Loops Practice

These questions follow Repetition. A loop is a bargain: describe one pass precisely and the computer does all of them. The two things worth getting exactly right are how many passes happen, and which lines are inside.

Reading

1. How many lines does this print, and what is the last one?

   for day in range(1, 8):
       print(f"Day {day}")

2. Trace it. Give the value of total after each pass, then say what prints.

   weights = [6, 11, 7]
   total = 0
   for weight in weights:
       total = total + weight
   print(total)

3. Find the fault. This prints 7 when it should print 24.

   weights = [6, 11, 7]
   for weight in weights:
       total = 0
       total = total + weight
   print(total)

4. How many lines does this print in total?

   for week in range(2):
       for day in range(3):
           print(f"Week {week}, day {day}")

Writing

5. Write a while loop that keeps asking How many bins? until the answer is made only of digits, then prints Recording 6 bins. Use .isdigit().
6. Add up every even number from 2 to 20 and print the total. Do it twice — once with a range that steps by 2, and once with a range over every number and an if inside.
7. Why does range(1, 8) give you a week, and range(1, 7) give you only six days? State the rule in your own words.
8. Challenge. Keep asking for weights until the user types done, then report the heaviest — without storing anything in a list.

Answers

Answer 1

Seven lines, ending with Day 7. range(1, 8) starts at 1 and stops before 8.

Answer 2

total is 6, then 17, then 24, and 24 prints. The print is outside the loop, so it runs once, after every pass is done. Move it inside and you get three lines instead of one.

Answer 3

total = 0 is inside the loop, so every pass throws away the running total and starts again — the printed value is only the last weight. Move that line above the for:

weights = [6, 11, 7]
total = 0
for weight in weights:
    total = total + weight
print(total)

An accumulator has to be created before the loop and used after it.

Answer 4

Six. The inner loop runs completely — three passes — for each of the outer loop’s two passes, so the count multiplies: 2 × 3. Nested loops are how you cover a table, a grid, or every day of every week.

Answer 5

answer = ""
 
while not answer.isdigit():
    answer = input("How many bins? ")
 
bins = int(answer)
print(f"Recording {bins} bins.")

Starting answer at "" matters: the condition is checked before the first pass, and an empty string is not made of digits, so the question gets asked at least once.

Answer 6

total = 0
for number in range(2, 21, 2):
    total = total + number
print(total)

total = 0
for number in range(1, 21):
    if number % 2 == 0:
        total = total + number
print(total)

Both print 110. The first says what you want; the second checks every number and throws half of them away. Prefer the first, but the second is the pattern you need the moment the test is more interesting than “is it even”.

Answer 7

range(a, b) includes a and stops before b, so the count is b - a. range(1, 8) is seven numbers, 1 through 7. Say it as “up to but not including”, out loud, until it sticks — this is the most common off-by-one error in the language.

Answer 8

highest = 0
entry = ""
 
while entry != "done":
    entry = input("Weight in kg (or 'done'): ")
    if entry.isdigit():
        weight = int(entry)
        if weight > highest:
            highest = weight
 
print(f"Heaviest: {highest} kg")

Entering 6, 11, 7, then done prints Heaviest: 11 kg. The .isdigit() check is what lets done pass through the loop without int() raising a ValueError. Notice the weakness of starting at 0: if every weight could be negative, nothing would ever beat the start value — the fix is to record the first value seen instead.