memory problem with string manipulation

This is a discussion on memory problem with string manipulation within the C++ Programming forums, part of the General Programming Boards category; I'm terrible when it comes to string manipulation, I always seem to find a way to segfaut or something. I'm ...

  1. #1
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    memory problem with string manipulation

    I'm terrible when it comes to string manipulation, I always seem to find a way to segfaut or something. I'm trying to return the string between 2 offsets of another string but I'm getting weird results for what should be a simple function.

    Code:
    char* substr( char *searchstr, int offset1, int offset2 ){
          int size = offset2-offset1;
          char tmpstr[size];
          int m = 0;
          for( int i = offset1; i < size; i++ ){
               tmpstr[m] = searchstr[i];
               m++;
          }
          char *str;
          str = (char*)malloc( sizeof( char ) * size );
          strcpy( str, tmpstr );
          return str;
    }
    Everything seems to work perfectly, however when I tried substr("wonderful", 0, 5) the returned result was "wonde +B" which leads me to believe I'm doing something wrong with the memory.

  2. #2
    Just Lurking Dave_Sinkula's Avatar
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    Last edited by Dave_Sinkula; 07-09-2005 at 01:16 AM.
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

  3. #3
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    Ahh, of course.. Thanks champ.

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    Still doesn't seem to be working properly, the output is not always correct. Sometimes it will just output the char 's' and there wasn't a single one in the given string.. Any ideas? I've modified the code slightly-

    Code:
    char* substr( char *searchstr, int offset1, int offset2 ){
          int size = offset2-offset1;
          char tmpstr[size];
          int m = 0;
          for( int i = offset1; i < size; i++ ){
               tmpstr[m] = searchstr[i];
               m++;
          }
          char *str;
          str = (char*)malloc( sizeof( char ) * size + 1 );
          strcpy( str, tmpstr );
          str[size] = '\0';
          return str;
    }
    I just don't understand how this is happening.

  5. #5
    C++ Witch laserlight's Avatar
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    hmm... shouldnt it be more like:
    Code:
    char* substr(char* searchstr, int offset1, int offset2) {
    	char* tempstr = new char[offset2-offset1+2];
    	int i = 0, j = offset1;
    	while (j <= offset2) {
    		tempstr[i++] = searchstr[j++];
    	}
    	return tempstr;
    }
    with the idea of using offset2-offset1+2 as b-a+1 gives the number of integers in range [a,b], adding 1 to account for '\0'
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  6. #6
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    Alright. Well now for the next problem. I'm trying to use that function with the following to return string between 2 delimiters.
    Code:
    char* strbetween( char* searchstr, char* delimiter1, char* delimiter2 ){
         char* str;
         int offset1 = -1;
         int offset2 = -1;
         int lendel1 = strlen( delimiter1 );
         int lendel2 = strlen( delimiter2 );
         int lensrch = strlen( searchstr );
         for( int i = 0; i < lensrch; i++ ){
              if( i+lendel1 < lensrch ){
                  if( strcmp( delimiter1, substr( searchstr, i, i+lendel1 ) ) == 0 ){
                      offset1 = i+lendel1+1;
                      break;
                  }
              }
         }
         for( int i = 0; i < lensrch; i++ ){
              if( i+lendel2 < lensrch )
                  if( strcmp( delimiter2, substr( searchstr, i, i+lendel2 ) ) == 0 ){
                      offset2 = i-1;
                      break;
                  }
         }
         if( offset1 > 0 && offset2 > 0 )
             str = substr( searchstr, offset1, offset2 );
         else
             str = NULL;
         return str;
    }
    [edit:] Umm.. the actual problem is, it always returns NULL.
    Last edited by Ichmael™; 07-09-2005 at 05:12 AM.

  7. #7
    C++ Witch laserlight's Avatar
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    Pretend that this function exists:
    Code:
    //returns index of 1st character of substring in haystack that matches needle
    //returns -1 if needle not found in haystack
    int findStr(char* haystack, char* needle);
    So one just uses findStr() on searchstr and delimiter1.
    If the index returned is not -1, then offset1 is set to the sum of the index returned and the length of delimiter1.
    Then one uses findStr() on searchstr and delimiter2.
    If the index returned is not -1, then offset2 is set to 1 less than the index.
    We then use substr() on the offsets, perhaps after a check that offset1 <= offset2.
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  8. #8
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    Ahh I have this problem of making things unecesseraily complicated. I'll see how I go with that, but I can see the idea is much more logical.
    Last edited by Ichmael™; 07-10-2005 at 02:26 AM.

  9. #9
    C++ Witch laserlight's Avatar
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    hmm... I decided to give implementing strbetween a try since I have so little experience with the C string functions.
    Here's my version:
    Code:
    #include <iostream>
    #include <cstring>
    
    //returns substring of searchstr between delimiter1 and delimiter2
    char* strbetween(char* searchstr, char* delimiter1, char* delimiter2) {
    	//start of delimiter1 in searchstr
    	//named betw as it contains substring to be returned
    	char* betw = strstr(searchstr, delimiter1);
    	//check that delimiter1 is found
    	if (betw == 0) {
    		return 0;
    	}
    	//start of delimiter2 in searchstr
    	char* after_betw = strstr(searchstr, delimiter2);
    	//check that delimiter2 is found
    	if (after_betw == 0) {
    		return 0;
    	}
    	//make betw point to start of substring to be returned
    	betw += strlen(delimiter1);
    	//check that betw is before delimiter2
    	if (betw < after_betw) {
    		int ret_len = after_betw - betw;
    		char* ret = new char[ret_len+1];
    		strncpy(ret, betw, ret_len);
    		return ret;
    	} else {
    		return 0;
    	}
    }
    
    int main() {
    	char* str = strbetween("Stephanie", "tep", "ie");
    	std::cout << str << std::endl;
    	delete[] str;
    	return 0;
    }
    The part that I'm wondering is this: strbetween() uses new[] to allocate memory for the string to be returned.
    Is it safe (i.e. no memory leak) to write main() as:
    Code:
    int main() {
    	std::cout << strbetween("Stephanie", "tep", "ie") << std::endl;
    	return 0;
    }
    considering that there is then no delete[] for the (temporary) string returned by strbetween()?
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  10. #10
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    Well I'd always assumed that sort of thing didn't cause a leak, it keeps reference to the memory till the end of that statement doesn't it?

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