Two-dimensional Arrays
Array: a numbered list of data items all of the same type.
An array is an example of a data structure. To find the appropriate element in the data
structure, we need ONE piece of information: the subscript.
2-D arrays
Sometimes we need more than one array in a program. For example, if I were keeping
track of the test scores for all of the students in this class, I would need three lists. Each
row represents a student. Each column represents a test. The first number in each pair
below represents the number of the student, and the second number represents the number
of the test.
[0, 0]
[0, 1]
[0, 2]
[1, 0]
[1, 1]
[1, 2]
[2, 0]
[2, 1]
[2, 2]
[3, 0]
[3, 1]
[3, 2]
[4, 0]
[4, 1]
[4, 2]
[5, 0]
[5, 1]
[5, 2]
[6, 0]
[6, 1]
[6, 2]
[7, 0]
[7, 1]
[7, 2]
One way to look at this data is that it is three lists.
Another way to look at it is that it is one table. However, if we have a table, we can no
longer find what we are looking for simply by using a single subscript. Now we need two
pieces of information: a ROW and a COLUMN.
C# provides us with a way to do this in our programs by allowing two-dimensional arrays.
A 2-dimensional array has both rows and columns, but it has no visual representation—it is
just stored in memory.
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To declare a 2-dimensional array
<type> [,] <arrayname>;
Example:
Create a new console program called Day10--2DArrays. The following creates an array of
integers called scores with rows numbered from 0 to 19 and columns numbered from 0 to 2.
const int maxStudents = 20;
const int maxTests = 3;
// In Main:
int[,] score;
score = new int[maxStudents, maxTests];
InitializeScores(score, maxStudents, maxTests);
LookupScore(score, maxStudents, maxTests);
Initialize the array with some dummy data:
static void InitializeScores(int[,] score, int students, int tests)
{
for (int r = 0; r < students; r++)
for (int c = 0; c < tests; c++)
score[r, c] = (r + 1) * 10 + c + 1;
}
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To process a 2-dimensional array
With a 2-dimensional array, we can process the data in several ways:
(1)
(2)
(3)
(4)
Look up one element
Process a row
Process a column
Process the entire array
Looking up a single element in a 2D array
static void LookupScore(int[,] score, int students, int tests)
{
int r, c;
Console.Write("Enter student number (0-" + (students-1) + "): ");
r = Convert.ToInt32(Console.ReadLine());
Console.Write("Enter test number (0-" + (tests-1) + "): ");
c = Convert.ToInt32(Console.ReadLine());
Console.WriteLine(score[r, c]);
Console.ReadLine();
}
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Process a row in a 2D array
Assume you want to process row 3. The following picture represents the cells that must be
visited to process row 3.
0
1
2
0
1
2
3
4
5
6
7
8
9
The cells to visit are: (3, 0), (3, 1), (3, 2). Note that the row number remains constant
while the column number goes from 0 to 2 (the last column). This can be processed by a for
loop that sets the row number to 3 and takes the column number from 0 to 2.
static void DisplayStudent(int[,] score, int students, int tests)
{
int student;
Console.Write("Which student (0-" + (students - 1) + ")? ");
student = Convert.ToInt32(Console.ReadLine());
for (int c = 0; c < tests; c++)
Console.WriteLine(score[student, c]);
Console.Write("Press Enter to continue...");
Console.ReadLine();
}
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Process a column in a 2D array
Assume you want to process column 2. The following picture represents the cells that must
be visited to process column 2.
0
1
2
0
1
2
3
4
5
6
7
8
9
The cells to visit are (0, 2), (1, 2), (2, 2), … (9, 2). Note that the column number remains
constant while the row number goes from 0 to 9 (the last row). This can be processed by a
for loop that sets the column number to 2 and takes the row number from 0 to 9.
static void DisplayTest(int[,] score, int students, int tests)
{
int test;
Console.Write("Which test (0-" + (tests - 1) + ")? ");
test = Convert.ToInt32(Console.ReadLine());
for (int r = 0; r < students; r++)
Console.WriteLine(score[r, test]);
Console.Write("Press Enter to continue...");
Console.ReadLine();
}
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Process ALL the rows and columns in a 2D array
To process all of the elements in a 2D array, you want to do the following: process row 0,
process row 1, process row 2, … process row 9. The indexes are:
0
1
2
0
1
2
3
4
5
6
7
8
9
So the main algorithm looks something like this:
for (int row = 0; row < students; row++)
process row
And, to process a row (see the algorithm above):
for (int col = 0; col < tests; col++)
process item at column col in this row
So the algorithm to process all of the items in a 2D array looks like this:
for (int row = 0; row < students; row++)
for (int col = 0; col < tests; col++)
process item at position (row, col)
The code looks like this
static void ShowAllStudentsAndTests(int[,] score, int students, int tests)
{
for (int r = 0; r < students; r++)
{
for (int c = 0; c < tests; c++)
{
Console.Write(score[r, c].ToString().PadLeft(5));
}
Console.WriteLine();
}
Console.Write("Press Enter to continue...");
Console.ReadLine();
}
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