DSA · Theory

Array

Theory

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Array sits in a class list. Dev’s job is marks[0..n-1]. Write that first.

Do Array once by hand. index access O(1). Change one input. Say the new result out loud.

Without Array, a class list gets messy and marks[0..n-1] is hard to trust.

Dev ships Array in a class list. That is the use case worth saying.

If marks[0..n-1] breaks under Array, check off-by-one on the last index first.

After Array, Dev should still remember off-by-one on the last index.

One breath for Array, then marks[0..n-1], then off-by-one on the last index. Sit down.

Diagram

  index →  0    1    2
  array → [10,  20,  30]

Exam tip

Board: marks[0..n-1]. Dry-run Array. Name the trap: off-by-one on the last index.

arr = [10, 20, 30, 40]
print(arr[2], arr[-1])
matrix = [[1, 2], [3, 4]]
print(matrix[1][0])  # 3

Arrays give O(1) index access; matrices are arrays of arrays.