Questions: Arrays and Lists

Question 1

Given a = [10, 20, 30, 40], what is the value of a[-2]?

• A. 10
• B. 20
• C. 30
• D. 40

Answer: (C)

Negative indices count backward from the end of the list. Index -1 is the last element (40), -2 is second-to-last (30), -3 is 20, and -4 is 10. Negative indexing is a convenience that avoids having to write a[len(a)-2]; it does not create a separate reversed copy of the list.

Question 2

After executing b = a (where a is the list [1, 2, 3]), the statement b[0] = 99 will also change a[0] to 99.

• T. True
• F. False

Answer: True

List assignment does not copy the list — it creates a second name that refers to the same object in memory. Both a and b point to the same list, so any mutation through b is visible through a. To get an independent copy, use b = a[:] or b = list(a). This is one of the most common sources of subtle bugs for new programmers coming from a background where assignment always copies the value.

Question 3

Describe the accumulation pattern for building a list of the squares of the integers 1 through 5 using a for loop.

Model answer: Initialize an empty list before the loop (e.g., squares = []). Inside a for loop over range(1, 6), append i**2 to the list each iteration. After the loop, squares holds [1, 4, 9, 16, 25].

The accumulation pattern separates creation from computation: start with an empty container, iterate over inputs, and collect results one at a time. It is the foundational pattern for transforming sequences and appears in list comprehensions, map operations, and reduce operations — all of which are built on this same idea.