Start with the recap, study the fully worked examples, then use the practice problems to
check your understanding of Array.
This page combines explanation, solved examples, and follow-up practice so you can move
from recognition to confident problem-solving in CS Thinking.
Concept Recap
An ordered collection of values stored together under a single name and accessed by their numeric index position. Arrays allow you to store, retrieve, and manipulate multiple related values efficiently using loops and index-based access.
A numbered list. Item 0, item 1, item 2... Access any by its number.
Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.
What to Focus On
Core idea:Arrays store multiple related values under one name, making it easy to iterate over them all.
Common stuck point:Most languages index from 0, not 1. The last element of a 3-item array is at index 2.
Sense of Study hint:When working with arrays, remember that indices start at 0 in most languages, so the last element of an array with n items is at index n−1. Use loops to process every element, and always check that your index is within bounds before accessing an element.
Worked Examples
Example 1
easy
An array `scores` contains [85, 92, 78, 95, 88]. What is scores[0]? What is scores[3]? How many elements are there?scores = [85, 92, 78, 95, 88] — which cells do scores[0] and scores[3] pick?
Answer
scores[0] = 85, scores[3] = 95, length = 5.
First step
1
Step 1: Arrays use zero-based indexing: scores[0] = 85.
Full solution
2
Step 2: scores[3] = 95 (the 4th element).
3
Step 3: The array has 5 elements (length = 5).
Arrays store multiple values in a single variable, accessed by index. Most programming languages use zero-based indexing, so the first element is at index 0.
Example 2
medium
Write pseudocode to find the sum of all elements in an array `nums = [4, 7, 2, 9]`.nums = [4, 7, 2, 9] — trace the accumulation loop over every index
Example 3
medium
Write pseudocode to count how many elements of `nums = [3, 7, 2, 9, 4]` are greater than 5. What is the count?
Example 4
hard
Given `a = [4, 9, 1, 7, 3]`, trace finding the min: keep `best = a[0]`, then compare to each later element. What is the final `best`?
Practice Problems
Try these problems on your own first, then open the solution to compare your method.
Example 1
medium
Given array `names = ['Alice', 'Bob', 'Charlie']`, what is names[1]? What happens if you access names[3]?
Example 2
medium
Given `scores = [12, 15, 9, 20]`, what are `scores[0]` and `scores[3]`, and what happens if code tries to read `scores[4]`?scores = [12, 15, 9, 20] — identify scores[0], scores[3], and what scores[4] means
Example 3
easy
Array `a = [10, 20, 30, 40]`. What is `a[0]`?a = [10, 20, 30, 40] — what value is at index 0?
Example 4
easy
Array `a = [10, 20, 30, 40]`. What is `a[2]`?a = [10, 20, 30, 40] — what value is at index 2?
Example 5
easy
Array `a = [5, 6, 7, 8, 9]` has 5 elements. What is the last valid index?a = [5, 6, 7, 8, 9] — what is the last valid index for length 5?
Example 6
easy
Array `a = [1, 2, 3]`. What happens with `a[3]`?
Example 7
easy
How many elements are in `a = [4, 8, 15, 16, 23, 42]`?a = [4, 8, 15, 16, 23, 42] — count the elements to find the length
Example 8
easy
Array `a = [3, 1, 4, 1, 5]`. After `a[1] = 9`, what is the array?After a[1] = 9, the array [3, 1, 4, 1, 5] becomes [3, 9, 4, 1, 5]
Example 9
easy
To loop over every element of an array of length n with indices, what range of indices do you use?
Example 10
easy
Why are arrays useful instead of separate variables x1, x2, x3, ...?
Example 11
medium
Array `a = [2, 4, 6, 8, 10]`. What is `a[a[0]]`?a = [2, 4, 6, 8, 10] — evaluate a[a[0]]: first read a[0]=2, then read a[2]
Example 12
medium
Sum every element of `a = [3, 5, 7]` by looping indices 0 to length-1. What is the sum?
Example 13
medium
Array `a = [9, 7, 5, 3, 1]`. What is `a[len(a) - 1]`?a = [9, 7, 5, 3, 1] — a[len(a)-1] = a[4]; what value does it hold?
