Coding Notes

2.2 Conditions

7 min readAndreas PatuzziAuroville Institute of Applied Technology

Conditions allow code to make decisions. Each individual decision is based on one boolean and the only two options are “run some code” or “don’t run the code”. The boolean is usually created using a series of comparison and logical operators.

Summary

  • If: Executes a block if the condition is truthy.
  • Else: Follows an if or elif, executing only if all prior conditions are falsy.
  • Elif: Allows checking multiple conditions. The first truthy condition is executed, and the rest are skipped.
  • Combined Usage: A conditional starts with if, can have multiple elif, and at most one else. Execution stops at the first truthy condition.
  • Ternary Operator: A short if-else expression (valueIfTrue if condition else valueIfFalse) used for conditional assignments.

If

The if statement checks a condition. If the condition is truthy, the code block inside is executed. If the condition is falsy, the code block inside is ignored/skipped.

if condition:
	# code inside executed if the condition is true
	# this is still inside 
# this is outside again, because of the indentation

The Python syntax needs a colon : after the condition and the correct indentation of the code inside. You often see parentheses () around the condition, however in Python they are not necessary and don’t change anything.

x = 5
if x > 0:  # condition is True the code inside is executed
	print("1 x is greater than 0")
	print("1 still inside")
 
if x > 10: # condition is False the code inside is ignored
	print("2 x is greater than 10")
	print("2 still inside")
 
if x:      # x is 5 which is truthy the code inside is executed
	print("3 x is truthy")
	print("3 still inside")
1 x is greater than 0
1 still inside
3 x is truthy
3 still inside

Else

The else statement is always immediately after the end of an if or elif statement and does not need its own condition. It executes only when all the if or elif conditions before evaluated to False. The else statement has to be on the same indentation level as its if/elif statement.

x = 12
y = 3
 
print("condition 1")
if x > 10:
		print("x is greater than 10")d
else:
	print("x is smaller than or equal to 10")
 
print("\ncondition 2")
if y > 10:
	print("y is greater than 10")
else:
	print("y is smaller than or equal to 10")
 
print("\ncondition 3")
if x < 10:
	print("x is smaller than 10")
else: 
	print("x is greater than or equal to 10")
	if x > y:
		print("x is greater than y")
	else:
		print("y is greater than or equal to x")
condition 1
x is greater than 10
 
condition 2
y is smaller than or equal to 10
 
condition 3
x is greater than or equal to 10
x is greater than y

Elif

The elif is a short alternative to nesting an if inside the else block. Therefore an elif condition is only checked if all previous if and elif statements were False. You can have as many elif statements as you want, which allows you to easily check multiple conditions in sequence. The following two Conditions are exactly the same, but the version using elif is much easier to write and read.

x = 15
 
print("nested if-else statements")
if x%2 == 0:
	print("x is divisible by 2")
else:
	if x%3 == 0:
		print("x is divisible by 3")
	else:
		if x%5 == 0:
			print("x is divisible by 5")
 
print("\nelif statements")
if x%2 == 0:
	print("x is divisible by 2")
elif x%3 == 0:
	print("x is divisible by 3")
elif x%5 == 0:
	print("x is divisible by 5")
nested if-else statements
x is divisible by 3
 
elif statements
x is divisible by 3

Combined Usage

A valid conditional statement consists of exactly one if then 0 - infinite elif and then 0 - 1 else. When executing the code the conditions (of the if and elif) are checked one by one. As soon as one of them evaluates to True, the code inside is executed, and afterward the program will jump to after the conditional statement. If all of them evaluate to False the code inside else is executed.

score = 75
print("Your grade is: ", end="")
if score > 88:
    print("A")
    print("great")
elif score > 76:
    print("B")
    print("good")
elif score > 64:
    print("C")
    print("solid")
elif score > 52:
    print("E")
    print("bad")
else:
    print("F")
    print("failed")
print(f"with {score}%")
Your grade is: C
solid
with 75%

You could explain what is happening similar to this. Create a new variable called “score” of type integer with the value 75. Output the message “Your grade is: ”, but without a “\n” after the print. The first if condition is False because 75 is not greater than 88. Next check the first elif condition, which is also False, because 75 is not greater than 76. Check the next elif condition (line 9), which is True, because 75 is greater than 64. Go inside the elif block and output “C”, which will be on the same line as the first message. Then output “solid” on a new line. Go after the conditional statement and output the message “with 75%” on a new line.

Ternary Operator

The ternary operator is a short way to write a if-else statement, that returns a value. The syntax for this is valueIfTrue if condition else valueIfFalse. This is often used to assign a value to a variable based on a condition. The following code shows you how to do this using the classic if-else and the ternary operator.

condition = True
 
# if-else
if condition:
	x = 10
else:
	x = 0
print(x)
 
#ternary operator
x = 10 if condition else 0
print(x)
10
10

The ternary operator does not support elif, but you can use nesting to achieve the same functionality.

age = 21
student = True
 
# if-else
message = "your discount is: "
if age < 15:
    message += "child"
elif age<30 and student:
    message += "student"
else:
    message += "none"
print(message)
 
#ternary operator
message = "your discount is: " + ("child" if age<15 else ("student" if(age<30 and student) else "none"))
print(message)
your discount is: student
your discount is: student

This is very helpful for compact conditional assignments but can get confusing when overdone.

Questions

  • What is the purpose of Conditions in programming?

  • Explain if in your own words.

  • Explain else in your own words.

  • Explain elif in your own words.

  • What is important about the syntax of if-elif-else?

  • Can there be an else or elif without an if statement? Can there be an else without an elif? Can there be an elif without an else?

  • When does the execution stop in a chain of if, elif, and else? Explain all possible scenarios.

  • Rewrite this nested if-else using elif. For an extra task to improve understanding, rewrite it only with if statements and without any else or elif.

    if x > 10:
    print("Big")
else:
    if x > 5:
        print("Medium")
    else:
        print("Small")

  • Write an if-elif-else statement that prints whether a number is positive, negative, or zero.

  • Explain the ternary operator in your own words.

  • How/When/Why is the ternary operator useful?

  • Can the ternary operator handle multiple conditions? How?

  • Convert this if-else statement into a ternary operator.

    if age < 18:
    discount = True
    category = "Minor"
else:
    discount = False
    category = "Adult"

Graph View