Lists Practice

These questions follow Lists. Every one of them uses the same starting list, so keep it in a file and edit as you go:

marks = [78, 91, 46, 63]

Positions and bounds

1. What does each print? marks[0], marks[3], marks[-1], len(marks).
2. What happens when you run print(marks[4]), and why is 4 a reasonable-looking mistake?
3. Starting from the original list each time, give the list after marks.append(70), then after marks.insert(1, 55), then after marks.remove(46) — applied one after the other.

Loops over lists

4. Write the loop that totals the marks and prints the average to one decimal place.
5. Write a linear search that reports the position of "Fifteen Dogs" in titles = ["Birdie", "Fifteen Dogs", "Indian Horse"], and prints -1 when the title is not there.
6. Count how many of weights = [6.5, 3.0, 11.2, 7.0] are above 6.0.
7. Find the largest value in weights without using max().
8. Challenge. Build a new list holding only the passing marks (50 or more) from marks, then print how many there are and what they are.

Answers

Answer 1

78, 63, 63, 4. Positions run 0 to 3, so marks[3] and marks[-1] are the same element reached from opposite ends, and len counts elements rather than naming the last position.

Answer 2

IndexError: list index out of range

Four elements, positions 0 to 3. 4 looks right because the list has four marks in it — but counting from zero means the last position is always len minus one.

Answer 3

[78, 91, 46, 63, 70], then [78, 55, 91, 46, 63, 70], then [78, 55, 91, 63, 70]. append adds to the end, insert puts the value at the position given and shifts everything after it along, and remove deletes the first matching value — not a position.

Answer 4

total = 0
for mark in marks:
    total = total + mark
print(f"Average: {total / len(marks):.1f}")

For the original four marks that prints Average: 69.5. Dividing by len(marks) rather than by 4 means the code still works when the list changes, which it always does.

Answer 5

titles = ["Birdie", "Fifteen Dogs", "Indian Horse"]
target = "Fifteen Dogs"
position = -1
 
for index in range(len(titles)):
    if titles[index] == target:
        position = index
 
print(position)

This prints 1. Looping over range(len(titles)) gives you positions rather than values, which is what a search has to report. Starting at -1 is the convention for “not found”, so the answer is still meaningful when nothing matches.

Answer 6

weights = [6.5, 3.0, 11.2, 7.0]
above = 0
for weight in weights:
    if weight > 6.0:
        above = above + 1
print(above)

3. A counter is an accumulator that grows by one instead of by the value — same skeleton, different update.

Answer 7

highest = weights[0]
for weight in weights:
    if weight > highest:
        highest = weight
print(highest)

11.2. Starting from weights[0] rather than from 0 is the part that matters: a list of negative numbers would defeat a starting value of zero, and the program would report a maximum that is not in the list.

Answer 8

passing = []
for mark in marks:
    if mark >= 50:
        passing.append(mark)
print(f"{len(passing)} passing: {passing}")

3 passing: [78, 91, 63]

Building a second list leaves the original untouched, which is usually what you want — the caller may still need the full set, and a function that quietly destroys its input is a hard bug to find later.