2-D arrays

Rows and columns

• A two-dimensional array is a grid. The declaration gives both bounds.
• Two indexes, separated by a comma, pick a cell: row, then column.

DECLARE Grid : ARRAY[1:2, 1:2] OF INTEGER
Grid[1, 1] ← 4
Grid[1, 2] ← 7
OUTPUT Grid[1, 2]

One loop inside another

• The outer loop walks the rows. The inner loop walks the columns of that row.
• NEXT names the counter of the loop it closes.

DECLARE Grid : ARRAY[1:2, 1:2] OF INTEGER
Grid[1, 1] ← 1
Grid[1, 2] ← 2
Grid[2, 1] ← 3
Grid[2, 2] ← 4
DECLARE Total : INTEGER
Total ← 0
FOR Row ← 1 TO 2
  FOR Column ← 1 TO 2
    Total ← Total + Grid[Row, Column]
  NEXT Column
NEXT Row
OUTPUT Total

Common mistakes

• Write Grid[1, 2], with a comma. Both indexes start at the declared lower bound.
• The inner NEXT comes before the outer one.

Now you try

• Read one cell of a 2-D array, then total the grid.