小编典典

莱文施泰因飞往达默劳-莱文施泰因

algorithm

我坐在这里,为Java主程序编写一些算法(到目前为止,这是第一个)。我对levenshtein算法进行了很好的编程,这要感谢Wiki对新手入门的伪代码非常友好,再加上一个不错的教程:D

然后,我决定升级到Damerau,并添加了额外的行,但随后我读到它不是DL算法,而是OptimalStringAlignmentDistance。我尝试阅读动作脚本代码以了解将其添加到DL时还需要添加的内容,但感到困惑。我去过不同的地方,代码看起来与Java相似,但是它们都使用了错误的伪代码。

度过了半天后,我放弃了,决定在这里问。有谁可以帮助我将此代码升级为Java中的Damerau-Levenshtein?

    public class LevensteinDistance {
        private static int Minimum(int a, int b, int c) {
            return Math.min(Math.min(a, b), c);
        }

        private static int Minimum (int a, int b) {
            return Math.min(a, b);
        }

        public static int computeLevensteinDistance(String s, String t){
            int d[][];
            int n; // length of s
            int m; // length of t
            int i; // iterates through s
            int j; // iterates through t
            char s_i; // ith character of s
            char t_j; // jth character of t
            int cost; // cost

            n = s.length ();
            m = t.length ();
            if (n == 0) {
                return m;
            }
            if (m == 0) {
                return n;
            }
            d = new int[n+1][m+1];

            for (i = 0; i <= n; i++) {
                d[i][0] = i;
            }

            for (j = 0; j <= m; j++) {
                d[0][j] = j;
            }

            for(i = 1; i <= n; i++) {
                s_i = s.charAt (i - 1);
                for(j = 1; j <= m; j++) {
                    t_j = t.charAt (j - 1);

                    if(s_i == t_j){
                        cost = 0;
                    }else{
                        cost = 1;
                    }
                    d[i][j] = Minimum(d[i-1][j]+1, d[i][j-1]+1, d[i-1][j-1] + cost);

                    if(i > 1 && j > 1 && s_i == t_j-1 && s_i-1 == t_j){ 
                        d[i][j] = Minimum(d[i][j], d[i-2][j-2] + cost);
                    }
                }
            }
        return d[n][m];
    }

    //    public static void main(String[] args0){
    //      String a = "I decided it was best to ask the forum if I was doing it right";
    //      String b = "I thought I should ask the forum if I was doing it right";
    //      System.out.println(computeLevensteinDistance(a, b));
    //    }
}

这是Damerau–Levenshtein距离算法的Wikipedia页面


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2020-07-28

共1个答案

小编典典

您的问题是在条件中引用字符串中的先前字符。在原始代码中,您具有:

if(i > 1 && j > 1 && s_i == t_j-1 && s_i-1 == t_j){ 
  d[i][j] = Minimum(d[i][j], d[i-2][j-2] + cost);
}

问题在于值 t_j-1s_i-1 。它们说s和t的第i个字符为减1,其中算法表示您要使用(第i个减1)字符。例如,如果字符串s为“
AFW”,而i为1,则

s_i - 1 = E; //the character value (s[1]='F') minus 1 = 'E'
s.charAt(i-1) = A; //i-1 = 0, s[0] = 'A'

因此您的条件应为:

if(i > 1 && j > 1 && s_i == t.charAt(j-1) && s.charAt(i-1) == t_j) { 
  d[i][j] = Minimum(d[i][j], d[i-2][j-2] + cost);
}

编辑:不幸的是,我从阅读代码后无法理解算法,但是这里是Java维基百科页面上ActionScript示例的翻译,它提供了与您的示例匹配的输出:

public static int damerauLevenshteinDistance(
      String a, String b, int alphabetLength) {
    final int INFINITY = a.length() + b.length();
    int[][] H = new int[a.length()+2][b.length()+2];  
    H[0][0] = INFINITY;
    for(int i = 0; i<=a.length(); i++) {
      H[i+1][1] = i;
      H[i+1][0] = INFINITY;
    }
    for(int j = 0; j<=b.length(); j++) {
      H[1][j+1] = j;
      H[0][j+1] = INFINITY;
    }      
    int[] DA = new int[alphabetLength];
    Arrays.fill(DA, 0);
    for(int i = 1; i<=a.length(); i++) {
      int DB = 0;
      for(int j = 1; j<=b.length(); j++) {
        int i1 = DA[b.charAt(j-1)];
        int j1 = DB;
        int d = ((a.charAt(i-1)==b.charAt(j-1))?0:1);
        if(d==0) DB = j;
        H[i+1][j+1] =
          min(H[i][j]+d,
              H[i+1][j] + 1,
              H[i][j+1]+1, 
              H[i1][j1] + (i-i1-1) + 1 + (j-j1-1));
      }
      DA[a.charAt(i-1)] = i;
    }
    return H[a.length()+1][b.length()+1];
  }

  private static int min(int ... nums) {
    int min = Integer.MAX_VALUE;
    for (int num : nums) {
      min = Math.min(min, num);
    }
    return min;
  }
2020-07-28