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Conditional Statements

Introduction​

For some problems, we are going to take different actions depending on the conditions. For example, take an umbrella when it's raining outside; otherwise don't need to.

Conditional statements are the instructions we give computers to make decisions and do something accordingly.

There are different kinds of conditional statements.

1. If statement:​

If a condition is true, it'll take action; otherwise, it'll do nothing.

For example,

if a number is equal to the pre-set number:
     output "You won!"

Python syntax:​

if condition:
    do something

Python implementation:​

#Assigning a number to the winning_number variable
winning_number = 65

#Asking user to type in a lucky number,
#input() function gets the entered number as a string from user so int() converts it to an integer
num = int(input("Pick a lucky number between 1 and 100: "))

if num == winning_number:
    print("You won!")

Output:

Pick a lucky number between 1 and 100: 65
You won!

2. If-Else statement:​

You can assign it to perform an action if a condition is true and a different action when the condition is false; therefore, different actions can be processed based on the condition.

For example, a math tutor evaluates if the answer to a math question is correct or not. It displays "Correct" if answer is correct. It displays "Incorrect" and the correct answer if the answer is incorrect.

Python syntax:​

if condition:
    do something
else:
    do something else

Python implementation:​

answer = int(input("What is the answer of 2 to the power of 8? "))

# The Python power operator ** raises the left number to the power of the right number
if answer == 2**8:
    print("Correct")
else:
    print("Incorrect. The answer is ", 2**8)

Output:

What is the answer of 2 to the power of 8? 64
Incorrect. The answer is  256
What is the answer of 2 to the power of 8? 256
Correct

3. If-Elif-Else statement:​

It has more than one condition, and it takes different actions depending on the condition. If all conditions do not match, it does the action of the "else" part.

For example, to find the letter grade ('A', 'B', 'C', 'D', or 'F') based on the following chart:

90-100 => "A"
80-89 => "B"
70-79 => "C"
60-69 => "D"
below 60 => "F"

Python syntax​

We can use the following syntax:

if condition:
    action1
elif condition:
    action2
else:
    other_action

Note: There can more than one elif condition.

Python implementation:​

score = int(input("Your score(0-100): "))

if score >= 90 and score <= 100:
    print("A")
elif score >= 80 and score < 90:
    print("B")
elif score >= 70 and score < 80:
    print("C")
elif score >= 60 and score < 70:
    print("D")
elif score >= 0 and score < 60:
    print("F")
else:
    print("invalid score")

Output:

Your score(0-100): 83
B
Your score(0-100): 59
F
Your score(0-100): -100
invalid score```

4. Python shorthand if-else:​

Python provides a shorthand method like the ternary operator of C language for writing if-else statements in one line.

Python syntax:​

action1 if condition else action2

Python implementation​

num1, num2 = 99, 100
print(num1, ">", num2) if num1 > num2 else print(num1, '<=', num2)

Output:

99 <= 100