Where语句检查符合条件的元素，并返回新的迭代器
1：基于谓词筛选值序列
IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
1.1：foreach解析：
//IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
//foreach解析：
//source:TSource->string
List<string> fruits15 = new List<string> { "apple", "passionfruit", "banana", "mango",
"orange", "blueberry", "grape", "strawberry" };
//predicate:TSource->item
Console.WriteLine("基于谓词筛选值序列foreach解析：");
foreach (string item in fruits15)
{
if (item.StartsWith("a"))//predicate:bool->item.StartsWith("a")
{
Console.Write(item + ",");
}
}1.2：原理图：
1.3：linq写法(查询语句)：
//2:linq写法(查询语句)：
var fruits16 = from f in fruits15.AsEnumerable()
where f.StartsWith("a")
select f;
Console.WriteLine("基于谓词筛选值序列linq写法：");
foreach (string item in fruits16)
{
Console.WriteLine(item);
}1.4：查询方法写法：
//3：查询方法写法：
var fruits17 = fruits15.AsEnumerable().Where(f=> f.StartsWith("a"));
Console.WriteLine("基于谓词筛选值序列查询方法写法：");
foreach (string item in fruits17)
{
Console.WriteLine(item);
}2：基于谓词筛选值序列。 将在谓词函数的逻辑中使用每个元素的索引。
IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, Func<TSource, int, bool> predicate)
2.1：foreach解析：
//IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, Func<TSource, int, bool> predicate)
//foreach解析：
//source:TSource->string
List<string> fruits25 = new List<string> { "apple", "passionfruit", "banana", "mango",
"orange", "blueberry", "grape", "strawberry" };
int index = 0;
//predicate:TSource->item
//predicate:int->index
Console.WriteLine("基于谓词筛选值序列foreach解析：");
foreach (string item in fruits25)
{
if (item.StartsWith("a"))//predicate:bool->item.StartsWith("a")
{
Console.Write(item + ",");
}
index++;
}2.2:原理解析：
2.2：linq写法(查询语句)：没有
2.3：查询方法写法：
//3：查询方法写法：
var fruits27 = fruits25.AsEnumerable().Where((f, index25) => f.StartsWith("a") && index25 > 0);
Console.WriteLine("基于谓词筛选值序列带索引查询方法写法：");
foreach (string item in fruits27)
{
Console.WriteLine(item);
}3：多层元素（类）的Where查询：
这里以搜索赛车手为例：
/// <summary>
/// 赛车手
/// </summary>
[Serializable]
public class Racer : IComparable<Racer>, IFormattable
{
/// <summary>
/// 名字
/// </summary>
public string FirstName { get; set; }
/// <summary>
/// 赛车手姓氏
/// </summary>
public string LastName { get; set; }
/// <summary>
/// 赛车手国家
/// </summary>
public string Country { get; set; }
/// <summary>
/// 夺冠次数
/// </summary>
public int Wins { get; set; }
public int Starts { get; set; }
/// <summary>
/// 赛车手夺冠用的车名称
/// </summary>
public IEnumerable<string> Cars { get; private set; }
/// <summary>
/// 赛车手夺冠时的年份
/// </summary>
public IEnumerable<int> Years { get; private set; }
public Racer(string firstName, string lastName, string country, int starts, int wins, IEnumerable<int> years, IEnumerable<string> cars)
{
this.FirstName = firstName;
this.LastName = lastName;
this.Country = country;
this.Starts = starts;
this.Wins = wins;
this.Years = new List<int>(years);
this.Cars = new List<string>(cars);
}
public Racer(string firstName, string lastName, string country, int starts, int wins) : this(firstName, lastName, country, starts, wins, null, null)
{
}
public int CompareTo(Racer other)
{
if (other == null) return -1;
//升序
//return string.Compare(this.LastName, other.LastName);
//降序
return string.Compare(other.LastName, this.LastName);
}
public string ToString(string format, IFormatProvider formatProvider)
{
switch (format)
{
case null:
case "N":
return ToString();
case "F":
return FirstName;
case "L":
return LastName;
case "C":
return Country;
case "S":
return Starts.ToString();
case "W":
return Wins.ToString();
case "A":
return String.Format("{0} {1}, {2}; starts: {3}, wins: {4}",
FirstName, LastName, Country, Starts, Wins);
default:
throw new FormatException(String.Format("Format {0} not supported", format));
}
}
public string ToString(string format)
{
return ToString(format, null);
}
public override string ToString()
{
return String.Format("{0} {1}", FirstName, LastName);
}
}
public static class Formula1
{
private static List<Racer> racers;
/// <summary>
/// 返回一组赛车手，包含了1950-2011年之间的所有一级方程式冠军
/// </summary>
/// <returns></returns>
public static IList<Racer> GetChampions()
{
if (racers == null)
{
racers = new List<Racer>(40);
racers.Add(new Racer("Nino", "Farina", "Italy", 33, 5, new int[] { 1950 }, new string[] { "Alfa Romeo" }));
racers.Add(new Racer("Alberto", "Ascari", "Italy", 32, 10, new int[] { 1952, 1953 }, new string[] { "Ferrari" }));
racers.Add(new Racer("Juan Manuel", "Fangio", "Argentina", 51, 24, new int[] { 1951, 1954, 1955, 1956, 1957 }, new string[] { "Alfa Romeo", "Maserati", "Mercedes", "Ferrari" }));
