给定一个这样的数据源:
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找到满足某一条件的第一辆车的指标?
编辑:
我可以想到这样的东西，但看起来很可怕:
int firstItem = someItems.Select((item, index) => new
{
ItemName = item.Color,
Position = index
}).Where(i => i.ItemName == "purple")
.First()
.Position;
用一个普通的循环来解决这个问题会是最好的吗?
当前回答
简单地做:
int index = List.FindIndex(your condition);
如。
int index = cars.FindIndex(c => c.ID == 150);
其他回答
myCars.Select((v, i) => new {car = v, index = i}).First(myCondition).index;
或者稍微短一点的
myCars.Select((car, index) => new {car, index}).First(myCondition).index;
或者稍微短一点的
myCars.Select((car, index) => (car, index)).First(myCondition).index;
我将在这里做出我的贡献……为什么?这是一个不同的实现，基于任何LINQ扩展，和一个委托。下面就是:
public static class Extensions
{
public static int IndexOf<T>(
this IEnumerable<T> list,
Predicate<T> condition) {
int i = -1;
return list.Any(x => { i++; return condition(x); }) ? i : -1;
}
}
void Main()
{
TestGetsFirstItem();
TestGetsLastItem();
TestGetsMinusOneOnNotFound();
TestGetsMiddleItem();
TestGetsMinusOneOnEmptyList();
}
void TestGetsFirstItem()
{
// Arrange
var list = new string[] { "a", "b", "c", "d" };
// Act
int index = list.IndexOf(item => item.Equals("a"));
// Assert
if(index != 0)
{
throw new Exception("Index should be 0 but is: " + index);
}
"Test Successful".Dump();
}
void TestGetsLastItem()
{
// Arrange
var list = new string[] { "a", "b", "c", "d" };
// Act
int index = list.IndexOf(item => item.Equals("d"));
// Assert
if(index != 3)
{
throw new Exception("Index should be 3 but is: " + index);
}
"Test Successful".Dump();
}
void TestGetsMinusOneOnNotFound()
{
// Arrange
var list = new string[] { "a", "b", "c", "d" };
// Act
int index = list.IndexOf(item => item.Equals("e"));
// Assert
if(index != -1)
{
throw new Exception("Index should be -1 but is: " + index);
}
"Test Successful".Dump();
}
void TestGetsMinusOneOnEmptyList()
{
// Arrange
var list = new string[] { };
// Act
int index = list.IndexOf(item => item.Equals("e"));
// Assert
if(index != -1)
{
throw new Exception("Index should be -1 but is: " + index);
}
"Test Successful".Dump();
}
void TestGetsMiddleItem()
{
// Arrange
var list = new string[] { "a", "b", "c", "d", "e" };
// Act
int index = list.IndexOf(item => item.Equals("c"));
// Assert
if(index != 2)
{
throw new Exception("Index should be 2 but is: " + index);
}
"Test Successful".Dump();
}
下面是投票最多的答案的实现，当项目没有找到时返回-1:
public static int FindIndex<T>(this IEnumerable<T> items, Func<T, bool> predicate)
{
var itemsWithIndices = items.Select((item, index) => new { Item = item, Index = index });
var matchingIndices =
from itemWithIndex in itemsWithIndices
where predicate(itemWithIndex.Item)
select (int?)itemWithIndex.Index;
return matchingIndices.FirstOrDefault() ?? -1;
}
简单地做:
int index = List.FindIndex(your condition);
如。
int index = cars.FindIndex(c => c.ID == 150);
IEnumerable不是有序集。 虽然大多数IEnumerables是有序的，但有些(如Dictionary或HashSet)不是。
因此，LINQ没有IndexOf方法。
但是，你可以自己写一个:
///<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)); }
