如何找到 Arrays 内部的差异值并记录差异的索引

Question

Happy New Year All!

But Sorry i want to ask a question, I have an array like this

[ 'even', 'even', 'odd', 'even', 'even' ]

I want the output will said 3, because the differences values inside of array is 'odd' and it laid in third index (the index is start from 1)

And also, when the question is changed into

[ 'odd', 'even', 'odd', 'odd', 'odd' ]

The output will get 2, because the differences values inside the array is 'even' and it laid in second index

Is there anyone could help me?

---

I have tried using javaScript method map, for loop, and other,... but i don't know, how to get the differences values of an array and print the indexed from it

Thanks

Answer 1

You can use findIndex on the main array and check if the element is unique by comparing its index and lastIndex

 const findIndexOfUnique = (arr) => arr.findIndex(x => arr.indexOf(x) === arr.lastIndexOf(x)) + 1 console.log(findIndexOfUnique([ 'even', 'even', 'odd', 'even', 'even' ]))

Heading:

indexOf method looks for an element from start to end. And lastIndexOf does the same from end to start. So if element is unique then both will result same otherwise they will be different.

findIndex() take a function as an argument and returns the index of the first element which satisfies the condition. So actually we are saying that. Find the first index of the element which matches the condition arr.indexOf(x) === arr.lastIndexOf(x) . Then we add 1 to get the required output

Answer 2

I can't comment yet but wanted to do so on Maheers answer, if you change all of the array entries to 'even' it returns 0 instead of -1, since you're adding one.

Best to update it:

function findIndexOfUnique(arr){
    let uniqueIndex = arr.findIndex(x => arr.indexOf(x) === arr.lastIndexOf(x));
    if (uniqueIndex >= 0) uniqueIndex++;
    return uniqueIndex;
}

console.log(findIndexOfUnique([ 'even', 'even', 'odd', 'even', 'even' ])) // 3
console.log(findIndexOfUnique([ 'even', 'even', 'even', 'even', 'even' ])) // -1

However, what if there are multiple unique entries? In this case it would simply return the first unique entry. Example:

[ 'even', 'even', 'even', 'odd', 'even', 'three', 'even' ]

Should it return [ 4, 6 ]?

function findUniqueIndexes(arr) {
  let uniqueIndexes = [];
  for (let index = 0; index < arr.length; index++) {
    if (index  === arr.lastIndexOf(arr[index]))
      uniqueIndexes.push(index + 1);
    }
  }
  return uniqueIndexes
}

And if you want to expand this further:

function findUniqueIndexes(arr) {
  let uniqueIndexes = [];
  for (let index = 0; index < arr.length; index++) {
    if (index  === arr.lastIndexOf(arr[index]))
      uniqueIndexes.push({"index" : index + 1, value: arr[index]});
    }
  }
  return uniqueIndexes
}

This last one would return:

[{ index: 4, value: 'odd' }, { index: 6, value: 'three' }]