Comparing strings and get the first place where they vary from eachother

Question

I want to get the first place where 2 string vary from each other. example: for these two strings: "AAAB" "AAAAC"

I want to get the result 4.

How do i do it in C#?

Answer 1

.NET 4:

string a1 = "AAAB";
string a2 = "AAAAC";

int index = a1.Zip(a2, (c1, c2) => c1 == c2).TakeWhile(b => b).Count() + 1;

Answer 2

    /// <summary>
    /// Compare two strings and return the index of the first difference.  Return -1 if the strings are equal.
    /// </summary>
    int DiffersAtIndex(string s1, string s2)
    {
        int index = 0;
        int min = Math.Min(s1.Length, s2.Length);
        while (index < min && s1[index] == s2[index]) 
            index++;

        return (index == min && s1.Length == s2.Length) ? -1 : index;
    }

Answer 3

string str1 = "AAAB";
string str2 = "AAAAC";

// returns the first difference index if found, or -1 if there's
// no difference, or if one string is contained in the other
public static int GetFirstDiffIndex(string str1, string str2)
{
    if (str1 == null || str2 == null) return -1;

    int length = Math.Min(str1.Length, str2.Length);

    for (int index = 0; index < length; index++)
    {
        if (str1[index] != str2[index])
        {
            return index;
        }
    }

    return -1;
}

Answer 4

static void Main(string[] args)
{
    Console.WriteLine("enter s1 :");
    string s1 = Console.ReadLine();
    Console.WriteLine("enter s2 :");
    string s2 = Console.ReadLine();

    Console.WriteLine("note: zero means there is *no* first dif index starting from s1 ");    
    Console.WriteLine("first dif index of s1 :{0}", findFirstDifIndex(s1, s2)+1);
}

private static int findFirstDifIndex(string s1, string s2)
{
    for (int i = 0; i <Math.Min(s1.Length, s2.Length); i++)
        if (s1[i] != s2[i]) 
            return i;

    return -1;
}

Answer 5

You can create an extension method to do the trick:

public static class StringExtensions {
    public static int IndexOfDifferenceFrom(this string source, string compareTo)
    {
        for(var i = 0; i < source.Length && i < compareTo.Length; ++i) {
            if (source[i] != compareTo[i]) {
                return i;
            }
        }

        return source.Length < compareTo.Length ? source.Length : compareTo.Length;
    }

}

Or, for a LINQy solution:

var index = string1.Where((ch, i) => string2[i] == ch).Select((ch, i) => i).DefaultIfEmpty(-1).First();

Answer 6

string one = "AAAB";
string two = "AAAAC";

int found = -1;
int index = 0;
while (one != two && found == -1 && one.Length > index && two.Length > index)
{
    if (one[index] != two[index]) found = index;
    index++;
}

Answer 7

int index;
int len = Math.Min(string1.Length, string2.Length);
for (index = 0; index < len; index++)
    if (string1[index] != string2[index])
        break;

This would provide "3" for your example (zero-based indexing), so just increment the result by one.

Answer 8

int compare( String a, String b ){

   for( int i = 0; i < min( a.length, b.length ); i++ ){

      if( a.getCharAt(i) != b.getCharAt(i) ){
         return i;
      }

  }

  return -1; //a contained in b, or b contained in a

} 

The code above doesn't check anything like nulls, etc.