如何使用linq将字符串列表分组在特定字符串上

Question

字符串列表示例：

var test = new List<string>{
    "hdr1","abc","def","ghi","hdr2","lmn","opq","hdr3","rst","xyz"
};

我想用"hdr*"来划分这个列表，这样每个组都包含元素.

"hdr1","abc","def","ghi"  
"hdr2","lmn","opq",  
"hdr3","rst","xyz"  

我试过：

var result = test.GroupBy(g => g.StartsWith("hdr"));

但这给了我两组

"hdr1","hdr2","hdr3"  
"abc","def"..."xyz"  

正确的LINQ语句是什么？让我强调一下，"hdr*"后面的字符串可以是任何东西。他们唯一的共同点就是遵循"hdr*"。

Answer 1

您可以得到两个组，因为一个组是以"hdr"开头的元素组，另一个组是不以"hdr"开头的元素组。StartsWith返回一个bool，因此这将导致两个组拥有密钥false和true。

可以在LINQ中使用语句块。这使我们能够：

string header = null;
var groups = test
    .Select(s => {
        if (s.StartsWith("hdr")) header = s;
        return s;
    })
    .Where(s => header != s)
    .GroupBy(s => header);

我们将最后一个头存储在header中。where子句消除标头本身，因为标头是组键。

接下来的测试..。

foreach (var g in groups) {
    Console.WriteLine(g.Key);
    foreach (var item in g) {
        Console.WriteLine("    " + item);
    }
}

..。用给定的列表打印该列表：

hdr1
    abc
    def
    ghi
hdr2
    lmn
    opq
hdr3
    rst
    xyz

相反，我们还可以创建以头作为第一个元素的列表：

string header = null;
IEnumerable<List<string>> lists = test
    .Select(s => {
        if (s.StartsWith("hdr")) {
            header = s;
        }
        return s;
    })
    .GroupBy(s => header)
    .Select(g => g.ToList());

这个测试..。

foreach (var l in lists) {
    foreach (var item in l) {
        Console.Write(item + " ");
    }
    Console.WriteLine();
}

..。指纹：

hdr1 abc def ghi
hdr2 lmn opq
hdr3 rst xyz

Answer 2

您可以制作一个花哨的可拓法 GroupWhen，当它找到匹配项时启动一个新组。就像IEnumerable.GroupBy一样，它将返回一个组的“列表”：

public static IEnumerable<IGrouping<int, T>> GroupWhen<T>(this IEnumerable<T> source, Func<T, bool> predicate)
{
    var i = 0;
    // This method "marks" which group each item belongs in
    // by creating a Tuple with the item and group number
    IEnumerable<(T Item, int GroupNum)> Iterate()
    {
        foreach (var item in source)
        {
            if (predicate(item)) i++; // Start new group
            yield return (item, i);
        }
    }
    // Group items by the "mark" from above and only
    // output the Item from the Tuple, since the
    // GroupNum will be the 'int' key of the group
    return Iterate().GroupBy(tup => tup.GroupNum, tup => tup.Item);
}

// Use like so:
var list = new List<string> {"hdr1","abc","def","ghi","hdr2","lmn","opq","hdr3","rst","xyz"};
var groups = list.GroupWhen(s => s.StartsWith("hdr"))
Console.WriteLine(string.Join(",", groups.First()))
// hdr1,abc,def,ghi

请查看这把小提琴进行测试运行。

Answer 3

是的，你可以用LINQ表达式来做。但我不认为它比前景循环可读性强。

var test = new List<string>{
    "hdr1","abc","def","ghi","hdr2","lmn","opq","hdr3","rst","xyz"
};

int groupid = 0;

var result = test.GroupBy(t =>
{
    if (t.StartsWith("hdr")) ++groupid;
    return groupid;
}).ToList();

result.Select(t => string.Join(' ', t)).ToList().ForEach(Console.WriteLine);

/*
 Outputs:
hdr1 abc def ghi
hdr2 lmn opq
hdr3 rst xyz
 */