matching characters in strings in visual C#

I'm working on visual C# to calculate the word error rate I have one textbox for the refrence which is the correct sentance and one for the hypothesis which is wrong one. in order to calculate WER I need to calculate : substitution : the word that has been changed which was my first question Insert : the words that had been inserted in the sentence Deleted: the words that had been deleted from the original sentence For EX: refrence: This is a NPL program. hypothesis: it is an NPL cool. it: substitution is: correct an :substitution NPL:correct program: deleted cool: inserted I tried the algorithm that dasblinkenlight proposed ( thank you so much by the way ) I worked but there is a runtime error I couldn't figure it out, in line ``` int x= Compute(buffer[j], buffer_ref[i]); ``` Index was outside the bounds of the array. and here is my code : ``` using System; using System.Collections.Generic; using System.ComponentModel; using System.Data; using System.Drawing; using System.Linq; using System.Text; using System.Windows.Forms; namespace WindowsFormsApplication1 { public partial class Form1 : Form { string [] hyp = new string[20]; string [] refrence = new string[20]; string [] Anser= new string[20]; string[] buffer = new string[20]; string[] buffer_ref = new string[20]; int count = 0; // number of words string ref2=" " ; string hyp2 = " "; string Anser2 = " "; string buffer2 = " "; int corecct_c=0; int corecct_d = 0; int corecct_i = 0; //==================================================================== public Form1() { InitializeComponent(); for (int i = 0; i <= 19; ++i) { hyp[i] = null; buffer[i] = null; } } private void textBox2_TextChanged(object sender, EventArgs e) { refrence = this.textBox2.Text.Split(' '); buffer_ref = this.textBox2.Text.Split(' '); } private void textBox1_TextChanged(object sender, EventArgs e) { hyp = this.textBox1.Text.Split(' '); buffer = this.textBox1.Text.Split(' '); //hyp = this.textBox1.Text; // fname1.Add(this.textBox1.Text); } public void correct(string[] R) { for (int i = 0; (i <= 19) && (R[i] != "."); ++i) { if (buffer[i] == refrence[i]) { buffer[i] = "0"; buffer_ref[i] = "0"; corecct_c = corecct_c + 1; Anser[i] = "C"; } } } // function that compute 2 strings public static int Compute(string s, string t) { int n = s.Length; int m = t.Length; int[,] d = new int[n + 1, m + 1]; // Step 1 if (n == 0) { return m; } if (m == 0) { return n; } // Step 2 for (int i = 0; i <= n; d[i, 0] = i++) { } for (int j = 0; j <= m; d[0, j] = j++) { } // Step 3 for (int i = 1; i <= n; i++) { //Step 4 for (int j = 1; j <= m; j++) { // Step 5 int cost = (t[j - 1] == s[i - 1]) ? 0 : 1; // Step 6 d[i, j] = Math.Min( Math.Min(d[i - 1, j] + 1, d[i, j - 1] + 1), d[i - 1, j - 1] + cost); } } // Step 7 return d[n, m]; } public void sub(){ for (int j = 0;j<=19;j++) { if (buffer[j].IndexOf("0") != -1) { for (int i = 0; i <= 19; i++) { if (buffer_ref[j].IndexOf("0") != -1) { int x= Compute(buffer[j], buffer_ref[i]); if (x > 3) { buffer[j] = "0"; Anser[j] = "S"; } }//end if } }//end if }//end for }// end fun private void button1_Click(object sender, EventArgs e) { correct(refrence); sub(); for (int i = 0; (i <= 19) && (refrence[i] != "."); ++i) { //loop intialize ref2 = ref2 + " " + refrence[i]; hyp2 = hyp2 + " " + hyp[i]; Anser2 = Anser2 + " " + Anser[i]; buffer2 = buffer2 + " " + buffer[i]; count++; } listBox1.Items.Add(" Refrence :" + ref2); listBox1.Items.Add(" HYp :" + hyp2); listBox1.Items.Add(" Anser:" + Anser2); listBox1.Items.Add(" buffer:" + buffer2); listBox1.Items.Add(count); } private void Form1_Load(object sender, EventArgs e) { } private void label1_Click(object sender, EventArgs e) { } private void button2_Click(object sender, EventArgs e) { } private void label2_Click(object sender, EventArgs e) { } private void listBox1_SelectedIndexChanged(object sender, EventArgs e) { } } } ``` can you help me please ?