Javascript -- Manipulating values in multidimensional array while iterating

Question

I'm given a matrix, and for every index that has a value of 0, I have to make the index directly below it to 0 as well.

for(let i = 0; i < matrix.length; i++) {
    for(let j = 0; j < matrix[i].length;j++) {
        //figure out what rooms not 0
        if(matrix[i][j] !== 0) {
            goodRooms.push(matrix[i][j])             
        } else {
            matrix[i + 1][j] = 0;
        }
    }
}

I keep getting the

TypeError: Cannot set property '0' of undefined

At the else statement, and i'm not sure why.

Because matrix[i+1] is undefined and j =0, then you try to set the property (undefined)[0] to an undefined object/array matrix[i+1]. — Peter Paul Kiefer
– Peter Paul Kiefer, Commented Nov 8, 2019 at 16:07

ibrahim mahrir · Accepted Answer · 2019-11-08 16:21:14Z

1

Change else to

else if(i < matrix.length - 1)

Otherwise if you are on the last row and the value is 0, you'll be accessing the next row which doesn't exist.

The final code should look like:

for(let i = 0; i < matrix.length; i++) {
    for(let j = 0; j < matrix[i].length; j++) {
        if(matrix[i][j] !== 0) {
            goodRooms.push(matrix[i][j]);
        } else if(i < matrix.length - 1) {
            matrix[i + 1][j] = 0;
        }
    }
}

Note: This will change the next row before the loop reaches that row. So if the first row contains a 0 value, the second row will be changed before the loop accesses it and thus when the loop reaches it the value will already be 0 and so the whole column will recursively be set to 0.

edited Nov 8, 2019 at 16:21

answered Nov 8, 2019 at 16:01

ibrahim mahrir

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7 Comments

fev3r Over a year ago

I tried it and it set the whole matrix to [[0]], it isn't setting the value at index below to 0

fev3r Over a year ago

I figured it out.. Instead of using else if, I rearranged the code to have the first if check if (i < matrix.length -1 && matrix[i][j] === 0)

ibrahim mahrir Over a year ago

@fev3r Read the note part. If that is not your desired result then please tell me how exactly do you want to handle that case?

ibrahim mahrir Over a year ago

@fev3r The && matrix[i][j] === 0 is redundant. It is already implied by the else if itself

fev3r Over a year ago

Ah okay, I see now. It was an issue with a reduce function that I was working on below the code.

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