javascript - missing array

Question

I need to write a JS function that creates an object from a given string. String characters are object keys. Repeated characters should not be included twice. Values of all keys are zero.

Example: input objFromStr("aaabc") should return {a: 0, b: 0, c: 0}

Here is my solution:

function objFromStr(inStr) {
  let charsArr = inStr.split("");

  let charsObj = {};
  var alreadyInArr = [];

  for (var i = 0; i < charsArr.length; i++) {
    if (!(alreadyInArr.includes(charsArr[i]))) {
      charsObj[charsArr[i]] = 0;
    } else {
      alreadyInArr.push(charsArr[i]);
    }
  }

  return charsObj;
}

This solution works as expected, but I don't understand why. I check chars duplication using alreadyInArr. However when I log my alreadyInArr, it's empty.

So after running this code:

function objFromStr(inStr) {
  let charsArr = inStr.split("");

  console.log("charsArr is:", charsArr);

  let charsObj = {};
  var alreadyInArr = [];

  for (var i = 0; i < charsArr.length; i++) {
    if (!(alreadyInArr.includes(charsArr[i]))) {
      charsObj[charsArr[i]] = 0;
    } else {
      alreadyInArr.push(charsArr[i]);
    }
  }

  console.log("alreadyInArr is:", alreadyInArr);

  return charsObj;
}

console.log(objFromStr("aaabc"));

My output is:

charsArr is: [ 'a', 'a', 'a', 'b', 'c' ]
alreadyInArr is: []
{ a: 0, b: 0, c: 0 }

Can someone explain, why alreadyInArr is empty and yet function still works as expected?

Here's a pen: https://codepen.io/t411tocreate/pen/bYReRj?editors=0012

You only add something to alreadyInArr if an item exists in alreadyInArr, but alreadyInArr is empty. — James
– James, Commented Nov 13, 2017 at 17:10
And why does it work? becuase you just overwrite the values.... — epascarello
– epascarello, Commented Nov 13, 2017 at 17:11
You can't define the same key more than once on an object. Your code can repeatedly set obj['a'] = 0, the result obj will still have one key named 'a', where a: 0. — IrkenInvader
– IrkenInvader, Commented Nov 13, 2017 at 17:12

Zakaria Acharki · Accepted Answer · 2017-11-13 17:18:52Z

3

Your else clause isn't reachable at all since you didn't pushed the element in the right place, you could add a console.log() inside the loop to see that the array still empty through all the iterations.

Instead, the element should be pushed in the if condition like :

for (var i = 0; i < charsArr.length; i++) {
    if (!alreadyInArr.includes(charsArr[i])) {
        charsObj[charsArr[i]] = 0;
        alreadyInArr.push(charsArr[i]);
    }
}

Hope this helps.

function objFromStr(inStr) {
  let charsArr = inStr.split("");

  console.log("charsArr is:", charsArr);

  let charsObj = {};
  var alreadyInArr = [];

  for (var i = 0; i < charsArr.length; i++) {
    if (!alreadyInArr.includes(charsArr[i])) {
      charsObj[charsArr[i]] = 0;
      alreadyInArr.push(charsArr[i]);
    }
  }

  console.log("alreadyInArr is:", alreadyInArr);

  return charsObj;
}

console.log(objFromStr("aaabc"));

edited Nov 13, 2017 at 17:18

answered Nov 13, 2017 at 17:11

Zakaria Acharki

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Comments

Jonas Wilms · Accepted Answer · 2017-11-13 17:14:28Z

3

You can shortify your code a bit:

const string = "aabbcc", result = {};

for(const char of string)
  result[char] = 0;

answered Nov 13, 2017 at 17:14

Jonas Wilms

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Comments

SLaks · Accepted Answer · 2017-11-13 17:10:13Z

2

Your else clause means you only push to that array if includes() returns true.

Therefore, you never actually push anything.

answered Nov 13, 2017 at 17:10

SLaks

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Comments

Pandelis · Accepted Answer · 2017-11-13 17:15:44Z

2

 if (!(alreadyInArr.includes(charsArr[i]))) {
      charsObj[charsArr[i]] = 0;
    } else {
      alreadyInArr.push(charsArr[i]);
    }

Your call to alreadyInArr.includes has a ! in front. This reads as so: If NOT(alreadyInarr has a letter CharrsArr[i] within it) then continue:

Which then becomes if the letter is not already in the array then do this.

Since the array is always empty because you don't push anything to it it will always be true and will never step into the else portion

answered Nov 13, 2017 at 17:15

Pandelis

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Nina Scholz · Accepted Answer · 2017-11-13 17:16:10Z

2

You could spread the carcters and map the objects with the wanted properties.

function objFromStr(inStr) {
    return Object.assign(...Array.from(inStr, k => ({ [k]: 0 })));
}

console.log(objFromStr('aaaabbc'));

answered Nov 13, 2017 at 17:16

Nina Scholz

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1 Comment

ktilcu Over a year ago

I love the solution but it doesn't answer the question. ;P

Matt · Accepted Answer · 2017-11-13 18:20:27Z

1

As others have already stated, you are not allowing access to push to the array so you are constantly overwriting your object and assigning 0.

You can cut down your code with the following:

var arr = "aabbc", obj = {};

function objFromStr(inStr) {        
    arr.split("").forEach(function(x){
        obj[x] !== undefined ? obj[x]++ : obj[x] = 1;
    });     
    return obj;
}

console.log(objFromStr(arr));

By doing it this way you don't have to push or track anything in a separate array. It will check the obj to see if the key already exists. If it does it will add 1 to it and if not then it will count it as 1.

answered Nov 13, 2017 at 18:20

Matt

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