Set Matrix Zeroes In Place in DSA Python - Execution Trace

Concept Flow - Set Matrix Zeroes In Place

Start: Given matrix

↓

Scan matrix for zeros

↓

Mark rows and columns to zero

↓

Update matrix cells to zero

↓

Done: Matrix updated in place

We scan the matrix to find zeros, mark which rows and columns to zero, then update the matrix in place.

Execution Sample

DSA Python

matrix = [
  [1,1,1],
  [1,0,1],
  [1,1,1]
]
# Set zeros in place

This code sets entire row and column to zero if any cell is zero, modifying the matrix directly.

Execution Table

Step	Operation	Nodes in Matrix	Pointer Changes	Visual State
1	Scan matrix for zeros	matrix[1][1] == 0 found	Mark row 1 and column 1 for zeroing	[[1,1,1], [1,0,1], [1,1,1]]
2	Mark rows and columns to zero	Rows to zero: {1}, Columns to zero: {1}	No pointer change	[[1,1,1], [1,0,1], [1,1,1]]
3	Update matrix cells to zero	Set matrix[1][0], matrix[1][1], matrix[1][2] = 0 Set matrix[0][1], matrix[1][1], matrix[2][1] = 0	No pointer change	[[1,0,1], [0,0,0], [1,0,1]]
4	Done	Matrix updated in place	No pointer change	[[1,0,1], [0,0,0], [1,0,1]]

💡 All rows and columns with zeros are updated; matrix is modified in place.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
matrix	[[1,1,1],[1,0,1],[1,1,1]]	[[1,1,1],[1,0,1],[1,1,1]]	[[1,1,1],[1,0,1],[1,1,1]]	[[1,0,1],[0,0,0],[1,0,1]]	[[1,0,1],[0,0,0],[1,0,1]]
rows_to_zero	{}	{1}	{1}	{1}	{1}
cols_to_zero	{}	{1}	{1}	{1}	{1}

Key Moments - 3 Insights

Why do we mark rows and columns first instead of setting zeros immediately?

If we set zeros immediately, new zeros would affect other rows and columns incorrectly. Marking first (see Step 2 in execution_table) ensures only original zeros cause changes.

How do we update the matrix in place without extra space for a copy?

We use sets to remember which rows and columns to zero, then update the matrix directly (Step 3). This avoids making a full copy.

What if the matrix has multiple zeros in different rows and columns?

All rows and columns with zeros are marked (Step 1 and 2). Then all those rows and columns are zeroed together (Step 3), ensuring correctness.

Visual Quiz - 3 Questions

Look at the execution_table at Step 1, which cell is found to be zero?

A matrix[0][0]

B matrix[2][2]

C matrix[1][1]

D matrix[0][2]

💡 Hint

Check the 'Nodes in Matrix' column in Step 1 of execution_table.

At which step does the matrix get updated with zeros?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Operation' and 'Visual State' columns in execution_table.

If there was a zero at matrix[0][2], which rows and columns would be marked after Step 2?

A Rows {0,1}, Columns {2}

B Rows {0,1}, Columns {1,2}

C Rows {0}, Columns {2}

D Rows {1}, Columns {1}

💡 Hint

Mark rows and columns for all zeros found; see variable_tracker for rows_to_zero and cols_to_zero.

Concept Snapshot

Set Matrix Zeroes In Place:
- Scan matrix to find zeros
- Mark rows and columns to zero
- Update matrix cells in place
- Avoid immediate zeroing to prevent cascading
- Use sets to track rows and columns
- Modify matrix directly without extra matrix copy

Full Transcript

We start with a matrix. We scan it to find cells with zero. When we find a zero, we mark its entire row and column to be zeroed later. We do not change the matrix immediately to avoid affecting other cells incorrectly. After scanning, we update all marked rows and columns by setting their cells to zero. This changes the matrix in place without extra space. The process ensures all rows and columns containing zeros are zeroed correctly.