Searching a 2D array

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Quick Questions

Shade in the A or B circle for each question.

1. Which line compares each element of a 2D array called Grid with the wanted value?

A IF Grid[Row, Column] = Wanted THEN

B IF Grid = Wanted THEN

2. To search every cell of a 2D array, which loop structure is needed?

A A FOR loop for rows with a FOR loop for columns inside it

B A single FOR loop that counts from 1 to Wanted

3. A search finds Wanted at Grid[3, 2]. Which assignments record where it was found?

A FoundRow ← Wanted and FoundColumn ← Wanted

B FoundRow ← Row and FoundColumn ← Column

4. Where should the IF Grid[Row, Column] = Wanted THEN line go?

A Before both FOR loops, so it only checks once

B Inside the inner FOR loop, so it checks every cell

5. How do you count how many times Wanted appears in a 2D array?

A Set Count ← 0 before the loops, then Count ← Count + 1 each time a cell matches

B Set Count ← 1 inside the inner loop each time a cell matches

Match the Pairs

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

Left column		Right column
1IF Grid[Row, Column] = Wanted THEN		AIF Grid[Row, Column] = " " THEN Empty ← TRUE
2FoundRow ← Row		B<- is the assignment arrow, so it cannot be used to compare values. Use = to compare: IF Grid[Row, Column] = Wanted THEN
3To count how many times a value appears in a 2D array, add 1 to a counter each time an element matches		CCompares the element at the current row and column with the wanted value inside nested loops
4To check a grid for an empty cell, compare each element with the empty marker such as a space		DStores the row number where the wanted value was found
5IF Grid[Row, Column] <- Wanted THEN		EA 2D array element needs both indexes. Write Grid[Row, Column] so one cell is compared with Wanted
6IF Grid[Row] = Wanted THEN		FIF Grid[Row, Column] = Wanted THEN Count ← Count + 1

Definitions

IF Grid[Row, Column] = Wanted THEN

Fill in the Blanks

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

FOR Column ← 1 TO 3 IF Grid[Row, Column] = ______ THEN

IF Grid[Row, Column] = Wanted THEN FoundRow ← Row FoundColumn ← ______

IF Grid[Row, Column] = Wanted THEN Count ← Count _ 1 ENDIF

FOR Column ← 1 TO 3 IF Grid[Row, Column] = "" ____ Empty ← TRUE

IF Grid[Row, 1] = "X" AND Grid[Row, 2] = "X" ___ Grid[Row, 3] = "X" THEN

Spot the Error

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

Question 1

A FOR R ← 1 TO 3

B FOR C ← 1 TO 4

C IF Grid[R, C] ← Want THEN

D FoundRow ← R

E FoundColumn ← C

F ENDIF

Question 2

A Count ← 0

B FOR R ← 1 TO 3

C FOR C ← 1 TO 3

D IF Grid[R] = Want THEN

E Count ← Count + 1

F ENDIF

Question 3

A // Grid: 3 rows, 4 columns

B FOR R ← 1 TO 4

C FOR C ← 1 TO 4

D IF Grid[R, C] = Want THEN

E FoundRow ← R

Question 4

A FOR R ← 1 TO 2

B FOR C ← 1 TO 3

C IF Grid[C, R] = " " THEN

D Empty ← TRUE

E ENDIF

Question 5

A Count ← 1

B FOR R ← 1 TO 3

C FOR C ← 1 TO 3

D IF Grid[R, C] = Want THEN

E Count ← Count + 1

Answer Key

Searching a 2D array

Section A: Quick Questions

A — IF Grid[Row, Column] = Wanted THEN

A — A FOR loop for rows with a FOR loop for columns inside it

B — FoundRow ← Row and FoundColumn ← Column

B — Inside the inner FOR loop, so it checks every cell

A — Set Count ← 0 before the loops, then Count ← Count + 1 each time a cell matches

Section B: Match the Pairs

IF Grid[Row, Column] = Wanted THEN → Compares the element at the current row and column with the wanted value inside nested loops

FoundRow ← Row → Stores the row number where the wanted value was found

To count how many times a value appears in a 2D array, add 1 to a counter each time an element matches → IF Grid[Row, Column] = Wanted THEN Count ← Count + 1

To check a grid for an empty cell, compare each element with the empty marker such as a space → IF Grid[Row, Column] = " " THEN Empty ← TRUE

IF Grid[Row, Column] <- Wanted THEN → <- is the assignment arrow, so it cannot be used to compare values. Use = to compare: IF Grid[Row, Column] = Wanted THEN

IF Grid[Row] = Wanted THEN → A 2D array element needs both indexes. Write Grid[Row, Column] so one cell is compared with Wanted

Section C: Definitions

IF Grid[Row, Column] = Wanted THEN — Inside nested FOR loops, this compares each 2D array element with the wanted value; when it matches, store Row and Column as FoundRow and FoundColumn.

Section D: Fill in the Blanks

FOR Column ← 1 TO 3
   IF Grid[Row, Column] = Wanted THEN

IF Grid[Row, Column] = Wanted THEN
   FoundRow ← Row
   FoundColumn ← Column

IF Grid[Row, Column] = Wanted THEN
   Count ← Count + 1
ENDIF

FOR Column ← 1 TO 3
   IF Grid[Row, Column] = "" THEN
      Empty ← TRUE

IF Grid[Row, 1] = "X" AND Grid[Row, 2] = "X"
   AND Grid[Row, 3] = "X" THEN

Section E: Spot the Error

C — ← assigns a value, it does not compare. A search must compare the element with the wanted value using =, so the line should be IF Grid[R, C] = Want THEN.

D — A 2D array needs both a row index and a column index. Grid[R] is missing the column, so the line should be IF Grid[R, C] = Want THEN.

B — The grid has only 3 rows, so the row loop must stop at 3. Going to 4 would use Grid[4, C], which is out of range. It should be FOR R ← 1 TO 3.

C — The row index must come first and the column index second. Grid[C, R] swaps them and could go out of range (the grid has 2 rows). It should be IF Grid[R, C] = " " THEN.

A — A counter must start at 0 before the loops. Starting at 1 makes the final count one too high. It should be Count ← 0.