Displaying a 2D array with nested loops

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Quick Questions

Shade in the A or B circle for each question.

1. Which nested loop layout displays a 2D array in row order?

A Outer loop for columns, inner loop for rows

B Outer loop for rows, inner loop for columns

2. Where should OUTPUT Grid[Row, Column] go to display every element once?

A Inside the inner loop

B After the outer loop has finished

3. A 2D array Grid[1:3, 1:4] is displayed with nested loops. How many times does OUTPUT Grid[Row, Column] run?

A 12 times

B 7 times

4. Which FOR line is the inner loop for Seats[1:5, 1:8] when the outer loop uses Row?

A FOR Column ← 1 TO 8

B FOR Column ← 1 TO 5

5. Which code outputs ONLY row 2 of Grid[1:3, 1:4]?

A FOR Row ← 1 TO 3 OUTPUT Grid[Row, 2] NEXT Row

B FOR Column ← 1 TO 4 OUTPUT Grid[2, Column] NEXT Column

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1FOR Row ← 1 TO 3		AThe OUTPUT line gives only one index. A 2D array needs both the row and the column, so it should be Grid[Row, Column].
2OUTPUT Grid[2, Column]		BInside a loop over Column, this displays every element of row 2 only.
3The inner loop moves across the columns, and OUTPUT inside it displays one element each time round.		CThe outer loop: it moves through each row of the 2D array, one row at a time.
4To display only column 3 of a 2D array, loop through the rows and keep the column index fixed.		DFOR Column ← 1 TO 4 OUTPUT Grid[Row, Column] NEXT Column
5FOR Row ← 1 TO 3 FOR Column ← 1 TO 4 OUTPUT Grid[Row] NEXT Column NEXT Row		EThe outer loop has no NEXT Row, so it is never closed. NEXT Row must be added after NEXT Column.
6FOR Row ← 1 TO 3 FOR Column ← 1 TO 4 OUTPUT Grid[Row, Column] NEXT Column		FFOR Row ← 1 TO 3 OUTPUT Grid[Row, 3] NEXT Row

Definitions

OUTPUT Name[Row, Column]

Fill in the Blanks

Complete each line of code by filling in the missing parts.

FOR Row ← 1 TO 3 FOR Column ← 1 TO 4 ______ Grid[Row, Column]

FOR Row ← 1 TO 3 FOR Column ← 1 TO 4 OUTPUT Grid[Row, ______]

FOR Row ← 1 TO 3 OUTPUT Grid[___, 2] NEXT Row

OUTPUT Grid[Row, Column] NEXT ______ NEXT Row

FOR Row ← 1 TO 3 FOR Column ← 1 __ 4 OUTPUT Seats[Row, Column]

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A FOR Row 1 TO 3

B FOR Col ← 1 TO 4

C OUTPUT Grid[Row, Col]

D NEXT Col

E NEXT Row

Question 2

A FOR Row ← 1 TO 3

B FOR Col ← 1 TO 4

C OUTPUT Grid[Row, Col]

D NEXT Row

E NEXT Row

Question 3

A FOR Student ← 1 TO 4

B FOR Test ← 1 TO 3

C OUTPUT Scores[Test, Student]

D NEXT Test

E NEXT Student

Question 4

A FOR Row ← 1 TO 5

B FOR Row ← 1 TO 8

C OUTPUT Seats[Row, Seat]

D NEXT Seat

E NEXT Row

Question 5

A FOR Day ← 1 TO 7

B FOR Hour ← 1 TO 3

C OUTPUT Temp[Day]

D NEXT Hour

E NEXT Day

Answer Key

Displaying a 2D array with nested loops

Section A: Quick Questions

B — Outer loop for rows, inner loop for columns

A — Inside the inner loop

A — 12 times

A — FOR Column ← 1 TO 8

B — FOR Column ← 1 TO 4 OUTPUT Grid[2, Column] NEXT Column

Section B: Match the Pairs

FOR Row ← 1 TO 3 → The outer loop: it moves through each row of the 2D array, one row at a time.

OUTPUT Grid[2, Column] → Inside a loop over Column, this displays every element of row 2 only.

The inner loop moves across the columns, and OUTPUT inside it displays one element each time round. → FOR Column ← 1 TO 4 OUTPUT Grid[Row, Column] NEXT Column

To display only column 3 of a 2D array, loop through the rows and keep the column index fixed. → FOR Row ← 1 TO 3 OUTPUT Grid[Row, 3] NEXT Row

FOR Row ← 1 TO 3 FOR Column ← 1 TO 4 OUTPUT Grid[Row] NEXT Column NEXT Row → The OUTPUT line gives only one index. A 2D array needs both the row and the column, so it should be Grid[Row, Column].

FOR Row ← 1 TO 3 FOR Column ← 1 TO 4 OUTPUT Grid[Row, Column] NEXT Column → The outer loop has no NEXT Row, so it is never closed. NEXT Row must be added after NEXT Column.

Section C: Definitions

OUTPUT Name[Row, Column] — Placed inside nested FOR loops (outer loop for rows, inner loop for columns), this line displays each element of a 2D array once, in row order.

Section D: Fill in the Blanks

FOR Row ← 1 TO 3
FOR Column ← 1 TO 4
OUTPUT Grid[Row, Column]

FOR Row ← 1 TO 3
FOR Column ← 1 TO 4
OUTPUT Grid[Row, Column]

FOR Row ← 1 TO 3
OUTPUT Grid[Row, 2]
NEXT Row

OUTPUT Grid[Row, Column]
NEXT Column
NEXT Row

FOR Row ← 1 TO 3
FOR Column ← 1 TO 4
OUTPUT Seats[Row, Column]

Section E: Spot the Error

A — The outer loop for the rows is missing the assignment arrow. It should be FOR Row ← 1 TO 3.

D — The inner loop is FOR Col, so it must end with NEXT Col. NEXT Row belongs on the last line, closing the outer loop.

C — The row index must come first and the column index second. Scores[Test, Student] swaps them, so the elements are not shown in row order. It should be Scores[Student, Test].

B — The inner loop reuses the variable Row, which clashes with the outer loop. The inner loop for the columns needs its own variable: FOR Seat ← 1 TO 8.

C — A 2D array needs two indexes, a row and a column. Temp[Day] gives only one. It should be OUTPUT Temp[Day, Hour].