C # / LINQ is the fastest way to compare two lists and assign a value

Question

I made code that basically compares two lists in C #. The first list contains the following properties:

• ItemID
• TOTALVIEWS

There are no values ​​for TotalView in the first list, so I assign them from the second list, which has these details:

• ItemID
• HitCount // this is a property for TotalViews to be assigned

The code is as follows:

foreach (var item in parsedMerchantData)
{
    var itemInB = HitCountItemIDS.FirstOrDefault(x => x.ItemID == item.ItemID);
    if (itemInB != null)
    {
        if (itemInB.HitCount != -1)
        {
            item.TotalViews = itemInB.HitCount;
        }
        else
        {
            item.TotalViews = 0;
        }
    }
}

Is there a more efficient way to write this using LINQ or implement a custom mapper that will work faster on large lists that contain at least 100,000 items on their own?

Answer 1

:

var arr1 = parsedMerchantData.OrderBy(x => x.ItemID).ToArray();
var arr2 = HitCountItemID.OrderBy(x => x.ItemID).ToArray();

var i, j = 0;
while(i + j < arr1.Length() + arr2.Length()) // or similar condition
{
    if (arr1[i].ItemID < arr2[j].ItemID) {
        if (i < arr1.Length() - 1) {
            i++;
        }
        continue;
    }

    if (arr1[i].ItemID > arr2[j].ItemID) {
        if (j < arr2.Length() - 1) {
            j++;
        }
        continue;
    }

    if (arr1[i].ItemID == arr2[j].ItemID) {
        arr1[i].TotalViews = arr2[j].HitCount != -1 ? arr2[j].HitCount : 0;
    }

    // Make sure you do not let i and j grow higher then lengths of arrays
}

MergeSort. , O (n * log (n)) , O (n). O (n * log (n)), , .

Answer 2

jdweng, , :

var hitCountsById = HitCountItemIDS.ToDictionary(x => x.ItemID, x => x.HitCount);
foreach (var item in parsedMerchantData)
{
    int hitCount;
    // We don't care about the return value of TryGetValue here...
    hitCountsById.TryGetValue(item.ItemID, out hitCount);
    item.HitCount = hitCount == -1 ? 0 : hitCount;
}

O (N + M), N - HitCountItemIDs, M - parsedMerchantData... , merge-sort , , , . ( , - .)

Answer 3

. , HitCountItemID. , "var dict":

Dictionary<string, ABC_TYPE> dict = HitCountItemID.GropupBy(x => x.ItemID, y => y).ToDictionary(x => x.Key, y => y.FirstOrDefault())
foreach (var item in parsedMerchantData)
{
    var itemInB = dict[item.ItemID];
    if (itemInB != null)
    {
        if (itemInB.HitCount != -1)
        {
            item.TotalViews = itemInB.HitCount;
        }
        else
        {
            item.TotalViews = 0;
        }
    }
}

Answer 4

, / 2 , . , , ID, .

, , ( n ^ 2 ), , , n.

int i = 0, j = 0;

while( i < parsedMerchantData.Count && j < HitCountItemIDS.Count)
{
    var item = parsedMerchantData[i];
    var itemInB = HitCountItemIDS[j];

    if (itemInB.ItemID == item.ItemID)
    {
        item.TotalViews = (itemInB.HitCount > 0) ? itemInB.HitCount : 0;
        i++;
        j++;
    }
    else if(itemInB.ItemID < item.ItemID)
        i++;
    else  //itemInB.ItemID > item.ItemID
        j++;
}

, . , ( , i j ).