An efficient way to get the difference between two arrays of objects?

by 
// Make hashtable of ids in B
var bIds = {}
b.forEach(function(obj){
    bIds[obj.id] = obj;
});

// Return all elements in A, unless in B
return a.filter(function(obj){
    return !(obj.id in bIds);
});

very minor addendum: If the lists are very large and you wish to avoid the factor of 2 extra memory, you could store the objects in a hashmap in the first place instead of using lists, assuming the ids are unique: a = {20:{etc:...}, 15:{etc:...}, 10:{etc:...}, 17:{etc:...}, 23:{etc:...}}. I’d personally do this. Alternatively: Secondly, javascript sorts lists in-place so it doesn’t use more memory. e.g. a.sort((x,y)=>x.id-y.id) Sorting would be worse than the above because it’s O(N log(N)). But if you had to sort it anyway, there is an O(N) algorithm that involves two sorted lists: namely, you consider both lists together, and repeatedly take the leftmost (smallest) element from the lists (that is examine, then increment a pointer/bookmark from the list you took). This is just like merge sort but with a little bit more care to find identical items… and maybe pesky to code. Thirdly, if the lists are legacy code and you want to convert it to a hashmap without memory overhead, you can also do so element-by-element by repeatedly popping the elements off of the lists and into hashmaps.

More Related Contents:

  1. JavaScript code to fetch range of dates from array of objects depending on one attribute of the array using array.reduce()
  2. For-each over an array in JavaScript
  3. Checking if a key exists in a JavaScript object?
  4. How do I check if a variable is an array in JavaScript?
  5. How to find the sum of an array of numbers
  6. Why is [1,2] + [3,4] = “1,23,4” in JavaScript?
  7. Firestore Update single item in an array field
  8. Why does typeof array with objects return “object” and not “array”? [duplicate]
  9. How to initialize an array’s length in JavaScript?
  10. How can I access and process nested objects, arrays or JSON?
  11. Better way to sum a property value in an array
  12. Remove last item from array
  13. Filter array of objects with another array of objects
  14. javascript extract certain properties from all objects in array
  15. Is a JavaScript array index a string or an integer?
  16. Javascript sort function. Sort by First then by Second
  17. Convert javascript object or array to json for ajax data
  18. Obtain smallest value from array in Javascript?
  19. No semicolon before [] is causing error in JavaScript
  20. How do I remove objects from a JavaScript associative array?
  21. Filter strings in Array based on content (filter search value)
  22. How to merge multiple array of object by ID in javascript?
  23. How to convert an array of key-value tuples into an object
  24. Extract certain properties from all objects in array
  25. Checking for duplicate strings in JavaScript array
  26. Sort an array to have specific items first in the array
  27. Sort an array of objects by multiple properties
  28. Using setInterval() with a .map method in array and returning as a promise
  29. How to create an array of object literals in a loop?
  30. Lodash create collection from duplicate object keys

Leave a Comment