c# string comparison method returning index of first non match

Question

Is there an exsting string comparison method that will return a value based on the first occurance of a non matching character between two strings?

i.e.

string A = "1234567890"

string B = "1234567880"

I would like to get a value back that would allow me to see that the first occurance of a matching break is A[8]

Is rolling your own not possible as (perhaps) an extension method? — glosrob, Dec 14 '11 at 12:15
+1 for checking if this already exists in standard library first. — Daren Thomas, Dec 14 '11 at 12:21
Totally. Nothing worse that the pride in polishing off a really good implementation falling when you realise it's been in the framework since 1.1! — Jon Hanna, Dec 14 '11 at 12:42

sll · Accepted Answer · 2011-12-14T12:56:25.570

7

/// <summary>
/// Gets a first different char occurence index
/// </summary>
/// <param name="a">First string</param>
/// <param name="b">Second string</param>
/// <param name="handleLengthDifference">
/// If true will return index of first occurence even strings are of different length
/// and same-length parts are equals otherwise -1
/// </param>
/// <returns>
/// Returns first difference index or -1 if no difference is found
/// </returns>
public int GetFirstBreakIndex(string a, string b, bool handleLengthDifference)
{
    int equalsReturnCode = -1;
    if (String.IsNullOrEmpty(a) || String.IsNullOrEmpty(b))
    {
        return handleLengthDifference ? 0 : equalsReturnCode;
    }

    string longest = b.Length > a.Length ? b : a;
    string shorten = b.Length > a.Length ? a : b;    
    for (int i = 0; i < shorten.Length; i++)
    {
        if (shorten[i] != longest[i])
        {
            return i;
        }
    }

    // Handles cases when length is different (a="1234", b="123")
    // index=3 would be returned for this case
    // If you do not need such behaviour - just remove this
    if (handleLengthDifference && a.Length != b.Length)
    {
        return shorten.Length;
    }

    return equalsReturnCode;
}

edited Dec 14 '11 at 12:56

answered Dec 14 '11 at 12:22

sll

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Is there a reason why you check `a.Equals(b)` rather than `a == b` ? Your code will break if `a` is null. – Kevin Gosse Dec 14 '11 at 12:34
The call to equals will iterate the whole string until the first break barring short-cut cases, before you then iterate the whole string. I'd call `ReferenceEquals` as a short-cut, but leave out the rest of `Equals` as its work will be duplicated anyway. – Jon Hanna Dec 14 '11 at 12:39
Now "abc" equals null? Or with the alternative suggested, null mismatches null on its first character? – Jon Hanna Dec 14 '11 at 12:50
@Jon : now null is considered as zero-length string so can be handled as difference at 0-index when `handleLengthDifference` is enabled – sll Dec 14 '11 at 13:00
if a is equals to b, then it returns -1 and must return the length of a or b, isn't it? – Marc Apr 25 '13 at 06:33

Francisco · Answer 2 · 2019-07-23T17:29:22.040

3

If you have .net 4.0 installed, this could be a way:

    string A = "1234567890";
    string B = "1234567880";

    char? firstocurrence = A.Zip(B, (p, q) => new { A = p, B = q })
        .Where(p => p.A != p.B)
        .Select(p => p.A)
        .FirstOrDefault();

edit:

Though, if you need the position:

    int? firstocurrence = A.Zip(B, (p, q) => new { A = p, B = q })
            .Select((p, i) => new { A = p.A, B = p.B, idx = i })
            .Where(p => p.A != p.B)
            .Select(p => p.idx)
            .FirstOrDefault();

edited Jul 23 '19 at 17:29

answered Dec 14 '11 at 12:31

Francisco

3,994
3
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That's not a requirement in the question. Kind of easy to check afterwards anyway, if two strings have different length – Francisco Dec 14 '11 at 21:11
1

Not really, because to find where "abc123432343234" mismatches "abcdefghijk" requires you to then do something that would have answered the question in the first place. – Jon Hanna Dec 14 '11 at 21:22
1

