i not defined in while loop

Question

I'm trying to add all the numbers for matching arrays and remove the duplicate names. It works for the first instance, but the while loop won't go past Apples.

function updateInventory(arr1, arr2) {

   function alphabetizer(a, b) {
   if (a[1] < b[1]) return -1;
   if (a[1] > b[1]) return 1;
   return 0;
   }

  var newInv = arr1.concat(arr2).sort(alphabetizer);


  for(var i = 0; i < newInv.length; i++) {
    while(newInv[i][1] === newInv[i++][1]) {
      newInv[i] += newInv[i++][0];
      newInv.push([newInv[i][0], newInv[i][1]]);
      newInv.splice(i,2);


   }
  } 

    return newInv;
}

// Example inventory lists
var curInv = [
    [21, "Bowling Ball"],
    [2, "Dirty Sock"],
    [1, "Hair Pin"],
    [5, "Microphone"],
    [10, "Apples"]
];

var newInv = [
    [9, "Apples"],
    [2, "Hair Pin"],
    [3, "Half-Eaten Apple"],
    [67, "Bowling Ball"],
    [7, "Toothpaste"]
];

updateInventory(curInv, newInv);

It keeps giving me the error TypeError: newInv[(+ i)] is undefined But I am not sure why since i is defined, and if I just try to run newInv[i] I get the first result.

JS Fiddle link

You are incrementing `i` two times in the while loop by doing `i++`, you probably don't want to do that. You probably want `i + 1` instead — Get Off My Lawn, Nov 22 '17 at 23:17
it's not `i` that is undefined - it's `newInv[i + 1]` - because at the end of the loop i == netInv.length - 1, and i + 1 (newInv[newInv.length]) references an index that doesn't exist in the array ... hence, undefined — Jaromanda X, Nov 22 '17 at 23:23
An extra info: https://stackoverflow.com/questions/971312/why-avoid-increment-and-decrement-operators-in-javascript — Victor, Nov 22 '17 at 23:24

score 1 · Accepted Answer · answered Nov 23 '17 at 00:06

There're couple places need fix. I updated this jsfiddle

function updateInventory(arr1, arr2) {

    function alphabetizer(a, b) {
        if (a[1] < b[1]) return -1;
        if (a[1] > b[1]) return 1;

        return 0;    
    }

    var newInv = arr1.concat(arr2).sort(alphabetizer);

    for (var i = 0; i < newInv.length; i++) {
        var j = i;
        while (newInv[j + 1] && newInv[j][1] == newInv[j + 1][1]) {
            newInv[j][0] += newInv[j + 1][0];
            newInv.splice(j + 1, 1);
            j++;
        }
    }

    document.getElementById('test').innerHTML = newInv.join('<br>');
}

// Example inventory lists
var curInv = [
    [21, "Bowling Ball"],
    [2, "Dirty Sock"],
    [1, "Hair Pin"],
    [5, "Microphone"],
    [10, "Apples"]
];

var newInv = [
    [9, "Apples"],
    [2, "Hair Pin"],
    [3, "Half-Eaten Apple"],
    [67, "Bowling Ball"],
    [7, "Toothpaste"]
];

updateInventory(curInv, newInv);

https://jsfiddle.net/c3j8p2zu/7/

You're increasing `i` and removing items at same time, which can skip some items in the array, `j` is for recording the old pointer position. And the main issue in the old code is `i++` could be bigger than the length of the array, hence `newInv[i++]` is undefined. — Hang Ye, Nov 29 '17 at 23:28

i not defined in while loop

1 Answers1