Two-dimensional arrays in Java - Step-by-Step Execution

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Concept Flow - Two-dimensional arrays

Declare 2D array

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Allocate memory for rows

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Allocate memory for columns in each row

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Access or modify element at [row

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Use nested loops to traverse rows and columns

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Process or print elements

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End

This flow shows how a two-dimensional array is declared, allocated, accessed, and traversed using nested loops.

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Execution Sample

Java

int[][] matrix = new int[2][3];
matrix[0][1] = 5;
for(int i=0; i<2; i++) {
  for(int j=0; j<3; j++) {
    System.out.print(matrix[i][j] + " ");
  }
  System.out.println();
}

This code creates a 2x3 integer array, sets one element, and prints all elements row by row.

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Execution Table

Step	Action	Indices (i,j)	Value Assigned/Accessed	Output	Notes
1	Declare 2D array	-	-	-	matrix created with 2 rows and 3 columns, all initialized to 0
2	Assign value	[0,1]	5	-	matrix[0][1] set to 5
3	Start outer loop	i=0	-	-	Begin first row
4	Start inner loop	i=0, j=0	-	0	Print matrix[0][0], default 0
5	Inner loop	i=0, j=1	-	5	Print matrix[0][1], assigned 5
6	Inner loop	i=0, j=2	-	0	Print matrix[0][2], default 0
7	End inner loop	i=0	-	Line break	End of first row print
8	Outer loop	i=1	-	-	Begin second row
9	Inner loop	i=1, j=0	-	0	Print matrix[1][0], default 0
10	Inner loop	i=1, j=1	-	0	Print matrix[1][1], default 0
11	Inner loop	i=1, j=2	-	0	Print matrix[1][2], default 0
12	End inner loop	i=1	-	Line break	End of second row print
13	End outer loop	-	-	-	All rows printed, loops end

💡 Outer loop i=2 is not less than 2, so loop ends

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Variable Tracker

Variable	Start	After Step 2	After Step 5	After Step 11	Final
matrix[0][0]	0	0	0	0	0
matrix[0][1]	0	5	5	5	5
matrix[0][2]	0	0	0	0	0
matrix[1][0]	0	0	0	0	0
matrix[1][1]	0	0	0	0	0
matrix[1][2]	0	0	0	0	0
i	-	0	0	1	2
j	-	-	2	2	-

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Key Moments - 3 Insights

▶Why do we need two loops to print a 2D array?

▶What happens if we try to access matrix[2][0]?

▶Why is matrix[0][1] 5 but others are 0?

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Visual Quiz - 3 Questions

Look at the execution_table at step 5, what value is printed for matrix[0][1]?

A 0

B 1

C 5

D Error

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Concept Snapshot

Two-dimensional arrays in Java are arrays of arrays.
Declare with: int[][] name = new int[rows][cols];
Access elements with name[row][col].
Use nested loops to traverse rows and columns.
Default int values are 0.
Indexing starts at 0 for both rows and columns.

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Full Transcript

This lesson shows how to work with two-dimensional arrays in Java. First, you declare and allocate the array with rows and columns. Then you can assign values to specific positions using two indices. To read or print all elements, you use nested loops: the outer loop goes through each row, and the inner loop goes through each column in that row. The example code creates a 2x3 array, sets one element to 5, and prints all elements. The execution table traces each step, showing how loops run and values are accessed or printed. The variable tracker shows how each element changes or stays the same. Key moments clarify why nested loops are needed, what happens if you access out-of-bounds indices, and why only one element is changed. The quiz tests understanding of values printed, loop variables, and array size effects. Remember, two-dimensional arrays are like tables with rows and columns, and you always use two indices to work with them.