An array stores many values of the same type under one name, reached by an index (position). Arrays are the foundation of searching, sorting and tables. This is a major ICSE Class 10 topic.
1Single dimensional arrays (SDA)
A single dimensional array is a row of values. Indices start at 0:
int[] marks = new int[5]; // 5 boxes: marks[0]..marks[4]
marks[0] = 92; // store
int x = marks[0]; // read
int[] a = {10, 20, 30}; // declare + initialise together
for (int i = 0; i < a.length; i++) // a.length is the size
System.out.println(a[i]);For an array of size n, the valid indices are 0 to n-1.
a.length gives the number of elements. Going outside this range causes an error.Key points
- An array stores many same-type values under one name, reached by an index starting at 0.
- For size n, valid indices are 0 to n-1; a.length gives the size.
- You can declare-and-initialise together: int[] a = {10, 20, 30};
2Searching: linear & binary
Searching looks for a value in an array (SDA):
| Linear search | Binary search | |
|---|---|---|
| How | Check each element in turn | Repeatedly halve a SORTED array |
| Array must be | Any order | Sorted |
| Speed | Slower for big arrays | Much faster |
// Linear search
for (int i = 0; i < a.length; i++)
if (a[i] == key) { found = i; break; }
// Binary search (a must be sorted)
int lo = 0, hi = a.length - 1;
while (lo <= hi) {
int mid = (lo + hi) / 2;
if (a[mid] == key) { found = mid; break; }
else if (a[mid] < key) lo = mid + 1;
else hi = mid - 1;
}
Key points
- Linear search checks each element in turn; it works on any order.
- Binary search repeatedly halves a SORTED array, so it's much faster.
- Binary search needs the array sorted first; linear search does not.
3Sorting: selection & bubble
Sorting arranges an SDA in order. Two classic methods:
- Selection sort — repeatedly find the smallest remaining element and place it at the front.
- Bubble sort — repeatedly compare neighbouring pairs and swap if out of order, so big values "bubble" to the end.
// Bubble sort (ascending)
for (int i = 0; i < n - 1; i++)
for (int j = 0; j < n - 1 - i; j++)
if (a[j] > a[j + 1]) {
int t = a[j]; a[j] = a[j + 1]; a[j + 1] = t; // swap
}Both use a nested loop and swap elements. Bubble sort compares adjacent pairs; selection sort selects the minimum each pass.
Key points
- Selection sort selects the smallest remaining element each pass and moves it to the front.
- Bubble sort compares adjacent pairs and swaps them, bubbling large values to the end.
- Both use nested loops and swap elements; swapping needs a temporary variable.
4Double dimensional arrays (DDA)
A double dimensional array is a grid (rows × columns) — like a table or matrix. You use two indices: a[row][col].
int[][] m = new int[3][3]; // 3 rows, 3 columns
m[0][0] = 5;
for (int i = 0; i < 3; i++)
for (int j = 0; j < 3; j++)
System.out.print(m[i][j] + " ");m.length gives the number of rows; m[0].length the number of columns. Common DDA tasks:
- Sum of a row — fix the row, loop the columns.
- Sum of a column — fix the column, loop the rows.
- Diagonal elements — where the row index equals the column index (
i == j).
Key points
- A double dimensional array is a grid; use a[row][col] with two indices.
- m.length is the number of rows; m[0].length is the number of columns.
- Diagonal elements are those where the row index equals the column index (i == j).
★ Practical: array programs
In BlueJ, write programs that:
- Read 5 numbers into an SDA and print the largest using a loop.
- Sort the array in ascending order using bubble sort.
- Search for a value with linear search and report its index.
- Fill a 3×3 DDA and print the sum of its diagonal elements (where i == j).
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