← IGCSE Computer Science Paper 2

Choosing a 1D or 2D array for a scenario

Use a 1D array for a simple list of values. Use a 2D array when data forms a table or grid, so each value needs two indexes: a row and a column.

Part of: IGCSE Computer Science 0478 — 8.2 Arrays — Module 38

Understanding Choosing a 1D or 2D array for a scenario in IGCSE Computer Science Paper 2

What it does: 30 students by 4 tests is a table with two ways of locating a value, so a 2D array fits. With parallel 1D arrays, index 5 in each array refers to the same student. Each array is still a simple list with one index. Two indexes inside the brackets (student 5, test 2) mean the array is two-dimensional. A game board is a grid, so each square needs two indexes (row and column). That calls for a 2D array. Data needing a row and a column must be stored in a 2D array. A 1D array only accepts one index.

Common mistakes: A list of 30 names is a 1D array, so each name needs only one index. It should be Names[5] ← "Zoe". A game board is a grid, so it is a 2D array and each square needs a row and a column. It should be Board[2,2] ← "X".

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This module's key terms are also in a printable crossword (Modules 479–488) and word search (Modules 479–499).

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Learning

Duo

A teacher stores the names of 30 students in one simple list. Which array fits best?

A 2D array, e.g. ARRAY[1:30, 1:30] OF STRINGA 1D array, e.g. ARRAY[1:30] OF STRING

Matches

DECLARE Marks : ARRAY[1:120] OF INTEGER Marks[3, 2] <- 75

↓

The array is 1D but is used with two indexes. Marks for students and tests form a table, so it should be declared as 2D: ARRAY[1:30, 1:4] OF INTEGER.

Applying

Meanings

Term

1D or 2D array

Meaning

Use a 1D array for a simple list of values. Use a 2D array when data forms a table or grid, so each value needs two indexes: a row and a column.

Practice

// 1D: a simple list of names

Testing

Gaps

DECLARE Names : ARRAY[] OF STRING

Check Answer

Spots

1Names[5,1] ← "Zoe"

2Names[6] ← "Leo"

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Quick Questions

1. A teacher stores the names of 30 students in one simple list. Which array fits best?

A. A 1D array, e.g. ARRAY[1:30] OF STRING ✓

B. A 2D array, e.g. ARRAY[1:30, 1:30] OF STRING

A simple list of names needs only one index to find each name, so a 1D array is the right choice.

2. A noughts and crosses game board has 3 rows and 3 columns. Which array fits best?

A. A 1D array, e.g. ARRAY[1:3] OF CHAR

B. A 2D array, e.g. ARRAY[1:3, 1:3] OF CHAR ✓

A game board is a grid. Each square is found using a row index and a column index, so a 2D array fits.

3. A program stores the marks of 30 students over 4 tests. Which array fits best?

A. One 1D array: ARRAY[1:30] OF INTEGER

B. One 2D array: ARRAY[1:30, 1:4] OF INTEGER ✓

The marks form a table: 30 rows (students) by 4 columns (tests). A 1D array has no way to store the 4 tests per student.

4. Which statement about choosing an array is correct?

A. Use a 2D array when each value needs a row and a column to locate it ✓

B. Use a 2D array whenever you have more than 10 values

The choice depends on the shape of the data, not on how many values there are. 100 names in a list still need only a 1D array.

5. Which array would you use to find a student's mark with Marks[5, 2]?

A. A 2D array, because two indexes are used ✓

B. A 1D array, because it is a list of marks

Two indexes inside the brackets (student 5, test 2) mean the array is two-dimensional.

Match the Pairs

DECLARE Names : ARRAY[1:30] OF STRING → A 1D array: one simple list of 30 names, where each name is found using a single index

DECLARE Board : ARRAY[1:3, 1:3] OF CHAR → A 2D array: a 3 by 3 game board grid, where each square is found using a row and a column

Marks for 30 students over 4 tests form a table, so each mark needs a student (row) and a test (column) → DECLARE Marks : ARRAY[1:30, 1:4] OF INTEGER

Parallel 1D arrays store related lists side by side, and the same index links the matching items → Names[5] <- "Ava" Scores[5] <- 88

DECLARE Marks : ARRAY[1:120] OF INTEGER Marks[3, 2] <- 75 → The array is 1D but is used with two indexes. Marks for students and tests form a table, so it should be declared as 2D: ARRAY[1:30, 1:4] OF INTEGER.

DECLARE Names : ARRAY[1:30, 1:1] OF STRING Names[7, 1] <- "Zoe" → A simple list of names needs only one index, so a 2D array is unnecessary. It should be DECLARE Names : ARRAY[1:30] OF STRING and Names[7] <- "Zoe".

Key Term

1D or 2D array

Use a 1D array for a simple list of values. Use a 2D array when data forms a table or grid, so each value needs two indexes: a row and a column.

Fill in the Blanks

1. DECLARE Names : ARRAY[1:30] OF STRING

Hint: A simple list of 30 names needs only one range. Type the bounds as lower:upper.

2. DECLARE Board : ARRAY[1:3, 1:3] OF CHAR

Hint: A 3 by 3 game board is a grid. Type the bounds for the second dimension (the columns).

3. DECLARE Marks : ARRAY[1:30, 1:4] OF INTEGER

Hint: Marks for 30 students over 4 tests form a table. Type the column bounds, one column for each test.

4. // mark for each student and test OUTPUT Marks[Student, Test]

Hint: Two indexes (student and test) are used, so type the name of the 2D array that holds the table of marks.

5. // parallel 1D arrays share one index OUTPUT Names[I], Marks[I]

Hint: Matching values in parallel arrays sit at the same position. Type the index variable that links them.

Spot the Error

Question 1

A Names[5,1] ← "Zoe"

B Names[6] ← "Leo"

C OUTPUT Names[5]

Line A — A list of 30 names is a 1D array, so each name needs only one index. It should be Names[5] ← "Zoe".

Question 2

A Board[2] ← "X"

B Board[1,3] ← "O"

C OUTPUT Board[2,2]

Line A — A game board is a grid, so it is a 2D array and each square needs a row and a column. It should be Board[2,2] ← "X".

Question 3

A Marks[1,1] ← 72

B Marks[1,2] ← 65

C Marks[2,1] ← 58

D Marks[2] ← 80

Line D — Marks for students across several tests form a table, so a 2D array. You need the student (row) and the test (column): Marks[2,2] ← 80.

Question 4

A Name[3] ← "Mia"

B Mark[4] ← 91

C OUTPUT Name[3], Mark[3]

Line B — With parallel 1D arrays, the same index must be used for the same student. Mia is at index 3 in Name, so her mark must be Mark[3] ← 91.

Question 5

A FOR r ← 1 TO 3

B FOR r ← 1 TO 3

C Board[r,c] ← "-"

D NEXT c

E NEXT r

Line B — A 2D board needs two different loop variables, one for rows and one for columns. The inner loop should be FOR c ← 1 TO 3, because Board[r,c] uses c.

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