如何使用 LINQ 获取索引? [英] How to get index using LINQ?

问题描述

给定这样的数据源:

var c = new Car[]
{
  new Car{ Color="Blue", Price=28000},
  new Car{ Color="Red", Price=54000},
  new Car{ Color="Pink", Price=9999},
  // ..
};

如何用LINQ找到满足特定条件的第一辆车的索引?

How can I find the index of the first car satisfying a certain condition with LINQ?

我可以想到这样的事情，但它看起来很糟糕:

I could think of something like this but it looks horrible:

int firstItem = someItems.Select((item, index) => new    
{    
    ItemName = item.Color,    
    Position = index    
}).Where(i => i.ItemName == "purple")    
  .First()    
  .Position;

用一个普通的旧循环来解决这个问题是最好的吗?

Will it be the best to solve this with a plain old loop?

推荐答案

IEnumerable 不是有序集.
尽管大多数 IEnumerable 是有序的，但有些(例如 Dictionary 或 HashSet)不是.

An IEnumerable is not an ordered set.
Although most IEnumerables are ordered, some (such as Dictionary or HashSet) are not.

因此，LINQ 没有 IndexOf 方法.

Therefore, LINQ does not have an IndexOf method.

但是，您可以自己编写一个:

However, you can write one yourself:

///<summary>Finds the index of the first item matching an expression in an enumerable.</summary>
///<param name="items">The enumerable to search.</param>
///<param name="predicate">The expression to test the items against.</param>
///<returns>The index of the first matching item, or -1 if no items match.</returns>
public static int FindIndex<T>(this IEnumerable<T> items, Func<T, bool> predicate) {
    if (items == null) throw new ArgumentNullException("items");
    if (predicate == null) throw new ArgumentNullException("predicate");

    int retVal = 0;
    foreach (var item in items) {
        if (predicate(item)) return retVal;
        retVal++;
    }
    return -1;
}
///<summary>Finds the index of the first occurrence of an item in an enumerable.</summary>
///<param name="items">The enumerable to search.</param>
///<param name="item">The item to find.</param>
///<returns>The index of the first matching item, or -1 if the item was not found.</returns>
public static int IndexOf<T>(this IEnumerable<T> items, T item) { return items.FindIndex(i => EqualityComparer<T>.Default.Equals(item, i)); }

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