获取字符串数组中匹配元素的所有索引

Question

I have this string array:

string[] stringArray = { "fg v1", "ws v2", "sw v3", "sfd v2"  };
string value = "v2";

How to get back all indexes of all occurrences value in the array?

so for this example we should get an array of int = [1,3] preferable without looping.

Answer 1

You can use the LINQ Where extension method to filter, and Select to get the index:

int[] indexesMatched = stringArray.Select((value, index) => new { Value = value, Index = index }
                                  .Where(x => x.Value.Contains("v2"))
                                  .Select(x => x.Index)
                                  .ToArray();

Answer 2

That's right. There is no method in the Array -class or extension method in the Enumerable -class which returns all indexes for a given predicate. So here is one:

public static IEnumerable<int> FindIndexes<T>(this IEnumerable<T> items, Func<T, bool> predicate)
{
    int index = 0;
    foreach (T item in items)
    {
        if (predicate(item))
        {
            yield return index;
        }

        index++;
    }
}

Here is your sample:

string[] stringArray = { "fg v1", "ws v2", "sw v3", "sfd v2" };
string value = "v2";

int[] allIndexes = stringArray.FindIndexes(s => s.Contains(value)).ToArray();

It uses deferred execution, so you don't need to consume all indexes if you don't want.

Answer 3

Linq approach

string[] stringArray = { "fg v1", "ws v2", "sw v3", "sfd v2" };
string value = "v2";

int[] result = stringArray.Select((x, i) => x.Contains(value) ? i : -1)
                          .Where(x => x != -1)
                          .ToArray();

Answer 4

Same result with Select followed by Where:

var indexes = stringArray.Select((x,i) => x.Contains(searchStr)? i.ToString() : "")
                         .Where(x=> x!="" ).ToList();

Working example