The LINQ is incorrect, it always returns position 0 due to the Where clause. The corred ending after Zip is: .Select((x, idx) => (item: x, idx)).Where(x => x.item.expected != x.item.actual).Select(x => x.idx).FirstOrDefault(); – dsschneidermann Jul 20 '19 at 18:09

glosrob · Answer 3 · 2011-12-14T12:27:41.977

2

An extension method along the lines of the below would do the job:

public static int Your_Name_Here(this string s, string other) 
{
    string first = s.Length < other.Length ? s : other;
    string second = s.Length > other.Length ? s : other;

    for (int counter = 0; counter < first.Length; counter++)
    {
        if (first[counter] != second[counter])
        {
            return counter;
        }
    }
    return -1;
}

edited Dec 14 '11 at 12:27

answered Dec 14 '11 at 12:19

glosrob

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What happens if `other` is shorter than `s`? – Oded Dec 14 '11 at 12:20
It bombs :) fair comment, will amend – glosrob Dec 14 '11 at 12:23
Names are awful - prefer answer below from @sll – glosrob Dec 14 '11 at 12:29
1

Returns a false -1 if the longer string starts with the shorter. "abc" mismatches "abcdef" on char 3 (the char "abc" doesn't have at all), but this will return -1 indicating a match. – Jon Hanna Dec 14 '11 at 12:44
This answer should have been deleted years ago. If the strings are different, but the lengths are the same, then both `first` and `second` will be assigned to s, and the return value will be -1. – Mark Feldman Oct 30 '20 at 03:08

Jon Hanna · Answer 4 · 2011-12-14T12:29:45.050

2

Not that I know of, but it's pretty trivial:

public static int FirstUnmatchedIndex(this string x, string y)
{
  if(x == null || y == null)
    throw new ArgumentNullException();
  int count = x.Length;
  if(count > y.Length)
    return FirstUnmatchedIndex(y, x);
  if(ReferenceEquals(x, y))
    return -1;
  for(idx = 0; idx != count; ++idx)
    if(x[idx] != y[idx])
      return idx;
  return count == y.Length? -1 : count;
}

This is a simple ordinal comparison. Ordinal case-insensitive comparison is an easy change, but culture-base is tricky to define; "Weißbier" mismatches "WEISSBIERS" on the final S in the second string, but does that count as position 8 or position 9?

edited Dec 14 '11 at 12:29

answered Dec 14 '11 at 12:22

Jon Hanna

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Hey... we all do that... SO should integrate an online compiler ;) – Oded Dec 14 '11 at 12:29
@Oded Serves me right for calling it "trivial". Well, it is, but there's still an imperfection in every answer's first draft. – Jon Hanna Dec 14 '11 at 12:45

score 1 · Answer 5 · edited May 23 '17 at 11:52

1

This topic is a duplicate of Comparing strings and get the first place where they vary from eachother, which contains a better, one line, solution using Linq

edited May 23 '17 at 11:52

Community

1
1

answered Nov 04 '15 at 12:19

Cathartis

119
5

score 0 · Answer 6 · answered Dec 14 '11 at 12:54

It possible to write string extension like

public static class MyExtensions
{
    public static IList<char> Mismatch(this string str1, string str2)
    {
        var char1 = str1.ToCharArray();
        var char2 = str2.ToCharArray();
        IList<Char> Resultchar= new List<char>();
        for (int i = 0; i < char2.Length;i++ )
        {
            if (i >= char1.Length || char1[i] != char2[i])
                Resultchar.Add(char2[i]);
        }
        return Resultchar;
    }
}

Use it like

var r = "1234567890".Mismatch("1234567880");

It is not an optimized algorithm for finding the mismatch.

If you are interested only to find the the first mismatch,

public static Char FirstMismatch(this string str1, string str2)
        {
            var char1 = str1.ToCharArray();
            var char2 = str2.ToCharArray();             
            for (int i = 0; i < char2.Length;i++ )
            {
                if (i >= char1.Length || char1[i] != char2[i])
                    return char2[i];
            }
            return ''c;
        }

c# string comparison method returning index of first non match

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