Extend zeroes in a matrix

Question

I built the Set Matrix Zeroes algorithm in JavaScript. I have a lot of loops in my code, so was wondering if there is a better way to implement this algorithm.

Given a m x n matrix, if an element is 0, set its entire row and column to 0. Do it in-place.

I loop through the matrix and when I find a 0, I set its entire row and column to a flag ('X'). I do not set 'X' if the position is a 0 because I might need to check this position in the future. At the end I replace all the 'X' to 0.

var setZeroes = function(matrix) {
 if(matrix === null || matrix.length === 0)
 return [];
 
 for(let i=0; i<matrix.length; i++){
 for(let j=0; j<matrix[i].length; j++){
 if(matrix[i][j] === 0){
 setFlag(matrix, i, j);
 }
 }
 }
 
 for(i=0; i<matrix.length; i++){
 for(j=0; j<matrix[i].length; j++){
 if(matrix[i][j] === 'X')
 matrix[i][j] = 0;
 }
 }
};
const setFlag = function(matrix, i, j) {
 matrix[i][j] = 'X';
 let tempI = i+1;
 while(tempI < matrix.length){
 if(matrix[tempI][j] !== 0)
 matrix[tempI][j] = 'X';
 tempI++;
 }
 tempI = i-1;
 while(tempI >= 0){
 if(matrix[tempI][j] !== 0)
 matrix[tempI][j] = 'X';
 tempI--;
 }
 let tempJ = j+1;
 while(tempJ < matrix[i].length){
 if(matrix[i][tempJ] !== 0)
 matrix[i][tempJ] = 'X';
 tempJ++;
 }
 tempJ = j-1;
 while(tempJ >= 0){
 if(matrix[i][tempJ] !== 0)
 matrix[i][tempJ] = 'X';
 tempJ--;
 }
}
```

Answer 1

From a short review;

• You probably want to encapsulate if(matrix === null || matrix.length === 0) into an isValidMatrix function for re-use
• I would drop the whole i/j thing and go for variables named after row and col
• I would collect all the rows and columns with a 0 in one set of loops, and then clear the matrix based on the collected rows and columns in another set of loops.
• let only declares in the block, so the for(i=0; i<matrix.length; i++){ creates a global i
• I much prefer function setFlag(matrix, i, j) { over const setFlag = function(matrix, i, j) {

This is my current proposal, I did not test it, but you should get the gist of it;

//This does not catch everything, but it'll do
function isValidMatrix(matrix){
 return !(!matrix || !matrix.length);
}
function setZeroes(matrix) {
 
 if(!isValidMatrix(matrix))
 return [];
 
 const rows = [], cols = [];
 
 for(let row = 0; row < matrix.length; row++){
 for(let col = 0; col < matrix[row].length; col++){
 if(matrix[row][col] === 0){
 rows.push(row);
 cols.push(col);
 }
 }
 }
 
 for(let row = 0; row < matrix.length; row++){
 for(let col = 0; col < matrix[row].length; col++){
 if(rows.includes(row) || cols.includes(col)){
 matrix[row][col] = 0;
 }
 }
 }
}

For further refinement, rows and cols might/will get multiple times the same value, so to deal with that gracefully you should use Sets for that.