c# - 如何使用指定行号的 LINQ 读取文本文件

Question

我有一个文本文件。我需要找到以“3”开头的行号总数，并且位置行中可用的文件中已经提供了以“7200”开头的行号总数 - 位置以 05 开头，长度为 6。类似的方式.总金额也可用在以“7200”开头的行中 - 位置以 21 开头，长度为 12。

211 87236486287346872837468724682871238483XYZ BANK             
1200ABCDEF                               8128361287AXTAKJ COLL
3270210000893281012870095628   00002500     8981273687jhgsjhdg

3270210000896281712870095628   00002500     1231273687jhgajhdj

3270210000891286712870095628   00002500     4561273687cxvnmbal

3270210000899283612870095628   00002500     7891273687nmkdkjhk

720000000400021000080000000100000000000000008128361287        
9000001000001000000010002100008000000010000000000000000       

例如:在我的文件中，以 3 开头的行总数在以“7”开头的行中可用，即“000004”

总金额以“7”开头，即“000000010000”

目前我正在使用下面的 C# 代码循环整个文件并导航到以 7 开头的行并读取上述位置可用的值，但由于文件记录数可能会花费太多时间太大了，像 200K

foreach (var line in FileLines)
{
    //// If line length is zero, then do nothing
    if (line.Length == 0)
    {
        continue;
    }

    switch (line.Substring(1, 1))
    {
        case 7:
            totalCount = int.Parse(line.Substring(4, 6));
            TotalAmount = line.Substring(20, 12);
            break;

        default:
            throw new Exception;
    }
}

有什么方法可以使用 LINQ 重写我的代码，从而获得更好的性能？

Answer 1

这是 Linq 语句。使它更有效的是它使用 Reverse 因为你提到你正在寻找的信息在页脚中。

static void Main(string[] args)
{
    var path = Path.Combine(
        Path.GetDirectoryName(Assembly.GetEntryAssembly().Location),
        "TextFile.txt");
    try
    {
        var count =
            int.Parse(
            File.ReadAllLines(path)
            .Reverse()
            .First(line => line.Any() && (line.First() == '7'))
            .Substring(4, 6));
        Console.WriteLine($"Count = {count}");
    }
    catch (Exception ex)
    {
         System.Diagnostics.Debug.Assert(false, ex.Message);
    }
}

编辑

你问了一个关于性能的好问题。最棒的是我们不必推测或猜测! 总有衡量绩效的方法。

这是我刚刚放在一起的基准。看，我做的真的很快，所以如果有人发现我遗漏了什么，请指出。但这是我得到的:

static void Main(string[] args)
{
    var path = Path.Combine(
        Path.GetDirectoryName(Assembly.GetEntryAssembly().Location),
        "TextFile.txt");
    try
    {
        // 200K lines of random guids
        List<string> builder = 
            Enumerable.Range(0, 200000)
            .Select(n => $"{{{System.Guid.NewGuid().ToString()}}}")
            .ToList();

        var footer =
            File.ReadAllLines(path);

        builder.AddRange(footer);

        var FileLines = builder.ToArray();

        var benchmark = new System.Diagnostics.Stopwatch();
        benchmark.Start();
        int totalCount = int.MinValue;
        foreach (var line in FileLines)
        {
            //// If line length is zero, then do nothing
            if (line.Length == 0)
            {
                continue;
            }
            // Original code from post
            // switch (line.Substring(1, 1))
            // Should be:
            switch (line.Substring(0, 1))
            {
                case "7":
                    totalCount = int.Parse(line.Substring(4, 6));
                    // This is another issue!! Breaking from the switch DOESN'T break from the loop
                    break;
                    // SHOULD BE: goto breakFromInner;
                    // One of the few good reasons to use a goto statement!!
            }
        }
        benchmark.Stop();
        Console.WriteLine($"200K lines using Original code: Elapsed = {benchmark.Elapsed}");
        Console.WriteLine($"Count = {totalCount}");


        benchmark.Restart();
        for (int i = FileLines.Length - 1; i >= 0; i--)
        {
            var line = FileLines[i];
            //// If line length is zero, then do nothing
            if (line.Length == 0)
            {
                continue;
            }
            // Original code from post
            // switch (line.Substring(1, 1))
            // Should be:
            switch (line.Substring(0, 1))
            {
                case "7":
                    totalCount = int.Parse(line.Substring(4, 6));
                    // One of the few good reasons to use a goto statement!!
                    goto breakFromInner;
            }
        }
        // See note
        breakFromInner:
        benchmark.Stop();
        Console.WriteLine($"200K lines using Original code with reverse: Elapsed = {benchmark.Elapsed}");
        Console.WriteLine($"Count = {totalCount}");

        benchmark.Restart();
        var count =
            int.Parse(
                FileLines
                .Reverse()
                .First(line => line.Any() && (line.First() == '7'))
                .Substring(4, 6));
        benchmark.Stop();
        Console.WriteLine($"200K lines using Linq with Reverse: Elapsed = {benchmark.Elapsed}");
        Console.WriteLine($"Count = {count}");
    }
    catch (Exception ex)
    {
        System.Diagnostics.Debug.Assert(false, ex.Message);
    }
}