假设我有一个字符串:
string mystring = "34234234d124";
我想得到这个字符串的最后四个字符是"d124"
。我可以使用SubString
,但它需要几行代码。
是否可以使用C#在一个表达式中获得此结果?
mystring.Substring(Math.Max(0, mystring.Length - 4)); //how many lines is this?
如果你是正面的,你的字符串长度至少为4,那么它甚至更短:
mystring.Substring(mystring.Length - 4);
mystring = mystring.Length > 4 ? mystring.Substring(mystring.Length - 4, 4) : mystring;
就是这样:
int count = 4;
string sub = mystring.Substring(mystring.Length - count, count);
与之前的一些答案相比,主要区别在于,当输入字符串为:时,这段代码会考虑:
这里是:
public static class StringExtensions
{
public static string Right(this string str, int length)
{
return str.Substring(str.Length - length, length);
}
public static string MyLast(this string str, int length)
{
if (str == null)
return null;
else if (str.Length >= length)
return str.Substring(str.Length - length, length);
else
return str;
}
}
对于任何长度的字符串,这都不会失败。
string mystring = "34234234d124";
string last4 = Regex.Match(mystring, "(?!.{5}).*").Value;
// last4 = "d124"
last4 = Regex.Match("d12", "(?!.{5}).*").Value;
// last4 = "d12"
这对于手头的任务来说可能有些过分,但如果需要进行额外的验证,则可以将其添加到正则表达式中。
编辑:我认为这个正则表达式会更有效:
@".{4}\Z"
使用通用的Last<T>
。这适用于任何IEnumerable
,包括字符串。
public static IEnumerable<T> Last<T>(this IEnumerable<T> enumerable, int nLastElements)
{
int count = Math.Min(enumerable.Count(), nLastElements);
for (int i = enumerable.Count() - count; i < enumerable.Count(); i++)
{
yield return enumerable.ElementAt(i);
}
}
还有一个特定的字符串:
public static string Right(this string str, int nLastElements)
{
return new string(str.Last(nLastElements).ToArray());
}
定义:
public static string GetLast(string source, int last)
{
return last >= source.Length ? source : source.Substring(source.Length - last);
}
用法:
GetLast("string of", 2);
结果:
的
我将从各种来源修改过的代码汇集在一起,这些代码将获得您想要的结果,并且还可以做更多的事情。我允许使用负int值,超出字符串长度的int值,以及结束索引小于起始索引。在最后一种情况下,该方法返回一个反序子字符串。有很多评论,但如果有什么不清楚或只是疯了,请告诉我。我正在玩这个,看看我可以用它做什么。
/// <summary>
/// Returns characters slices from string between two indexes.
///
/// If start or end are negative, their indexes will be calculated counting
/// back from the end of the source string.
/// If the end param is less than the start param, the Slice will return a
/// substring in reverse order.
///
/// <param name="source">String the extension method will operate upon.</param>
/// <param name="startIndex">Starting index, may be negative.</param>
/// <param name="endIndex">Ending index, may be negative).</param>
/// </summary>
public static string Slice(this string source, int startIndex, int endIndex = int.MaxValue)
{
// If startIndex or endIndex exceeds the length of the string they will be set
// to zero if negative, or source.Length if positive.
if (source.ExceedsLength(startIndex)) startIndex = startIndex < 0 ? 0 : source.Length;
if (source.ExceedsLength(endIndex)) endIndex = endIndex < 0 ? 0 : source.Length;
// Negative values count back from the end of the source string.
if (startIndex < 0) startIndex = source.Length + startIndex;
if (endIndex < 0) endIndex = source.Length + endIndex;
// Calculate length of characters to slice from string.
int length = Math.Abs(endIndex - startIndex);
// If the endIndex is less than the startIndex, return a reversed substring.
if (endIndex < startIndex) return source.Substring(endIndex, length).Reverse();
return source.Substring(startIndex, length);
}
/// <summary>
/// Reverses character order in a string.
/// </summary>
/// <param name="source"></param>
/// <returns>string</returns>
public static string Reverse(this string source)
{
char[] charArray = source.ToCharArray();
Array.Reverse(charArray);
return new string(charArray);
}
/// <summary>
/// Verifies that the index is within the range of the string source.
/// </summary>
/// <param name="source"></param>
/// <param name="index"></param>
/// <returns>bool</returns>
public static bool ExceedsLength(this string source, int index)
{
return Math.Abs(index) > source.Length ? true : false;
}
因此,如果你有一个像“这是一个扩展方法”这样的字符串,这里有一些例子和结果。
var s = "This is an extension method";
// If you want to slice off end characters, just supply a negative startIndex value
// but no endIndex value (or an endIndex value >= to the source string length).
Console.WriteLine(s.Slice(-5));
// Returns "ethod".
Console.WriteLine(s.Slice(-5, 10));
// Results in a startIndex of 22 (counting 5 back from the end).
// Since that is greater than the endIndex of 10, the result is reversed.
// Returns "m noisnetxe"
Console.WriteLine(s.Slice(2, 15));
// Returns "is is an exte"
希望这个版本对某人有帮助。如果您不使用任何负数,它就像正常一样运行,并为超出范围的参数提供默认值。
string var = "12345678";
if (var.Length >= 4)
{
var = var.substring(var.Length - 4, 4)
}
// result = "5678"
假设您希望字符串位于一个字符串之间,该字符串与最后一个字符相距10个字符,而您只需要3个字符。
让我们说StreamSelected = "rtsp://72.142.0.230:80/SMIL-CHAN-273/4CIF-273.stream"
在上面,我需要提取我将在数据库查询中使用的"273"
//find the length of the string
int streamLen=StreamSelected.Length;
//now remove all characters except the last 10 characters
string streamLessTen = StreamSelected.Remove(0,(streamLen - 10));
//extract the 3 characters using substring starting from index 0
//show Result is a TextBox (txtStreamSubs) with
txtStreamSubs.Text = streamLessTen.Substring(0, 3);
你可以使用extension method:
public static class StringExtension
{
public static string GetLast(this string source, int tail_length)
{
if(tail_length >= source.Length)
return source;
return source.Substring(source.Length - tail_length);
}
}
然后打电话:
string mystring = "34234234d124";
string res = mystring.GetLast(4);
好的,所以我看到这是一篇旧帖子,但为什么我们要重写已在框架中提供的代码?
我建议您添加对框架DLL“Microsoft.VisualBasic”的引用
using Microsoft.VisualBasic;
//...
string value = Strings.Right("34234234d124", 4);
string mystring = "34234234d124";
mystring = mystring.Substring(mystring.Length-4)
使用Substring实际上非常简短和可读:
var result = mystring.Substring(mystring.Length - Math.Min(4, mystring.Length));
// result == "d124"
你所要做的就是......
String result = mystring.Substring(mystring.Length - 4);
这是另一种不应该表现得太差的选择(因为deferred execution):
new string(mystring.Reverse().Take(4).Reverse().ToArray());
虽然mystring.Last(4)
的扩展方法显然是最干净的解决方案,尽管还有一些工作要做。
你可以简单地使用C#的Substring
方法。对于前者
string str = "1110000";
string lastFourDigits = str.Substring((str.Length - 4), 4);
它将返回结果0000。
一个简单的解决方案是:
string mystring = "34234234d124";
string last4 = mystring.Substring(mystring.Length - 4, 4);