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Question

I seem to be having a problem writing a string search function. strlength,and strmid are both previously written functions that have been tested and are working.

int strfind(char * string1, char * string2)
{
   /* if string1 contains the substring string2 returns
      the starting position of string2 in string1
      otherwise returns -1
      e.g.  strinc("hello world","wor") returns 6
            strinc("hello world","war") returns -1
   */
   int begPos = 0, endPos, count = 0, match = 1;
   char *tempStr;
   endPos = strlength(string2)-1;

   while (endPos <= strlength(string1)-1)
   {
        strmid(string1, begPos, endPos, tempStr);

        while (match == 1)
        {
             if (tempStr[count] == string2[count]) {
                 if (count < strlength(string2)) count++;
                 else break;
              }
              else match = 0;
        }

        if ( match == 1 ) return begPos;
        else { begPos++;
        endPos++; }
   }

   return -1;
}

The algorithm should be something like

• Get the segment of the string between begPos and endPos
• Compare this segment with string2
• If they're the same, increment the count and check the next array address
• If not, then the strings do not match and you haven't found the segment in your string yet and match becomes 0.
• If match not found, move the beginning and end position 1 array cell over.
• If match = 1 then return begPos
• Repeat the 6 previous steps until endPos reaches the end of string1.
• If endPos reaches the end of string1 without having found string2 within string1, return -1.

The problem I'm having is that

while (match == 1)
{
    if (tempStr[count] == string2[count]) {
         if (count < strlength(string2)) count++;
         else break;
    }
    else match = 0;
}

never seems to reach the else clause. The value returned always seems to be the value that begPos is initialized with. This is a homework piece, but I've rewritten it several times using different methods such as for loops, and done multiple dry runs and cannot seem to work out the problem. Any light you can shed would be greatly appreciated.

Cheers,

espSquall

strmid function

void strmid(char * string1, int start, int end, char * string2)
{
   /* copies the elements of string1 from start to end
      to string2  */

   int len, count2 = 0;

   for (len = start; len <= end; len++)
   {
       string2[count2] = string1[len];
       count2++;
   }

   string2[count2] = '\0';
}

Answer 1

I have only one point to raise.

How do you think that tempStr is getting set to anything useful?

The line char *tempStr; sets it to whatever happened to be on the stack at that time while C, with its lack of "proper" pass by reference, cannot change it with the call:

strmid(string1, begPos, endPos, tempStr);

In order to change the pointer, you would have to pass in &tempStr rather than tempStr .

So, it appears to me that your tempStr is not pointing to anything usable.

---

And, based on your added strmid function, this program is definitely in the "undefined behaviour" class. A quick fix, though kludgy, would be to change:

char *tempStr;

to:

char tempStr[1000];

That might not fix all your problems (and it'll introduce a potential for buffer overflow) but it'll at least give you a well-defined program.

Answer 2

2 points:

• you need to reinitialie count to zero before the inner while
• you need to reinitialise match to 1 before the inner while

If you don't do this, then if the first iteration doesn't match, match will never be 1 again, and count will have you checking memory you don't know the content of.

This is in addition to the issue noted by paxdiablo.

Answer 3

Look at this code:

   if (tempStr[count] == string2[count]) {
       if (count < strlength(string2)) count++;
      else break; }
   else match = 0;

Can you see how match will never get set to zero?

Answer 4

if ( match == 1 ) return begPos; else { begPos++; endPos++; }

match will always be 1 for it to get here

Answer 5

Just re set match to 1 and count to zero at the start of the loop and allocate sufficient memory for tempStr* :

int endPos = strlen(string2)-1; char* tempStr = (char*)malloc((size_t)(endPos+1));

while (endPos <= strlength(string1)-1) { count=0; match=1; ...