C# take a substring of a string

Question

I am taking a line from a file only if that file doen't have a specific pattern.. and i want to take from that line the last 3 chars... my code is:

        while (!line.Contains(pattern))
        {
             String num = line.Substring((line.Length - 3), (line.Length - 2));
             System.Console.WriteLine(num);
        }

but i get an error..

Index and length must refer to a location within the string. Parameter name: length

why i get that? i am starting the new string 3 chars before the end of the line and i stop 2 chars before.. :\

Starting 3 chars before, stopping 2 chars before makes 1 character. Do you want 3 characters or 1 character? — BoltClock
– BoltClock, Commented Nov 11, 2011 at 17:25
How long is your line? If you don't check the length before substring you will certainly create issues. — SliverNinja
– SliverNinja, Commented Nov 11, 2011 at 17:26
i dont know the lines length cause it is not the same all the times... i just want the last 3 chars... — tequilaras
– tequilaras, Commented Nov 11, 2011 at 17:28
Just tossing this out, but there's probably a nifty RegEx way to just parse all the lines at once and match the last 3 chars on each line. — Mike Christensen
– Mike Christensen, Commented Nov 11, 2011 at 17:40

Mike Christensen · Accepted Answer · 2011-11-11 17:31:31Z

9

Substring takes an offset and then a number of characters to return:

http://msdn.microsoft.com/en-us/library/aa904308%28v=VS.71%29.aspx

So:

String num = line.Substring((line.Length - 3), 3);

This of course assumes that line.Length > 3. You could check with:

String num = (line.Length < 3) ? line : line.Substring((line.Length - 3), 3);

edited Nov 11, 2011 at 17:31

answered Nov 11, 2011 at 17:25

Mike Christensen

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5 Comments

Adam Ralph Over a year ago

+1 - this Q was always going to attract a flood of answers but you got in there first ;-) Some other answers seem to think that 4 should be subtracted from the length, which is clearly incorrect.

rotman Over a year ago

That's incorrect. Arrays are indexed starting from 0, so there is no char which index is line.Length

Adam Ralph Over a year ago

@rotman, think of a string with length 3. If I take length -3 then I get zero which is the first character.

Vlad Over a year ago

@rotman: could you than please give an example when this code fails?

Mike Christensen Over a year ago

Yea when I was writing my answer, I almost typed - 4 and actually had to draw out the array on paper to make sure I was right :)

rotman · Accepted Answer · 2011-11-11 17:33:41Z

2

Second argument of Substring is how many chars it have to take starting from first argument. It should just look like that: String num = line.Substring(line.Length - 3, 3);

edited Nov 11, 2011 at 17:33

answered Nov 11, 2011 at 17:28

rotman

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2 Comments

Russell McClure Over a year ago

Ah, the old no-comment answer edit. So now I have to comment on my comment so I don't look like an idiot. Thank you for changing the -4 to a -3.

rotman Over a year ago

My mistake once again. Sorry mate, got to remember about that.

FailedDev · Accepted Answer · 2011-11-11 17:33:56Z

1

This is dangerous. What if the length of the line is < 3? You should probably check this otherwise you will get an exception.

In addition you should use the substring method as depicted here :

String num = line.Substring((line.Length - 3), 3);

edited Nov 11, 2011 at 17:33

answered Nov 11, 2011 at 17:28

FailedDev

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1 Comment

Russell McClure Over a year ago

@AdamRalph: What a finely worded and insightful comment. ;)

Jacob Sobus · Accepted Answer · 2015-06-08 08:57:19Z

1

The problem is that you try to get more characters then your array have.

Extensions are best for problems like this one ;) Mine have some dirty name but everyone know what it would do - this is exception safe substring:

public static string SubstringNoLongerThenSource(this string source, int startIndex, int maxLength)
{
    return source.Substring(startIndex, Math.Min(source.Length - startIndex, maxLength));
}

So in your exact problem it should be like that:

String num = line.SubstringNoLongerThenSource((line.Length - 3), 3);
System.Console.WriteLine(num);

So num will have max 3 letters if the string you provide to function have enough letters :)

edited Jun 8, 2015 at 8:57

answered Jun 7, 2015 at 10:44

Jacob Sobus

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Comments

BLUEPIXY · Accepted Answer · 2011-11-11 18:33:34Z

0

String num = line.Substring(line.Length - 3)

answered Nov 11, 2011 at 18:33

BLUEPIXY

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2 Comments

Pubby Over a year ago

Explaining your snippet would be much more helpful.

BLUEPIXY Over a year ago

If you specify only the starting position of the substring, the substring is taken as a whole after it. Since this case must be followed by last three letters, (Length-3) is sufficient to specify.

Digbyswift · Accepted Answer · 2011-11-14 10:41:43Z

-1

This is happening because the last parameter of Substring() should be the length of the string to extract.

In your case it should be 3

, and not line.Length - 2

The first parameter should also be:

line.Length - 3

edited Nov 14, 2011 at 10:41

answered Nov 11, 2011 at 17:26

Digbyswift

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1 Comment

Digbyswift Over a year ago

You're correct, no idea why I put that. I have edited my answer.

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