Session 4 · Operators, conditions and input()#

📖 Based on the blog post Python: Operators, Conditionals and Input().

The big idea#

Three tools that make programs useful:

  • Operators do the work: add, compare, combine.

  • Conditionals (if, elif, else) let the program choose what to do.

  • input() lets the user talk to the program.

Everyday example: a traffic signal. It compares (is it red?), decides (stop or go), and reacts to input (a pedestrian presses the button).

Arithmetic operators#

a, b = 7, 2
print(a + b, a - b, a * b)    # add, subtract, multiply
print(a / b)                  # division always gives a float
print(a // b)                 # floor division: round DOWN to a whole number
print(a % b)                  # modulus: the remainder
print(a ** b)                 # power: 7 squared

9 5 14
3.5
3
1
49

% is more useful than it looks: n % 2 == 0 means n is even; n % 10 is the last digit.

Note

// rounds down, not towards zero: -7 // 2 is -4. And computers store decimals in binary, so 0.1 + 0.2 gives 0.30000000000000004. Use round() when you print money-like values.

Comparison operators#

They always give True or False:

a, b = 5, 3
print(a == b, a != b)     # equal, not equal
print(a > b, a < b)       # greater, less
print(a >= 5, b <= 2)     # greater-or-equal, less-or-equal

False True
True False
True False

Logical operators#

Combine conditions with and, or, not:

a, b = 5, 3
print(a > b and a > 0)    # both must be True
print(a < b or a > 0)     # at least one True
print(not a > 0)          # flips True/False

True
True
False

Python also lets you chain comparisons like in maths: 0 < age < 120.

Conditionals: if / elif / else#

if / elif / else Python checks the conditions from top to bottom and runs the first block whose condition is True. else runs only if none matched. number = float(input()) if number > 0 True print("… positive.") False elif number < 0 True print("… negative.") False else print("… zero.")
Python checks the conditions from top to bottom and runs the first block whose condition is True. else runs only if none matched.
a, b = 3, 5
if a > b:
    print("a is greater than b")
elif a == b:
    print("a is equal to b")
else:
    print("a is less than b")

a is less than b

  • The indentation (4 spaces) is what puts a line inside the if. It isn’t decoration.

  • Only the first matching branch runs, then Python skips the rest.

  • You can have any number of elifs, and else is optional.

input(): asking the user#

input() shows a prompt, waits for the user to type and press Enter, and returns text:

name = input("What is your name? ")
print("Hello, " + name + "!")

To do maths, convert the text to a number first:

age = int(input("How old are you? "))
print(f"Next year you'll be {age + 1}.")

Putting it together#

Is a number positive, negative or zero?

number = float(input("Enter a number: "))

if number > 0:
    print("The number is positive.")
elif number < 0:
    print("The number is negative.")
else:
    print("The number is zero.")

Enter a number: The number is positive.

(When you run it yourself, you type the number after the prompt.)

Common mistakes#

  • = instead of == in a condition: if x = 5: is a syntax error.

  • Forgetting int(): input() returns text, so input() + 1 raises TypeError, and "10" > "9" is False (text compares letter by letter).

  • Missing colon or indentation: every if, elif, else line ends with : and its block is indented.

  • Typing “ten” when int() expects digits: ValueError. You’ll handle this with try/except in Session 12.

Hands-on exercises#

Exercise 1 · Even or odd. Ask for a number and print whether it’s even or odd.

Solution 
n = int(input("Enter a number: "))
if n % 2 == 0:
    print(f"{n} is even")
else:
    print(f"{n} is odd")

Exercise 2 · Grade calculator. Ask for marks (0–100) and print A (90+), B (75–89), C (50–74) or F.

Solution 
marks = int(input("Marks: "))
if marks >= 90:
    grade = "A"
elif marks >= 75:
    grade = "B"
elif marks >= 50:
    grade = "C"
else:
    grade = "F"
print("Grade:", grade)

Order matters: check the highest band first, so 95 doesn’t stop at >= 50.

Exercise 3 · Leap year. A year is a leap year if it’s divisible by 4, except centuries, unless divisible by 400. Check 2024, 1900 and 2000.

Solution 
for year in [2024, 1900, 2000]:
    is_leap = (year % 4 == 0 and year % 100 != 0) or year % 400 == 0
    print(year, is_leap)

2024 True, 1900 False, 2000 True.

Exercise 4 · Biggest of three. Ask for three numbers and print the largest, using if (not max()).

Solution 
a = float(input("a: "))
b = float(input("b: "))
c = float(input("c: "))
if a >= b and a >= c:
    largest = a
elif b >= c:
    largest = b
else:
    largest = c
print("Largest:", largest)

Exercise 5 · Ticket price. Children under 5 are free, 5–17 pay ₹50, 60 and above pay ₹60, everyone else ₹100. Ask for the age and print the price.

Solution 
age = int(input("Age: "))
if age < 5:
    price = 0
elif age <= 17:
    price = 50
elif age >= 60:
    price = 60
else:
    price = 100
print(f"Ticket: ₹{price}")

Exercise 6 · Simple calculator. Ask for two numbers and an operator (+ - * /) and print the result. Print a message instead of crashing when dividing by zero.

Solution 
a = float(input("First number: "))
op = input("Operator (+ - * /): ")
b = float(input("Second number: "))

if op == "+":
    print(a + b)
elif op == "-":
    print(a - b)
elif op == "*":
    print(a * b)
elif op == "/":
    if b == 0:
        print("Can't divide by zero")
    else:
        print(a / b)
else:
    print("Unknown operator")