Locate the nearest index within the array

Question

Locate the nearest index within the array

I'm currently working with an array of "events" where the key assigned to each event corresponds to the Unix Timestamp of that particular event. To illustrate, consider the following array of event objects in JS:

var MyEventsArray=[];
MyEventsArray[1513957775]={lat:40.671978333333, lng:14.778661666667, eventcode:46};
MyEventsArray[1513957845]={lat:40.674568332333, lng:14.568661645667, eventcode:23};
MyEventsArray[1513957932]={lat:41.674568332333, lng:13.568661645667, eventcode:133};
and so forth for many rows...

The data is transmitted via an Ajax call and encoded in JSON to be processed using JS. Upon receiving the dataset, if I have a Unix Timestamp such as 1513957845 from another source and need to identify the event corresponding to that time stamp... it's pretty straightforward by simply selecting the element from the array with the matching index (the one second in the list above). Now here's the dilemma: what if the specified index isn't found (let's say we're searching for UXTimestamp=1513957855) and this index doesn't exist within the array? In such cases, I aim to retrieve the event with the closest index (in our example, MyEventsArray[1513957845] would be chosen since its index, 1513957845, is the nearest to 1513957855). How can I achieve this desired outcome? My challenge lies in managing array indexes particularly when I receive the array without knowing the starting point of the index.

How does the system handle scenarios like this? Does the machine allocate memory, potentially wasting space, for vacant/dummy elements interspersed throughout the rows or does the compiler possess mechanisms to construct its own index and optimize storage efficiency? Essentially, is it safe to manipulate indexes as we are currently doing, or would it be better to initialize the array as follows:

var MyEventsArray=[];
MyEventsArray['1513957775']={lat:40.671978333333, lng:14.778661666667, eventcode:46};
MyEventsArray['1513957845']={lat:40.674568332333, lng:14.568661645667, eventcode:23};
MyEventsArray['1513957932']={lat:41.674568332333, lng:13.568661645667, eventcode:133};
and so on for thousands of entries...

In this setup, the key and index are distinctly separate allowing us to access the first element using MyArray[0] even if the specific key value is unknown. Is this alternative approach more resource-intensive (since both index and key must be stored), in terms of memory usage, or do the implications remain unchanged for the compiler?

javascript arrays json ajax associative-array

Answer 1

Answer №1

When looking at MyEventsArray[1513957775] compared to MyEventsArray['1513957775'], they are essentially the same. This is because array indexes are essentially just property names, which are treated as strings.

As for concerns about sparse indices leading to a large number of empty cells being allocated, rest assured that this will not occur. Sparse arrays only store the values that are actually assigned to them, without wasting space on empty elements.

To quickly find a specific key within the array, one approach is to get an array of keys, sort them, and then locate the desired key:

var MyEventsArray=[];
MyEventsArray[1513957775]={lat:40.671978333333, lng:14.778661666667, eventcode:46};
MyEventsArray[1513957845]={lat:40.674568332333, lng:14.568661645667, eventcode:23};
MyEventsArray[1513957932]={lat:41.674568332333, lng:13.568661645667, eventcode:133};

var target = 1513957855;

var closest= Object.keys(MyEventsArray)
                   .map(k => ({ k, delta: Math.abs(target - k) }))  
                   .sort((a, b) => a.delta - b.delta)[0].k;

console.log(closest);

Answer 2

When looking at MyEventsArray[1513957775] compared to MyEventsArray['1513957775'], they are essentially the same. This is because array indexes are essentially just property names, which are treated as strings.

As for concerns about sparse indices leading to a large number of empty cells being allocated, rest assured that this will not occur. Sparse arrays only store the values that are actually assigned to them, without wasting space on empty elements.

To quickly find a specific key within the array, one approach is to get an array of keys, sort them, and then locate the desired key:

var MyEventsArray=[];
MyEventsArray[1513957775]={lat:40.671978333333, lng:14.778661666667, eventcode:46};
MyEventsArray[1513957845]={lat:40.674568332333, lng:14.568661645667, eventcode:23};
MyEventsArray[1513957932]={lat:41.674568332333, lng:13.568661645667, eventcode:133};

var target = 1513957855;

var closest= Object.keys(MyEventsArray)
                   .map(k => ({ k, delta: Math.abs(target - k) }))  
                   .sort((a, b) => a.delta - b.delta)[0].k;

console.log(closest);

Answer 3

Answer №2

If you want to efficiently exit an iteration when the delta exceeds a certain threshold, you can use Array#some.

var array = [];
array[8937458761] = { lat: 40.671978333333, lng: 14.778661666667, eventcode: 46 };
array[8937458835] = { lat: 40.674568332333, lng: 14.568661645667, eventcode: 23 };
array[8937458922] = { lat: 41.674568332333, lng: 13.568661645667, eventcode: 133 };

var key = 0,
    search = 8937458855;

Object.keys(array).some(function (k) {
    if (Math.abs(k - search) > Math.abs(key - search)) {
        return true;
    }
    key = k;
});

console.log(key);

Answer 4

If you want to efficiently exit an iteration when the delta exceeds a certain threshold, you can use Array#some.

var array = [];
array[8937458761] = { lat: 40.671978333333, lng: 14.778661666667, eventcode: 46 };
array[8937458835] = { lat: 40.674568332333, lng: 14.568661645667, eventcode: 23 };
array[8937458922] = { lat: 41.674568332333, lng: 13.568661645667, eventcode: 133 };

var key = 0,
    search = 8937458855;

Object.keys(array).some(function (k) {
    if (Math.abs(k - search) > Math.abs(key - search)) {
        return true;
    }
    key = k;
});

console.log(key);

Answer 5

Answer №3

If you want to retrieve an array of keys from MyEventsArray, you can utilize Object.keys(MyEventsArray). The keys in this array are represented as strings, so you will need to iterate through them to find the closest match.

var MyEventsArray=[];
MyEventsArray[1513957775]={lat:40.671978333333, lng:14.778661666667, eventcode:46};
MyEventsArray[1513957845]={lat:40.674568332333, lng:14.568661645667, eventcode:23};
MyEventsArray[1513957932]={lat:41.674568332333, lng:13.568661645667, eventcode:133};
Object.keys(MyEventsArray)

["1513957775", "1513957845", "1513957932"]

You can refer to the official Mozilla documentation on Arrays in JavaScript for more information.

Answer 6

If you want to retrieve an array of keys from MyEventsArray, you can utilize Object.keys(MyEventsArray). The keys in this array are represented as strings, so you will need to iterate through them to find the closest match.

var MyEventsArray=[];
MyEventsArray[1513957775]={lat:40.671978333333, lng:14.778661666667, eventcode:46};
MyEventsArray[1513957845]={lat:40.674568332333, lng:14.568661645667, eventcode:23};
MyEventsArray[1513957932]={lat:41.674568332333, lng:13.568661645667, eventcode:133};
Object.keys(MyEventsArray)

["1513957775", "1513957845", "1513957932"]

You can refer to the official Mozilla documentation on Arrays in JavaScript for more information.

Locate the nearest index within the array

Answer №1

Answer №2

Answer №3

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