Trim whitespace from the end of a StringBuilder without calling ToString().Trim() and back to a new SB
The following is an extension method, so you can call it like this:
sb.TrimEnd();
Also, it returns the SB instance, allowing you to chain other calls (sb.TrimEnd().AppendLine()
).
public static StringBuilder TrimEnd(this StringBuilder sb)
{
if (sb == null || sb.Length == 0) return sb;
int i = sb.Length - 1;
for (; i >= 0; i--)
if (!char.IsWhiteSpace(sb[i]))
break;
if (i < sb.Length - 1)
sb.Length = i + 1;
return sb;
}
Notes:
If Null or Empty, returns.
If no Trim is actually needed, we're talking a very quick return time, with probably the most expensive call being the single call to
char.IsWhiteSpace
. So practically zero expense to callTrimEnd
when not needed, as opposed to theseToString().Trim()
back to SB routes.Else, the most expensive thing, if trim is needed, is the multiple calls to
char.IsWhiteSpace
(breaks on first non-whitespace char). Of course, the loop iterates backwards; if all are whitespace you'll end up with aSB.Length
of 0.If whitespaces were encountered, the
i
index is kept outside the loop which allows us to cut the Length appropriately with it. InStringBuilder
, this is incredibly performant, it simply sets an internal length integer (the internalchar[]
is kept the same internal length).
Update: See excellent notes by Ryan Emerle as follows, which correct some of my misunderstandings (the internal workings of SB are a little more complicated than I made it out to be):
The
StringBuilder
is technically a linked list of blocks ofchar[]
so we don't end up in the LOH. Adjusting the length isn't technically as simple as changing the end index because if you move into a different chunk the Capacity must be maintained, so a new chunk may need to be allocated. Nevertheless, you only set the Length property at the end, so this seems like a great solution. Relevant details from Eric Lippert: https://stackoverflow.com/a/6524401/62195
Also, see this nice article discussing the .NET 4.0 new StringBuilder
implementation: http://1024strongoxen.blogspot.com/2010/02/net-40-stringbuilder-implementation.html
Update: Following illustrates what happens when a StringBuilder
Length is altered (the only real operation done to the SB here, and that only when needed):
StringBuilder sb = new StringBuilder("cool \t \r\n ");
sb.Capacity.Print(); // 16
sb.Length.Print(); // 11
sb.TrimEnd();
sb.Capacity.Print(); // 16
sb.Length.Print(); // 4
You can see the internal array (m_ChunkChars
) stays the same size after changing the Length, and in fact, you can see in the debugger it doesn't even overwrite the (in this case whitespace) characters. They are orphaned is all.
You can try this:
StringBuilder b = new StringBuilder();
b.Append("some words");
b.Append(" to test ");
int count = 0;
for (int i = b.Length - 1; i >= 0; i--)
{
if (b[i] == ' ')
count++;
else
break;
}
b.Remove(b.Length - count, count);
string result = b.ToString();
It will just iterate through the end while there are whitespaces then breaking out of the loop.
Or even like this:
StringBuilder b = new StringBuilder();
b.Append("some words");
b.Append(" to test ");
do
{
if(char.IsWhiteSpace(b[b.Length - 1]))
{
b.Remove(b.Length - 1,1);
}
}
while(char.IsWhiteSpace(b[b.Length - 1]));
string get = b.ToString();