

height = int(input("Enter your height in inches: "))

if height < 60:
    print("You are not tall enough for this ride.")
else:
    print("You are tall enough for this ride.")

Enter your height in inches: 61
You are tall enough for this ride.

and in the lab, we saw nested conditional statements

num = int(input("Enter a number: "))

#this code checks if a number is divisible by both 2 and 3
if num % 2 == 0:
    print(num,"is even")
else:
    if num % 3 == 0:
        print(num,"is odd and divisble by 3")
    else:
        print(num,"is odd and not divisible by 3")

Enter a number: 9
9 is odd and divisble by 3

Group Exercise: Comparing Strings¶

Type the following program into Thonny, and run it several times. What does it do?

name1 = input("Enter a name: ")
name2 = input("Enter another name: ")

if name1 < name2:
    print(name1,"is first")
else:
    print(name2,"is first")

Enter a name: Eric
Enter another name: Manley
Eric is first

Is string comparison in Python case-sensitive? E.g., is "Eric" the same as "eric"? How could we test it?

Character representation¶

Every text character is represented by a numerical code in your computer's memory.

They came up with in the 1960's as part of the ASCII standard (https://en.wikipedia.org/wiki/ASCII)

Here's some examples

code	character
33	A
34	B
35	C
36	D
37	E

code	character
65	A
66	B
67	C
68	D
69	E

code	character
97	a
98	b
99	c
100	d
101	e

You can use the built-in ord() function to look up the code for any given character. Use chr() to do the opposite.

ord("A")

65

chr(65)

'A'

Other conditional statement options¶

There are also chained conditional statements when you have more than two choices

elif a new keyword meaning "else if" which allows you to add as many more options to a conditional statement as you want.

temp = float(input("What's the temperature? "))

if temp >= 70:
    print("Let's go hiking.")
elif temp >= 30:
    print("Let's put on jackets and go hiking.")
elif temp >= 10:
    print("Let's go skiing.")
else:
    print("It's too cold. Let's stay in and code.")

What's the temperature? 40
Let's put on jackets and go hiking.

Note:

else part is still optional but gives you the opportunity for "default" behavior
at most one block in a if-elif-else statement will run

a common chained-conditional logic error¶

temp = float(input("What's the temperature? "))

if temp >= 10:
    print("Let's go skiing.")
elif temp >= 30:
    print("Let's put on jackets and go hiking.")
elif temp >= 70:
    print("Let's go hiking.")
else:
    print("It's too cold. Let's stay in and code.")

What's the temperature? 80
Let's go skiing.

Make sure to put the conditions in the order you want to check them - the first one to evalute to True will be the one that runs.

Logical Operators¶

Logical operators are operators that take Booleans as operands: and, or, not

temp = float(input("What's the temperature? "))
raining = input("Is it raining? ")

if temp < 50 or raining == 'yes':
    print("Grab your coat.")

if not (raining == 'yes') and temp > 70:
    print("Grab your mitt.")

What's the temperature? 60
Is it raining? yes
Grab your coat.

a = 2
b = 4
c = 6

See if you can work out the values below before the answer is revealed

a == 4 or b > 2

6 <= c and a > 3

1 != b and c != 3

a >= -1 or a <= b

not a > 2

Logical operators are pretty intuitive because we use them in our natural language, but they can sometimes be tricky when you get complex expressions.

a = 2
b = 4
c = 6
result = a == 2 or b != 2 and c != 6 or a == b + c
print(result)

Rule of thumb: Always use parentheses when you have more than one logical operator