MASTER YOUR LOGIC BUILDING:Phase 2 - Level 5: Logical Loop Combinations

Phase 2 - Level 5: Logical Loop Combinations

Why This Matters

Real programs don't just repeat blindly — they repeat with rules. "Add up all donations, but skip any $0 entries." "Find every number between 1 and 100 whose digits sum to an even number." These problems combine loops with the if logic you learned in Phase 1.

This is where everything comes together — and where programming starts to feel powerful.

What You'll Learn

• How to filter items inside a loop using if
• How to accumulate results only when a condition is met
• How continue skips one iteration without stopping the loop
• How break exits a loop early when you're done

Three Powerful Loop Patterns

1. Filter — process only what matches

for num in range(1, 101):
    if digit_sum_is_even(num):
        print(num)

Loop through everything, but act only on items that pass the test.

2. Accumulate with a condition — sum only the good ones

total = 0
for num in numbers:
    if num > 0:
        total += num

Keep a running total, but only add when the condition is true.

3. Skip with continue — ignore specific items

for i in range(5):
    num = int(input("Enter a number: "))
    if num == 0:
        continue        # skip the rest of this iteration
    total += num

continue says "skip to the next number" — the loop keeps going.

Step-by-Step: Even Digit Sum

Problem: Find numbers 1–100 whose digits add up to an even total.

1. Loop through each number
2. Extract digits and compute their sum (inner while loop)
3. Check if the sum is even (sum % 2 == 0)
4. If yes, print the number

for num in range(1, 101):
    digit_sum = 0
    temp = num
    while temp > 0:
        digit_sum += temp % 10
        temp //= 10
    if digit_sum % 2 == 0:
        print(num, end=" ")

Think: outer loop picks the number, inner loop processes its digits, if decides whether to keep it.

continue vs break

Common Mistakes

• Forgetting to reset variables — digit_sum must start at 0 inside the outer loop for each new number
• Using `break` when you mean `continue` — break stops everything; continue just skips one item
• Not initializing the accumulator — set total = 0 before the loop
• Putting the `if` outside the loop — the check must be inside, once per item

Tips

• Break the problem into two parts: "loop through items" and "check a condition"
• Name your conditions clearly: digit_sum % 2 == 0 reads better than a mystery variable
• Trace with 3–4 small inputs before running on 1–100
• Combine patterns: filter + accumulate is one of the most common real-world loops

Practice Focus

Tackle the examples below:

• Find numbers whose digit sum is even — nested loop + condition
• Sum five inputs while skipping zeros — practice continue
• Try writing the plain-English rule first, then map it to filter / accumulate / skip