How to locate the duplicate elements within an array using JavaScript

Question

How to locate the duplicate elements within an array using JavaScript

There are two different sets of numbers

Numbers1=[ 0, 1, 2, 0, 2 ];

Numbers2=[ 0, 0, 1, 2, 2 ];

The goal is to determine the index of each element in Numbers2 from Numbers1 and create a new array like [0,3,1,2,4];

If you would like to see the code I wrote, check it out here. However, please note that it currently does not account for duplicate values in the arrays.

var index = [];
for (i = 0; i <= Numbers2.length - 1; i++) {
  index.push(Numbers1.indexOf(Numbers2[i]));
}

javascript arrays

Answer 1

Answer №1

To find the second element and all repeating elements after the first index, use the fromIndex argument in the Array#indexOf method to specify the starting index for the search.

// Use a reference object to set the from index for duplicates
var ref = {};

var index = [];

for (i = 0; i < arr2.length; i++) {
  // Set the from index using the reference object, if not found, set as 0
  var i1 = arr1.indexOf(arr2[i], ref[arr2[i]] || 0);

  // Push the index into the array
  index.push(i1);

  // Update the from index for the current element
  ref[arr2[i]] = i1 + 1;
}

var ref = {};

var arr1 = [0, 1, 2, 0, 2],
  arr2 = [0, 0, 1, 2, 2];

var ref = {};

var index = [];

for (i = 0; i < arr2.length; i++) {
  var i1 = arr1.indexOf(arr2[i], ref[arr2[i]] || 0);
  index.push(i1);
  ref[arr2[i]] = i1 + 1;
}

console.log(index);

Use the Array#map method to generate the index array.

var index = arr2.map(function(v, i) {
  // Find the index of the element, starting the search after a specific index for repeat occurrences
  var i1 = arr1.indexOf(v, this[v] || 0);

  // Update the reference index 
  this[v] = i1 + 1;

  // Return the index value
  return i1;
  // Set this argument as an object for reference to from index
}, {});

var arr1 = [0, 1, 2, 0, 2],
  arr2 = [0, 0, 1, 2, 2];

var index = arr2.map(function(v, i) {
  var i1 = arr1.indexOf(v, this[v] || 0);
  this[v] = i1 + 1;
  return i1;
}, {});

console.log(index);

Answer 2

To find the second element and all repeating elements after the first index, use the fromIndex argument in the Array#indexOf method to specify the starting index for the search.

// Use a reference object to set the from index for duplicates
var ref = {};

var index = [];

for (i = 0; i < arr2.length; i++) {
  // Set the from index using the reference object, if not found, set as 0
  var i1 = arr1.indexOf(arr2[i], ref[arr2[i]] || 0);

  // Push the index into the array
  index.push(i1);

  // Update the from index for the current element
  ref[arr2[i]] = i1 + 1;
}

var ref = {};

var arr1 = [0, 1, 2, 0, 2],
  arr2 = [0, 0, 1, 2, 2];

var ref = {};

var index = [];

for (i = 0; i < arr2.length; i++) {
  var i1 = arr1.indexOf(arr2[i], ref[arr2[i]] || 0);
  index.push(i1);
  ref[arr2[i]] = i1 + 1;
}

console.log(index);

Use the Array#map method to generate the index array.

var index = arr2.map(function(v, i) {
  // Find the index of the element, starting the search after a specific index for repeat occurrences
  var i1 = arr1.indexOf(v, this[v] || 0);

  // Update the reference index 
  this[v] = i1 + 1;

  // Return the index value
  return i1;
  // Set this argument as an object for reference to from index
}, {});

var arr1 = [0, 1, 2, 0, 2],
  arr2 = [0, 0, 1, 2, 2];

var index = arr2.map(function(v, i) {
  var i1 = arr1.indexOf(v, this[v] || 0);
  this[v] = i1 + 1;
  return i1;
}, {});

console.log(index);

Answer 3

Answer №2

Here is a JavaScript code snippet that compares two arrays and returns the indexes of common elements:

var arr1 = [ 0, 1, 2, 0, 2 ];
var arr2 = [ 0, 0, 1, 2, 2 ]
var index = [];
var hash = {};
for (i = 0; i < arr2.length; i++) {
  var ind_temp;
  if(arr2[i] in hash){
    //console.log("here");
    ind_temp = arr1.indexOf(arr2[i],hash[arr2[i]] + 1);
    index.push(ind_temp);
    hash[arr2[i]] = ind_temp;
  }
  else{
    ind_temp = arr1.indexOf(arr2[i]);
    index.push(ind_temp);
    hash[arr2[i]] = ind_temp;
  }

console.log(index);

