Loops Checkpoint

Optional

20 min · Beginner

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A review of this module. Skip it if you feel confident; the next module does not need it.

Fix the mistakes

The slips that cost marks in this module. Spot each one, then check the fix.

Consolidate your loops mastery by solving standard placement technical coding round problems.

Prime number check with for-else

Prime number is a whole number greater than 1 that has no divisors other than 1 and itself.

A number is prime if it has no divisors other than 1 and itself. We only need to test divisors up to √N.

Python

1n = 292if n < 2:3    print(f"{n} is not prime")4else:5    for d in range(2, int(n ** 0.5) + 1):6        if n % d == 0:7            print(f"{n} is not prime")8            break9    else:10        print(f"{n} is prime")

Because no divisor between 2 and 5 evenly divides 29, the loop finishes without breaking and the else block executes.

Mathematical digit reversal

Digit extraction splits a number into its digits with two operations: n % 10 gives the last digit and n // 10 removes it.

Extract the last digit using % 10 and strip it using // 10 in a while loop to reverse a number without string conversions.

Python

1num = 12342rev = 03while num > 0:4    rev = (rev * 10) + (num % 10)5    num //= 106print(f"Reversed: {rev}")

Example 1 · Is 121 a palindrome?

Python

1num = 1212original = num3rev = 04while num > 0:5    rev = rev * 10 + num % 106    num //= 107if rev == original:8    print(f"{original} is a palindrome")9else:10    print(f"{original} is not a palindrome")

Output

121 is a palindrome

What this code does

A palindrome number reads the same from both ends, so its reverse equals the number.
original keeps the number, because the loop uses num up until it is 0.
The reverse of 121 is 121, so the if branch runs.

The loop mathematically extracts 4, 3, 2, 1, building 4321 in O(log10 N) time.

Example 2 · Factorial of 5

Python

n = 5fact = 1for i in range(2, n + 1):    fact *= iprint(f"{n}! = {fact}")

Output

5! = 120

What this code does

fact starts at 1, because starting at 0 would make every product 0.
The loop multiplies by 2, 3, 4 and 5, which gives 120.

Example 3 · Fibonacci series

Python

1n_terms = 52a, b = 0, 13
4print("Fibonacci:", end="")5for _ in range(n_terms):6    print(f" {a}", end="")7    a, b = b, a + b8print()

Output

Fibonacci: 0 1 1 2 3

What this code does

Uses tuple swapping a, b = b, a + b on each iteration.
Prints the first 5 Fibonacci numbers on one line: 0, 1, 1, 2, 3.

Finish up

Mark complete & continue

Next: Must-do problems: Loops

1n = 292if n < 2:3 print(f"{n} is not prime")4else:5 for d in range(2, int(n ** 0.5) + 1):6 if n % d == 0:7 print(f"{n} is not prime")8 break9 else:10 print(f"{n} is prime")

1num = 1212original = num3rev = 04while num > 0:5 rev = rev * 10 + num % 106 num //= 107if rev == original:8 print(f"{original} is a palindrome")9else:10 print(f"{original} is not a palindrome")

Example 3 · Fibonacci series

Python

1n_terms = 52a, b = 0, 13
4print("Fibonacci:", end="")5for _ in range(n_terms):6    print(f" {a}", end="")7    a, b = b, a + b8print()

Output

Fibonacci: 0 1 1 2 3

What this code does

Uses tuple swapping a, b = b, a + b on each iteration.
Prints the first 5 Fibonacci numbers on one line: 0, 1, 1, 2, 3.