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📘 CIE IGCSE Computer Science

💻 Arrays – Detailed Notes

1️⃣ What is an Array?

An array is a data structure that stores multiple values of the same data type under one name.

Instead of writing:

score1 ← 45
score2 ← 60
score3 ← 75

We use:

DECLARE score : ARRAY[1:3] OF INTEGER

Then access using an index:

score[1] ← 45
score[2] ← 60
score[3] ← 75

🔹 Key Features of Arrays​

  • All elements must be the same data type
  • Each value is stored in a numbered position
  • The position number is called the index
  • The first index can be 0 or 1 (depends on language)

2️⃣ Why Do We Use Arrays?

Arrays are used when:

  • You need to store many values of the same type
  • The number of values is known
  • You need to process data using loops

Example Uses:​

  • Student marks
  • Monthly temperatures
  • Product prices
  • Game scores

3️⃣ One-Dimensional (1D) Arrays

A 1D array is like a single row or list.

Declaration (Index starts at 1)​

DECLARE marks : ARRAY[1:5] OF INTEGER

Declaration (Index starts at 0)​

DECLARE marks : ARRAY[0:4] OF INTEGER

⚠️ Always check what the question specifies.

🔹 Writing Values into a 1D Array​

Using iteration:

FOR i ← 1 TO 5
INPUT marks[ i ]
NEXT i

Here:

  • i is the index variable
  • This avoids writing five separate statements

🔹 Reading Values from a 1D Array​

FOR i ← 1 TO 5
OUTPUT marks[ i ]
NEXT i

🔹 Totalling Array Values​

total ← 0
FOR i ← 1 TO 5
total ← total + marks[ i ]
NEXT i

🔹 Counting Values in Array​

count ← 0
FOR i ← 1 TO 5
IF marks[ i ] >= 50 THEN
count ← count + 1
ENDIF
NEXT i

🧠 Exam Tips for 1D Arrays​

✔ Use a loop
✔ Initialise totals and counters
✔ Use correct index range
✔ Avoid going out of bounds

4️⃣ Two-Dimensional (2D) Arrays

A 2D array is like a table (rows and columns).

Think of it like:

Col1Col2
Row1
Row2

🔹 Declaration​

DECLARE grid : ARRAY [1:3,1:2] OF INTEGER

This means:

  • 3 rows
  • 2 columns

🔹 Accessing a 2D Array​

grid[2,1]

This means:

  • Row 2
  • Column 1

5️⃣ Nested Iteration (Very Important)

To process a 2D array, you MUST use nested loops.

🔹 Writing Values into 2D Array​

FOR row ← 1 TO 3
FOR col ← 1 TO 2
INPUT grid[row,col]
NEXT col
NEXT row

🔹 Reading Values from 2D Array​

FOR row ← 1 TO 3
FOR col ← 1 TO 2
OUTPUT grid[row,col]
NEXT col
NEXT row

🔹 Totalling 2D Array​

total ← 0

FOR row ← 1 TO 3
FOR col ← 1 TO 2
total ← total + grid[row,col]
NEXT col
NEXT row

🧠 Exam Tip​

Outer loop = rows
Inner loop = columns

Indent properly to avoid losing marks.

6️⃣ Using Variables as Indexes

Instead of:

marks[1]
marks[2]

We use:

Marks[ i ]

This allows:

  • Efficient processing
  • Dynamic access
  • Fewer lines of code

7️⃣ Common Array Exam Questions

You may be asked to:

  1. Trace an array
  2. Fill missing loop code
  3. Write code to total/count values
  4. Identify errors
  5. Complete nested loops
  6. Identify index mistakes

8️⃣ Very Common Mistakes

❌ Going out of array bounds
❌ Forgetting to initialise total
❌ Using wrong index range
❌ Mixing up row and column order
❌ Not using nested loops for 2D
❌ Declaring wrong array size

9️⃣ Example Exam Question

Question:​

Store 5 temperatures in an array and output the highest value.

Answer:​

DECLARE temp : ARRAY[1:5] OF REAL
DECLARE highest : REAL

FOR i ← 1 TO 5
INPUT temp[ i ]
NEXT i

highest ← temp[1]

FOR i ← 2 TO 5
IF temp[ i ] > highest THEN
highest ← temp[ i ]
ENDIF
NEXT i

OUTPUT highest

🔟 Why Arrays Improve Programs

✔ Reduce repetition
✔ Make code cleaner
✔ Allow efficient processing
✔ Enable searching and sorting
✔ Essential for structured programming

⭐ Final Summary Table

TypeStructureLoops Needed
1DSingle listOne loop
2DTable (rows & columns)Nested loops

🎯 Final Exam Advice

✔ Always check starting index (0 or 1)
✔ Always use loop variables as indexes
✔ Carefully track row and column order
✔ Keep indentation neat
✔ Initialise totals and counters