Answer 4

Here is a JavaScript code snippet that compares two arrays and returns the indexes of common elements:

var arr1 = [ 0, 1, 2, 0, 2 ];
var arr2 = [ 0, 0, 1, 2, 2 ]
var index = [];
var hash = {};
for (i = 0; i < arr2.length; i++) {
  var ind_temp;
  if(arr2[i] in hash){
    //console.log("here");
    ind_temp = arr1.indexOf(arr2[i],hash[arr2[i]] + 1);
    index.push(ind_temp);
    hash[arr2[i]] = ind_temp;
  }
  else{
    ind_temp = arr1.indexOf(arr2[i]);
    index.push(ind_temp);
    hash[arr2[i]] = ind_temp;
  }

console.log(index);

Answer 5

Answer №3

To search through an array and match it with another array, you can use the map function while ensuring the first occurrence is set to undefined. Keep in mind that this method may not be effective if undefined is a value you need to locate.

let updatedArray = originalArray.map(item => {
  if (!item) return undefined;
  let index = secondArray.indexOf(item);
  if (index) secondArray[index] = undefined;
  return index;
})

Answer 6

To search through an array and match it with another array, you can use the map function while ensuring the first occurrence is set to undefined. Keep in mind that this method may not be effective if undefined is a value you need to locate.

let updatedArray = originalArray.map(item => {
  if (!item) return undefined;
  let index = secondArray.indexOf(item);
  if (index) secondArray[index] = undefined;
  return index;
})

Answer 7

Answer №4

Here's a helpful solution for dealing with arrays containing only positive numbers:

let copy = originalArr.slice(0); // Create a copy of the original array

let indices = [];

additionalArr.forEach(element => {
    let idx = copy.indexOf(element);
    indices.push(idx);
    if (idx > -1) {
        copy[idx] = -1;
    }
});

console.log(indices);

Answer 8

Here's a helpful solution for dealing with arrays containing only positive numbers:

let copy = originalArr.slice(0); // Create a copy of the original array

let indices = [];

additionalArr.forEach(element => {
    let idx = copy.indexOf(element);
    indices.push(idx);
    if (idx > -1) {
        copy[idx] = -1;
    }
});

console.log(indices);

Answer 9

Answer №5

To achieve this, iterate through arr2 and store the index found in arr1 in a variable. If an element in arr2 matches the previous element, compare it starting from the saved index + 1 using the second parameter of the indexOf method.

var duplicate = [0, 1, 2, 0, 2];
var newiter = [0, 0, 1, 2, 2];
var indexArray = []; // STORED INDEX ARRAY

var newiterSorted = newiter.sort(); // ENSURE NEWITER IS SORTED

var i = -1;
for (var j = 0; j < newiterSorted.length; j++) {

    // Check if current element is different from previous, reset i to -1
    if (j > 0 && newiterSorted[j] != newiterSorted[j - 1]) {
        i = -1;
    }

    // Find index in 'duplicate' array starting from i+1
    i = duplicate.indexOf(newiterSorted[j], i + 1);
    indexArray.push(i);
}

console.log(indexArray);

Answer 10

To achieve this, iterate through arr2 and store the index found in arr1 in a variable. If an element in arr2 matches the previous element, compare it starting from the saved index + 1 using the second parameter of the indexOf method.

var duplicate = [0, 1, 2, 0, 2];
var newiter = [0, 0, 1, 2, 2];
var indexArray = []; // STORED INDEX ARRAY

var newiterSorted = newiter.sort(); // ENSURE NEWITER IS SORTED

var i = -1;
for (var j = 0; j < newiterSorted.length; j++) {

    // Check if current element is different from previous, reset i to -1
    if (j > 0 && newiterSorted[j] != newiterSorted[j - 1]) {
        i = -1;
    }

    // Find index in 'duplicate' array starting from i+1
    i = duplicate.indexOf(newiterSorted[j], i + 1);
    indexArray.push(i);
}

console.log(indexArray);

How to locate the duplicate elements within an array using JavaScript

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

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