Search in 2D Matrix in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of search in sorted 2D matrix

What is the output of the following JavaScript code that searches for the number 5 in a sorted 2D matrix?

DSA Javascript

const matrix = [
  [1, 3, 5],
  [7, 9, 11],
  [13, 15, 17]
];

function searchMatrix(matrix, target) {
  let row = 0;
  let col = matrix[0].length - 1;
  while (row < matrix.length && col >= 0) {
    if (matrix[row][col] === target) {
      return [row, col];
    } else if (matrix[row][col] > target) {
      col--;
    } else {
      row++;
    }
  }
  return [-1, -1];
}

console.log(searchMatrix(matrix, 5));

A [0, 2]

B [2, 0]

C [-1, -1]

D [1, 0]

💡 Hint

Trace the search starting from the top-right corner of the matrix.

The search starts at the top-right element (0,2) which is 5. Since it matches the target, the function returns [0, 2].

❓ Predict Output

intermediate

Result of searching for a missing element

What does the following code print when searching for 8 in the given matrix?

DSA Javascript

const matrix = [
  [2, 4, 6],
  [8, 10, 12],
  [14, 16, 18]
];

function searchMatrix(matrix, target) {
  let row = 0;
  let col = matrix[0].length - 1;
  while (row < matrix.length && col >= 0) {
    if (matrix[row][col] === target) {
      return [row, col];
    } else if (matrix[row][col] > target) {
      col--;
    } else {
      row++;
    }
  }
  return [-1, -1];
}

console.log(searchMatrix(matrix, 8));

A [-1, -1]

B [0, 2]

C [1, 0]

D [2, 1]

💡 Hint

Check the matrix elements row by row and column by column as per the search logic.

The search finds 8 at position (1, 0) in the matrix and returns [1, 0].

❓ Predict Output

advanced

Output of search in matrix with duplicates

What is the output of this code searching for 3 in a matrix containing duplicates?

DSA Javascript

const matrix = [
  [1, 2, 3],
  [3, 4, 5],
  [5, 6, 7]
];

function searchMatrix(matrix, target) {
  let row = 0;
  let col = matrix[0].length - 1;
  while (row < matrix.length && col >= 0) {
    if (matrix[row][col] === target) {
      return [row, col];
    } else if (matrix[row][col] > target) {
      col--;
    } else {
      row++;
    }
  }
  return [-1, -1];
}

console.log(searchMatrix(matrix, 3));

A [0, 2]

B [1, 0]

C [-1, -1]

D [2, 0]

💡 Hint

The search stops at the first found occurrence from top-right.

The search starts at (0,2) which is 3, so it returns [0, 2] immediately.

🧠 Conceptual

advanced

Why start search from top-right corner?

Why is the top-right corner chosen as the starting point for searching a target in a sorted 2D matrix where each row and column is sorted ascending?

A Because starting from top-right guarantees the target is found in the first step.

B Because top-right is always the smallest element in the matrix.

C Because top-right corner is the middle element of the matrix.

D Because from top-right, moving left decreases values and moving down increases values, allowing efficient elimination.

💡 Hint

Think about how values change when moving left or down from the top-right.

Starting at top-right allows moving left to smaller values and down to larger values, efficiently narrowing the search space.

🔧 Debug

expert

Identify the error in this 2D matrix search code

What error will this code produce when searching for 10 in the matrix? Identify the exact error type.

DSA Javascript

const matrix = [
  [1, 4, 7],
  [8, 10, 12],
  [14, 16, 18]
];

function searchMatrix(matrix, target) {
  let row = 0;
  let col = matrix[0].length; // Note the difference here
  while (row < matrix.length && col >= 0) {
    if (matrix[row][col] === target) {
      return [row, col];
    } else if (matrix[row][col] > target) {
      col--;
    } else {
      row++;
    }
  }
  return [-1, -1];
}

console.log(searchMatrix(matrix, 10));

A RangeError: Maximum call stack size exceeded

B TypeError: Cannot read property 'undefined' of undefined

C ReferenceError: col is not defined

D TypeError: Cannot read property 'length' of undefined

💡 Hint

Check the initial value of 'col' and how it is used as an index.

Setting col = matrix[0].length sets col to 3, which is out of bounds for indices 0-2. Accessing matrix[row][3] is undefined, causing a TypeError when trying to compare or access properties.