Find Missing Values in Array of Objects

Question

I have a json array that has multiple missing numbers and is out of sequence. What is the best way to find which numbers are missing?

My First thought was to iterate through and construct a new temporary array in order (so if the first key is 50, it goes to arr[50]) and then find out which do not have a key. Unfortunately this seems extremely inefficient.

Update: Here's a bit of my json:

"groups": [
 {
 "group_id": "1",
 "group_name": "AABYODAADAAAW6KAAA",
 },
 {
 "group_id": "5",
 "group_name": "AABYODAADAAAW6KAAB",
 },
 {
 "group_id": "2",
 "group_name": "AABYODAADAAAW6KAAC",
 },
 {
 "group_id": "3",
 "group_name": "AABYODAADAAAW6KAAAD",
 },
 {
 "group_id": "6",
 "group_name": "AABYODAADAAAW6KAAAE",
 }
]

and I'm sorting group_id, but the array length is over 2,000.

• Phrogz: Is this a JS Array, a JS Object, a JSON Array, or a JSON Object? (Hint: JSON is a string.)
• wajiw: you may want to go accept some of your previous answers to get more help.
• Marcel Korpel: @Phrogz: A JS Array is still a JS Object. ;) SortingHat: Please read benalman.com/news/2010/03/theres-no-such-thing-as-a-json And do you have a bit of code to show us?
• Marcel Korpel: It can't be a ‘JSON Array’, ‘JSON’ and ‘Array’ are mutually exclusive. Please read the linked article.
• Marcel Korpel: You have superfluous commas at the end of each object within your array; that will cause lots of pain in several browsers.

Answer 1

Assuming this is a JS object you're talking about (and not a JS Array or a JSON Array or a JSON Object), you'll have to loop twice:

var max;
for (var key in obj) if (obj.hasOwnProperty(key) && (!max || key>max)) max = key;
for (var i=0;i<=max;++i) if (obj[i]==undefined){
 console.log("Missing: "+i);
}

Edit: Based on your updated sample, it appears that you have an array of objects whose keys are strings that represent integers, and you want to figure out keys might be missing. Here's code that would do that:

var groups = myObj.groups;
var groupNames = [];
for (var i=0,len=groups.length;i<len;++i){
 groupNames[groups[i].group_id] = groups[i].group_name;
}
for (i=0,len=groupNames.length;i<len;++i){
 var name = groupNames[i];
 if (name==undefined){
 console.log("Oops, no name for group_id: "+i);
 }else{
 // Do what you want
 }
}

• Phrogz: @SortingHat I've updated my answer based on your sample code.
• Marcel Korpel: You don't have to declare i and len locally twice (the first var i... will suffice).

Answer 2

Perhaps you could construct a separate array with the numbers in the correct order, then iterate through the first array and have it remove ones from the secod array that match. What's leftin the newer array should be the missing numbers, in order.