if i have array
array[0] = "jack"; array[1] = "jill"; array[2] = "lisa"; array[2] = "jackie";
and i want to find all elements with "ack" in it.
in this case it would return
"jack", "jackie".
what is the fastest way of doing this?
1
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1Are you sure you want the fastest, or do you want the easiest? Also, how many strings will be in your array in the real world? How many times will you be finding all elements in the array containing a particular substring? Is your array of words the same all the time, or is the substring the same all the time? The answers to these questions will prompt different solutions, if you really want the fastest one.Greg Hewgill– Greg Hewgill2009-10-19 01:17:34 +00:00Commented Oct 19, 2009 at 1:17
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4 Answers
array.Where(s => s.Contains("ack"));
(Cheerfully ignoring any localisation / string collation / case sensitivity issues.)
3 Comments
leora
is there anyway to make this case insensitive
ChaosPandion
Not that it matters but my original answer was the same as this but with .ToLower() included. :(
itowlson
Or s.IndexOf("ack", StringComparison.OrdinalIgnoreCase) >= 0. The benefit of using IndexOf >= 0 with a StringComparison is that it allows you to control whether the comparison is ordinal or culture-respecting (e.g. if a string contained an accented upper-case A, would it match the "a" of "ack"?).
This should be a little bit faster than a LINQ solution.
var list = new List<string>();
foreach(string s in array)
{
if ((s ?? "").Contains("ack"))
{
list.Add(s);
}
}
5 Comments
Yuriy Faktorovich
using String.Empty instead of "" might make it a little faster
itowlson
I don't think there'll be any appreciable speed difference -- the LINQ Where operator results in almost exactly the same code, just with an additional iterator class stuck in to handle deferred enumeration. Good catch on handling nulls though!
ChaosPandion
That depends really. Some people consider shaving microseconds a good thing.
Yuriy Faktorovich
Although it is worth it to note that my crappy benchmarking tests showed it to be faster than the LINQ method, anyone know why it is? I used the following for the LINQ: array.Where(s => s.Contains("ack")).ToList()
ChaosPandion
Using Where creates a class called WhereArrayIterator which adds a layer of abstraction upon what I wrote here.
I don't really know C#. Here is a basic low-level approach in pseudo-code:
function boolean contains_string(string haystack, string needle)
int needleIndex
int haystackIndex
for haystackIndex from 0 to haystack.length-needle.length
for needleIndex from 0 to needle.length
if haystack[haystackIndex+needleIndex] != needle[needleIndex]
break
end if
end for
if needleIndex == needle.length-1
return TRUE
end if
end for
return FALSE
end function
for each element in array
if contains_string(element, "ack")
new_array.push element
end if
end for
Almost certainly contains bugs.
3 Comments
Joel
I guess you probably don't need to write your own Contains method. If feel a bit silly for posting this now. Maybe I should leave the C# questions to C# experts.
ChaosPandion
Looks like a mixture of VB, C, JavaScript. Very interesting.
Yuriy Faktorovich
It's too bad they didn't offer a reason.
I believe this should be faster than the LINQ solution.
IEnumerable<string> Containing(string[] xs, string sub) {
foreach(string s in array)
if (s.Contains(sub))
yield return s;
}
I am assuming no null strings in xs.
1 Comment
ChaosPandion
I don't think this calls for yield. The odds are they will call .ToArray() anyway.