在数组中查找多个索引

Question

Say I have an array like this

  string [] fruits = {"watermelon","apple","apple","kiwi","pear","banana"};

Is there an built in function that allows me to query all the index of "apple"? For example,

  fruits.FindAllIndex("apple"); 

will return an array of 1 and 2

If there is not, how should I implement it?

Thanks!

Answer 1

LINQ version:

var indexes = fruits.Select((value, index) => new { value, index })
                    .Where(x => x.value == "apple")
                    .Select(x => x.index)
                    .ToList();

Non-LINQ version, using Array<T>.IndexOf() static method:

var indexes = new List<int>();
var lastIndex = 0;

while ((lastIndex = Array.IndexOf(fruits, "apple", lastIndex)) != -1)
{
    indexes.Add(lastIndex);
    lastIndex++;
}

Answer 2

One way would be to write like this:

var indices = fruits
                .Select ((f, i) => new {f, i})
                .Where (x => x.f == "apple")
                .Select (x => x.i);

Or the traditional way:

var indices = new List<int>();
for (int i = 0; i < fruits.Length; i++)
    if(fruits[i] == "apple")
        indices.Add(i);

Answer 3

Pretty easy with an extension method.

var fruits = new[] { "watermelon","apple","apple","kiwi","pear","banana" };
var indexes = fruits.FindAllIndexes("apple");

public static class Extensions
{
  public static int[] FindAllIndexes(this string[] array, string search) => array
      .Select((x, i) => (x, i))
      .Where(value => value.x == search)
      .Select(value => value.i)
      .ToArray();
}