Example 14
medium
Reverse `a = [1, 2, 3, 4]` by swapping a[i] with a[n-1-i]. What is `a[3]` paired with for swapping when i=0?
Example 15
medium
Array `a = [10, 20, 30]`. Loop `for i in 0..3 (inclusive): print(a[i])`. What goes wrong?
Example 16
medium
To insert a value at the front of an array of length n, how many existing elements must shift right?
Example 17
medium
Array `a = [5, 3, 8, 1]`. After sorting ascending, what is `a[0]`?After sorting [5, 3, 8, 1] ascending → [1, 3, 5, 8]; what is a[0]?
Example 18
medium
Array of length n. How many valid index values are there?
Example 19
challenge
A 2D array (matrix) is stored as `m` with rows of length 4. Element at row r, column c maps to flat index `r*4 + c`. What flat index is row 2, column 3?
Example 20
challenge
Array `a = [1, 2, 3, 4, 5]`. You delete `a[1]` while iterating forward by index. Why might you skip an element?
Example 21
challenge
To find the max of an unsorted array of length n, what is the minimum number of comparisons needed?
Example 22
medium
Array `a = [7, 14, 21, 28]`. What is `a[1] + a[3]`?a = [7, 14, 21, 28] — read a[1] and a[3] then add them
Example 23
easy
Array `a = [11, 22, 33, 44, 55]`. What is `a[0]`?a = [11, 22, 33, 44, 55] — what value is at index 0?
Example 24
easy
Array `a = [11, 22, 33, 44, 55]`. What is `a[4]`?a = [11, 22, 33, 44, 55] — what value is at the last index 4?
Example 25
easy
Array `a = [9, 8, 7]`. What does `a[3]` produce?
Example 26
easy
How many elements are in `a = [1, 1, 2, 3, 5, 8, 13]`?
Example 27
easy
Array `a = [5, 5, 5, 5]`. After `a[2] = 9`, what is the array?After a[2] = 9, array [5, 5, 5, 5] becomes [5, 5, 9, 5]
Example 28
easy
Array `a = [4, 4, 4]`. What is the sum of all elements?
Example 29
medium
Array `a = [3, 1, 4, 1, 5, 9]`. What is `a[1] + a[4]`?
Example 30
medium
Array `a = [10, 20, 30, 40, 50]`. What is `a[a[0]/10]`?
Example 31
medium
Array `a = [6, 2, 8, 4]`. What is `max(a)`?a = [6, 2, 8, 4] — which cell holds the maximum value?
Example 32
medium
Reverse `a = [1,2,3,4,5]` by swapping `a[i]` with `a[n-1-i]`. After processing i=0 and i=1, what is the array?
Example 33
medium
Array `a = [2, 4, 6, 8]`. Loop `for i in 0..3 (inclusive)` prints `a[i]`. How many values are printed and what are they?
Example 34
medium
To delete the LAST element of an array of length n, how many existing elements must shift?
Example 35
medium
Array `a = [9, 3, 7, 1, 5]`. After ascending sort, what is `a[2]`?After sorting [9, 3, 7, 1, 5] ascending → [1, 3, 5, 7, 9]; what is a[2]?
Example 36
medium
Sum every element of `a = [10, 20, 30, 40]` using indices 0 to length-1. What is the sum?
Example 37
medium
Array `a = [3, 6, 9, 12]`. What is `a[len(a) - 2]`?a = [3, 6, 9, 12] — a[len(a)-2] = a[2]; what value does it hold?
Example 38
medium
To insert a value at index 1 in an array of length 6 (no resize cost), how many existing elements must shift right?
Example 39
hard
A 2D array (matrix) stored row-major with row length 5. What flat index corresponds to row 3, column 2?
Example 40
hard
Linear search needs at most how many comparisons to find a value in an array of length n (worst case)?
Example 41
hard
Array `a = [1, 2, 3, 4, 5, 6]`. Iterating forward by index and deleting `a[i]` whenever `a[i]` is even — which elements remain after one full pass?
Example 42
hard
You want to swap the FIRST and LAST elements of an array of length 6 without overwriting either. What is the minimum number of assignments needed?
Example 43
challenge
An array stores a 2D 4x4 grid in row-major order. The value at flat index 11 corresponds to which (row, column)?