Skip to content
Codextroop TechnologiesCodexLab

Arrays

The structure behind every list in your code: elements side by side, instant access by index, but costly to squeeze a new value into the middle.

Class 11 to 12beginner30 min2 lessons1 interactive lab
By the end you will be able to
  • Explain why indexed access is constant time
  • Trace the element shifting caused by an insert or delete
  • Choose between an array and a linked list for a given task

Finish first

Complexity & Big-O

Cost at a glance

access
O(1)
search
O(n)
insert
O(n)
delete
O(n)
Topic contents

Lesson 1 of 2

Side by side in memory

An array stores its elements in one continuous block, in order. Because every element is the same size and the block is unbroken, the computer can jump straight to element 5 instead of hunting from the start. That is why array access is O(1).

The cost shows up when you change the shape. There is no gap to insert into, so making room means sliding every element after the insertion point one place to the right. Try it in the lab and count the moves.

Exam questions often ask: fast lookup or fast insert in the middle? Arrays win lookup. Linked lists, coming soon, win insert at a known position.

Interactive lab

Array operations

Insert, delete, search and traverse, one controllable step at a time, with the shifting made visible.

Array operations

access O(1) · insert/delete/search O(n)
10
0
20
1
30
2
40
3
1 / 5

Start

Traversal visits every element exactly once, in index order.

Continue

Lesson 2 of 2

Watch the shift happen

Use the visualizer to insert a value at index 2. Step through it one frame at a time and notice the order of events: the last element moves right first, then the one before it, and so on back to the insertion point. If you shifted left-to-right instead, you would overwrite values you still needed.

Then delete at index 2 and watch the direction reverse, elements move left to close the gap, and the array shrinks by one.

Continue

Worked examples

Read the code, then change it

Copy any example into the playground and break it on purpose. That is the fastest way to learn what each line is holding up.

Insert at an index by handJavaScript
function insertAt(values, index, value) {
  // Walk backwards so we never overwrite an element we still need.
  for (let i = values.length; i > index; i--) {
    values[i] = values[i - 1];
  }
  values[index] = value;
  return values;
}

console.log(insertAt([10, 20, 30, 40], 2, 25));
The same idea in PythonPython
def insert_at(values, index, value):
    values.append(None)
    for i in range(len(values) - 1, index, -1):
        values[i] = values[i - 1]
    values[index] = value
    return values

print(insert_at([10, 20, 30, 40], 2, 25))

Practice

Work these out yourself

No answer key here on purpose: these are the questions worth thinking through before you move on. Open one and work it out.

0/3 attempted

Assessment

Check your understanding

Answer each question, then read the explanation. That is where the learning is.

0/5
  1. Question 1: Inserting at the front of an array of n elements costs:
    Question 1 / 5

    Inserting at the front of an array of n elements costs:

    Select an option first
  2. Question 2: When shifting elements right to make room, which element should move first?
    Question 2 / 5

    When shifting elements right to make room, which element should move first?

    Select an option first
  3. Question 3: Searching an unsorted array of 1,000 elements, worst case, checks:
    Question 3 / 5

    Searching an unsorted array of 1,000 elements, worst case, checks:

    Select an option first
  4. Question 4: Which array operation does not require shifting any elements?
    Question 4 / 5

    Which array operation does not require shifting any elements?

    Select an option first
  5. Question 5: A dynamic array is full and one more element is appended. What happens?
    Question 5 / 5

    A dynamic array is full and one more element is appended. What happens?

    Select an option first
5 of 5 questions left.

Finished this topic?

0 of 2 lessons marked done. Marking the topic complete ticks the rest.