Unit 4 Cycle 2 Day 15: 2D Array: Border Elements
Share
2D Array: Border Elements
Section 4.11 — 2D Array Algorithms
Key Concept
Border elements of a 2D array are those in the first row, last row, first column, or last column. Identifying border elements requires checking: r == 0 or r == grid.length - 1 or c == 0 or c == grid[0].length - 1. An efficient approach processes each border separately with four loops rather than checking every element. The AP exam tests border element operations like summing, counting, or replacing border values. Watch for corner elements being counted twice if borders are processed separately.
Consider the following code segment.
What is printed?
Answer: (B) 40
Border: Row 0 all (1+2+3=6), Row 1 edges (4+6=10), Row 2 all (7+8+9=24). Total=40. Only center (5) excluded.
Why Not the Others?
(A) 45 = all elements. Center (5) is excluded.
(C) 25 does not match.
(D) 20 is too low.
Common Mistake
Border check: r==0 || r==rows-1 || c==0 || c==cols-1. Interior elements are excluded.
AP Exam Tip
Border detection uses OR conditions on row/column boundaries.