Chapter 5 code modification. Counting the number of times a certain string occurs.

scolty · April 26th, 2011, 06:31 PM

Morning, ive been working through all the examples upto chapter 5 and i decided to try and modify q3 in the exercises by trying to determine the number of times a string ie "here" was present in a larger string.

Unfortunately i seem to have bitten off more than i can chew. What i have atm, which doesnt work, is as follows.

Code:

    private void button1_Click(object sender, EventArgs e)
    {
        string temp;
        String tempx;
        int number123;
        int count;
        string wordError;
        int x;
        bool flag;

        wordError = txtError.Text;
        wordCorrect = txtCorrect.Text;
        count = 0;



        tempx = txtInput.Text;
        temp = txtInput.Text.Length.ToString();

        flag = int.TryParse(temp, out number123);
        if (flag == false)
        {
            MessageBox.Show("Text data only");
            txtInput.Focus();
            return;
        }

        for (x = 0; x <= number123; x++)
        {

            if (tempx.Equals(wordError))
            {
                count += 1;
            }
        }

        txtResult.Text =count.ToString();
   }
}

if someone could comment as to how i should be doing this i would appreciate it. (Im assuming the above code is comparing the whole string to the wordError variable??)

Thanks in advance

DrPurdum · April 28th, 2011, 12:00 AM

I wrote an article on this some time ago, mainly to show how minor algorithm changes lowered the pattern search time from about 45 seconds to just over 3 seconds. (Computers were A LOT slower back then!) See: "Pattern Matching in C", Computer Language, Nov., 1987. The changes would be easy to rewrite in C#. I'm out of town right now, but will try to dig out the article when I get home.

DrPurdum · April 28th, 2011, 06:52 AM

I thought I'd give you something to work with until I get home. Try creating two textboxes, one for the input string to search and one for the pattern. Create a third textbox to show the number of matches found. Now try:

Code:

privatevoid btnSearch_Click(object sender, EventArgs e)
{
int matchCounter = 0;
int i;
int inputLength;
int patternLength;
string buffer = txtInputString.Text.ToLower(); // Make lower case
string target = txtTargetPattern.Text.ToLower();
 
inputLength = buffer.Length;             // Get string lengths
patternLength = target.Length;
 
i = 0;
while (i < inputLength)
{
 i = buffer.IndexOf(target, i);
 
 if (i == -1)         // We're done...
    break;
 matchCounter++;      //...up the counter...
 i += patternLength;  //...and go to the end of the pattern
}                      // End while (i...
 
txtCount.Text = matchCounter.ToString();
}

First we convert both strings to lower case. You could add code to preserve case based on the state of a checkbox indicating the user wants the case preserved. I'll leave that to you (as well as error checking). Next, we get the lengths of the two strings, using the variable i and patterLength to control the while loop. The method IndexOf() returns an index telling where a match on the pattern is found. If no (further) match is found, the index returned is -1. Note that we use that to break out of the loop. If we do find a match, we record the match in matchCounter and then advance past the match to look for additional matches.

I think if you single-step through the code, you'll see how it works.

scolty · April 28th, 2011, 08:29 AM

Afternoon Dr Purdum,

Thanks very much for the sample code. Ive had a read through it and i think i understand how it works. I will experiment with it tonight after work + attempt to implement your suggest homework exercise.

Regarding your article on "Pattern Matching in C", if it doesnt require a large amount of digging around then id be interesting in having a look. Ill send you a PM with my email address. (I see u dont have an email address so mine is [email protected])

Cheers again for your help, much appreciated.

Rgds

Scolty.