CODEWITHSUNDEEP
c#containslastindexof
user3430150
user3430150

Reputation: 53

String.Contains and String.LastIndexOf C# return different result?

I have this problem where String.Contains returns true and String.LastIndexOf returns -1. Could someone explain to me what happened? I am using .NET 4.5.

    static void Main(string[] args)
    {
        String wikiPageUrl = @"http://it.wikipedia.org/wiki/ʿAbd_Allāh_al-Sallāl";

        if (wikiPageUrl.Contains("wikipedia.org/wiki/"))
        {

            int i = wikiPageUrl.LastIndexOf("wikipedia.org/wiki/");

            Console.WriteLine(i);

        }
    }

Upvotes: 5

Views: 1150

Answers (4)

Andrew Whitaker
Andrew Whitaker

Reputation: 126052

While @sa_ddam213's answer definitely fixes the problem, it might help to understand exactly what's going on with this particular string.

If you try the example with other "special characters," the problem isn't exhibited. For example, the following strings work as expected:

string url1 = @"http://it.wikipedia.org/wiki/»Abd_Allāh_al-Sallāl";
Console.WriteLine(url1.LastIndexOf("it.wikipedia.org/wiki/")); // 7

string url2 = @"http://it.wikipedia.org/wiki/~Abd_Allāh_al-Sallāl";
Console.WriteLine(url2.LastIndexOf("it.wikipedia.org/wiki/")); // 7

The character in question, "ʿ", is called a spacing modifier letter1. A spacing modifier letter doesn't stand on its own, but modifies the previous character in the string, this case a "/". Another way to put this is that it doesn't take up its own space when rendered.

LastIndexOf, when called with no StringComparison argument, compares strings using the current culture.

When strings are compared in a culture-sensitive manner, the "/" and "ʿ" characters are not seen as two distinct characters--they're processed into one character, which does not match the parameter passed in to LastIndexOf.

When you pass in StringComparison.Ordinal to LastIndexOf, the characters are treated as distinct, due to the nature of Ordinal comparison.

Another way to make this work would be to use CompareInfo.LastIndexOf and supply the CompareOptions.IgnoreNonSpace option:

Console.WriteLine(
    CultureInfo.CurrentCulture.CompareInfo.LastIndexOf(
        wikiPageUrl, @"it.wikipedia.org/wiki/", CompareOptions.IgnoreNonSpace));
// 7

Here we're saying that we don't want combining characters included in our string comparison.

As a sidenote, this means that @Partha's answer and @Noctis' answer only work because the character is being applied to a character that doesn't appear in the search string that's passed to LastIndexOf.

Contrast this with the Contains method, which by default performs an Ordinal (case sensitive and culture insensitive) comparison. This explains why Contains returns true and LastIndexOf returns false.

For a fantastic overview of how strings should be manipulated in the .NET framework, check out this article.

1: Is this different than a combining character or is it a type of combining character? would appreciate if someone would clear that up for me.

Upvotes: 5

SomeUser
SomeUser

Reputation: 390

The thing is C# lastindexof looks from behind.

And wikipedia.org/wiki/ is followed by ' which it takes as escape sequence. So either remove ' after wiki/ or have an @ there too.

The following syntax will work( anyone )

string wikiPageUrl = @"http://it.wikipedia.org/wiki/Abd_Allāh_al-Sallāl";

string wikiPageUrl = @"http://it.wikipedia.org/wiki/@ʿAbd_Allāh_al-Sallāl";

int i = wikiPageUrl.LastIndexOf("wikipedia.org/wiki");

All 3 works

If you want a generalized solution for this problem replace ' with @' in your string before you perform any operations.

Upvotes: 2

sa_ddam213
sa_ddam213

Reputation: 43596

Try using StringComparison.Ordinal

This will compare the string by evaluating the numeric values of the corresponding chars in each string, this should work with the special chars you have in that example string

 string wikiPageUrl = @"http://it.wikipedia.org/wiki/ʿAbd_Allāh_al-Sallāl";
 int i = wikiPageUrl.LastIndexOf("http://it.wikipedia.org/wiki/", StringComparison.Ordinal);

// returns 0;

Upvotes: 3

Noctis
Noctis

Reputation: 11763

the ' characters throws it off.

This should work, when you escape the ' as \': 

wikiPageUrl = @"http://it.wikipedia.org/wiki/\'Abd_Allāh_al-Sallāl";

if (wikiPageUrl.Contains("wikipedia.org/wiki/"))
{
    "contains".Dump();
  int i = wikiPageUrl.LastIndexOf("wikipedia.org/wiki/");

  Console.WriteLine(i);

}

figure out what you want to do (remove the ', escape it, or dig deeper :) ).

Upvotes: 0

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