For Loop Scoping with 2D Array

December 23, 2021

I am working on an Advent Code Challenge and have run into a hall. I solved this error before, but in this instance, I am stuck. The following code is giving me a Uncaught TypeError: Cannot read properties of undefined. in relation to switch (diagArray[i][j]).

My thought is that diagArray is out of scope. Is that true?

Please, any help is great!

const diagArray = [
  [0,1,0,0,0,1,1,1,0,0,0,1],
  [1,1,0,1,0,0,0,0,0,0,0,1],
  [1,1,1,0,0,1,0,0,1,0,1,1]
 ]; // example data
  
  for(let i = 0; i < diagArray.length; i++) {
  for (let j = 0; j < diagArray.length; i++) {
    let gammaRate = ''
    let epsilonRate = ''
    let ones = 0
    let zeros = 0
    let rates = []

    switch (diagArray[i][j]) {
      case 1:
        ones++
        break
      case 0:
        zeros++
        break
      default:
        break
    }

    if (ones > zeros) {
      gammaRate+=1
      epsilonRate+=0
    } else {
      gammaRate+=0
      epsilonRate+=1
    }

    rates.push([gammaRate, epsilonRate])
    // console.log(rates)
  }
}

>Solution :

for(let i = 0; i < diagArray.length; i++) {
  for (let j = 0; j < diagArray.length; i++) {

This here has multiple problems.

The first for loop is fine, but the second one needs be bounded by the length of the inner array:

diagArray[i].length

And it’s also currently incrementing the first loops counts with i++. That needs to be j++

Working example:

const diagArray = [
  [0,1,0,0,0,1,1,1,0,0,0,1],
  [1,1,0,1,0,0,0,0,0,0,0,1],
  [1,1,1,0,0,1,0,0,1,0,1,1]
 ]; // example data
  
for (let i = 0; i < diagArray.length; i++) {
  for (let j = 0; j < diagArray[i].length; j++) {
    let gammaRate = ''
    let epsilonRate = ''
    let ones = 0
    let zeros = 0
    let rates = []

    switch (diagArray[i][j]) {
      case 1:
        ones++
        break
      case 0:
        zeros++
        break
      default:
        break
    }

    if (ones > zeros) {
      gammaRate+=1
      epsilonRate+=0
    } else {
      gammaRate+=0
      epsilonRate+=1
    }

    rates.push([gammaRate, epsilonRate])
    console.log(rates)
  }
}

Also it’s usually not advisable to have loops like this for iterating arrays, since, as you’ve discovered, it’s very easy to screw up the indices.

I recommend interating over the values of the arrays instead:

for (const row of diagArray) {
  for (const value of row) {

const diagArray = [
  [0,1,0,0,0,1,1,1,0,0,0,1],
  [1,1,0,1,0,0,0,0,0,0,0,1],
  [1,1,1,0,0,1,0,0,1,0,1,1]
 ]; // example data
  
for (const row of diagArray) {
  for (const value of row) {
    let gammaRate = ''
    let epsilonRate = ''
    let ones = 0
    let zeros = 0
    let rates = []

    switch (value) {
      case 1:
        ones++
        break
      case 0:
        zeros++
        break
      default:
        break
    }

    if (ones > zeros) {
      gammaRate+=1
      epsilonRate+=0
    } else {
      gammaRate+=0
      epsilonRate+=1
    }

    rates.push([gammaRate, epsilonRate])
    console.log(rates)
  }
}