racers.Add(new Racer("Mike", "Hawthorn", "UK", 45, 3, new int[] { 1958 }, new string[] { "Ferrari" }));
racers.Add(new Racer("Phil", "Hill", "USA", 48, 3, new int[] { 1961 }, new string[] { "Ferrari" }));
racers.Add(new Racer("John", "Surtees", "UK", 111, 6, new int[] { 1964 }, new string[] { "Ferrari" }));
racers.Add(new Racer("Jim", "Clark", "UK", 72, 25, new int[] { 1963, 1965 }, new string[] { "Lotus" }));
racers.Add(new Racer("Jack", "Brabham", "Australia", 125, 14, new int[] { 1959, 1960, 1966 }, new string[] { "Cooper", "Brabham" }));
racers.Add(new Racer("Denny", "Hulme", "New Zealand", 112, 8, new int[] { 1967 }, new string[] { "Brabham" }));
racers.Add(new Racer("Graham", "Hill", "UK", 176, 14, new int[] { 1962, 1968 }, new string[] { "BRM", "Lotus" }));
racers.Add(new Racer("Jochen", "Rindt", "Austria", 60, 6, new int[] { 1970 }, new string[] { "Lotus" }));
racers.Add(new Racer("Jackie", "Stewart", "UK", 99, 27, new int[] { 1969, 1971, 1973 }, new string[] { "Matra", "Tyrrell" }));
racers.Add(new Racer("Emerson", "Fittipaldi", "Brazil", 143, 14, new int[] { 1972, 1974 }, new string[] { "Lotus", "McLaren" }));
racers.Add(new Racer("James", "Hunt", "UK", 91, 10, new int[] { 1976 }, new string[] { "McLaren" }));
racers.Add(new Racer("Mario", "Andretti", "USA", 128, 12, new int[] { 1978 }, new string[] { "Lotus" }));
racers.Add(new Racer("Jody", "Scheckter", "South Africa", 112, 10, new int[] { 1979 }, new string[] { "Ferrari" }));
racers.Add(new Racer("Alan", "Jones", "Australia", 115, 12, new int[] { 1980 }, new string[] { "Williams" }));
racers.Add(new Racer("Keke", "Rosberg", "Finland", 114, 5, new int[] { 1982 }, new string[] { "Williams" }));
racers.Add(new Racer("Niki", "Lauda", "Austria", 173, 25, new int[] { 1975, 1977, 1984 }, new string[] { "Ferrari", "McLaren" }));
racers.Add(new Racer("Nelson", "Piquet", "Brazil", 204, 23, new int[] { 1981, 1983, 1987 }, new string[] { "Brabham", "Williams" }));
racers.Add(new Racer("Ayrton", "Senna", "Brazil", 161, 41, new int[] { 1988, 1990, 1991 }, new string[] { "McLaren" }));
racers.Add(new Racer("Nigel", "Mansell", "UK", 187, 31, new int[] { 1992 }, new string[] { "Williams" }));
racers.Add(new Racer("Alain", "Prost", "France", 197, 51, new int[] { 1985, 1986, 1989, 1993 }, new string[] { "McLaren", "Williams" }));
racers.Add(new Racer("Damon", "Hill", "UK", 114, 22, new int[] { 1996 }, new string[] { "Williams" }));
racers.Add(new Racer("Jacques", "Villeneuve", "Canada", 165, 11, new int[] { 1997 }, new string[] { "Williams" }));
racers.Add(new Racer("Mika", "Hakkinen", "Finland", 160, 20, new int[] { 1998, 1999 }, new string[] { "McLaren" }));
racers.Add(new Racer("Michael", "Schumacher", "Germany", 287, 91, new int[] { 1994, 1995, 2000, 2001, 2002, 2003, 2004 }, new string[] { "Benetton", "Ferrari" }));
racers.Add(new Racer("Fernando", "Alonso", "Spain", 177, 27, new int[] { 2005, 2006 }, new string[] { "Renault" }));
racers.Add(new Racer("Kimi", "Räikkönen", "Finland", 148, 17, new int[] { 2007 }, new string[] { "Ferrari" }));
racers.Add(new Racer("Lewis", "Hamilton", "UK", 90, 17, new int[] { 2008 }, new string[] { "McLaren" }));
racers.Add(new Racer("Jenson", "Button", "UK", 208, 12, new int[] { 2009 }, new string[] { "Brawn GP" }));
racers.Add(new Racer("Sebastian", "Vettel", "Germany", 81, 21, new int[] { 2010, 2011 }, new string[] { "Red Bull Racing" }));
}
return racers;
}
}示例：这里以1.1的方法变体查询
找出至少赢得15场比赛的巴西和奥地利赛车手
3.1：foreach解析：
//IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
//foreach解析：
//source:TSource->string
var gc1 = Formula1.GetChampions();
//predicate:TSource->item
foreach (var r in gc1)
{
//bool的判断使用lambda表达式
//predicate:bool->r.Wins >= 15 && (r.Country == "Brazil" || r.Country == "Austria")
if (r.Wins >= 15 && (r.Country == "Brazil" || r.Country == "Austria"))
{
Console.WriteLine("2.1.1:{0:A}", r);
}
}3.2：linq写法(查询语句)：
//2.1:找出至少赢得15场比赛的巴西和奥地利赛车手
//2.1.1：语法糖
var racers = from r in Formula1.GetChampions()
where r.Wins >= 15 && (r.Country == "Brazil" || r.Country == "Austria")
select r;
foreach (Racer r in racers)
{
Console.WriteLine("2.1.1:{0:A}", r);
}3.3：查询方法写法：
//写法2：2.1.3
var racers3 = Formula1.GetChampions().Where(r => r.Wins >= 15 && (r.Country == "Brazil" || r.Country == "Austria")).Select(r => r);
//与写法2一样，select方法可以舍弃
var racers35 = Formula1.GetChampions().Where(r => r.Wins >= 15 && (r.Country == "Brazil" || r.Country == "Austria"));
foreach (Racer r in racers3)
{
Console.WriteLine("2.1.3:{0:A}", r);
}