Conditional

Comparison Operators

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Comparison operations compare some value or operand and based on a condition, produce a Boolean.

Most of us have been seeing these operators for decades. Nothing new in Python, (just note the use of = as it is not a comparison operator, but an assignment), when comparing two values you can use these operators:

• equal: ==
• not equal: !=
• greater than: >
• less than: <
• greater than or equal to: >=
• less than or equal to: <=
• I’ll put this here even though it is NOT a comparison operator, but it is an assignment: =

I will not go into much detail here, just one quick example

a = 5   # Assign the integer 5 to the variable a
a == 6  # Then check its value compared to 6

False

Note: operators can be applied to strings just as well as they are applied to numbers

Here we see the corresponding ASCII values of letters

Char.	ASCII	Char.	ASCII	Char.	ASCII	Char.	ASCII
A	65	N	78	a	97	n	110
B	66	O	79	b	98	o	111
C	67	P	80	c	99	p	112
D	68	Q	81	d	100	q	113
E	69	R	82	e	101	r	114
F	70	S	83	f	102	s	115
G	71	T	84	g	103	t	116
H	72	U	85	h	104	u	117
I	73	V	86	i	105	v	118
J	74	W	87	j	106	w	119
K	75	X	88	k	107	x	120
L	76	Y	89	l	108	y	121
M	77	Z	90	m	109	z	122

• So from the table above we can see that B > A = TRUE
• Also note that lower case letters have a larger value than upper case letters

'B' > 'A'

True

• If we have multiple letters we start off with the first letter and then compare the second letter and so on….
• In the case below the first letters are equal, second letter B < C so the first statement is FALSE

'AB' > 'AC'

False

Branching

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Allows us to run different statements for different inputs. This is your typical if statement, if you’re familiar with other programming languages. Usually conditional comparisons and operators are used to test a condition prior to executing a command (or several commands)

if

Note: Indentation is extremely important in Python, in R the command(s) to be executed are enclosed in {} so there is no mistake what is done if the condition is met. In Python, the indentation tells Python what needs to be executed if the condition is met. So all commands need to be indented if they are conditional commands

• Syntax: if condition statement:
• In R {} are used, here : instead but the principals are the same
• Pay attention to the indentation
• What’s indented is part of the Branching/If statement and will only run if the condition is met
• What’s not indented after the If statement will run regardless of the condition in the if statement
• Here is a simple example

Note: if is lower case

age = 19
if age > 18:
        print("Yeehaw, I can drink. Oh wait that was 30 years ago!")
        
print("This will print regardless of condition because it is NOT indented")

Yeehaw, I can drink. Oh wait that was 30 years ago!
This will print regardless of condition because it is NOT indented

nested if

• You can nest if statements inside other if statements like this example below

user_choice = "Withdraw Cash"
if user_choice == "Withdraw Cash":
    amount = input("Enter the amount to withdraw: ")
    if amount % 10 == 0:
        dispense_cash(amount)
    else:
        print("Please enter a multiple of 10.")
else:
    print("Thank you for using the ATM.")

else

• else runs a block of code if none of the conditions are True before this else statement\
• Note the : in the else:
• Note the indentation again, same apply for block of code that needs to be executed with else needs to be indented

age = 17
if age > 18:
        print("Yeehaw, I can drink. Oh wait that was 30 years ago!")
else:
        print("Grow up")
        
print("This will print regardless of condition because it is NOT indented")

Grow up
This will print regardless of condition because it is NOT indented

elif

• elif is short for else if, gives us another way to string along additional branches/conditional statements before exiting out to the else:
• If the condition in the elif is True then that block of code is executed

age = 18

if age > 18:
        print("Ok I'm getting old, so what now?")
elif age == 18:
        print("Made it, huh!")
else:
        print("I need to grow up")
        
print("Done for now")

Made it, huh!
Done for now

Logical Operators

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Sometimes you want to check more than one condition at once. For example, you might want to check if one condition and another condition are both True. Logical operators allow you to combine or modify conditions. I’m not going to spend much time on this, as it’s logic 101 learned before you were allowed to drink.

• and
• or
• not

Note: use lower case (unlike R)

album_year = 1990

if(album_year < 1980) or (album_year > 1989):
    print ("Album was not made in the 1980's")
else:
    print("The Album was made in the 1980's ")

Album was not made in the 1980's