Quick search & replace with c-strings?

[proteusx]

I'm in the middle of a project using a mysql database and spanning several languages. A problem has arisen that we need a piece of it to be in C, that reads in a query, replaces some placeholders (wrapped in '?' chars), and then runs said query.

The problem here is, C is not one of my strengths In fact, I'm almost illiterate in it. This is what I have so far, its poorly put together and doesnt even work. What it should do, is replace "?value?" with the value of repl_val.

#include <stdio.h>
#include <string.h>

int main ()
{
    char* query = "SELECT ?value? FROM Table";
    char newquery[100];
    char* repl_val = "VariableFieldName";
    
    char* pch;
    int start,end;
    int count;
    
    // get start and end indices...
    pch=strchr(query,'?');
    if (pch!=NULL)
    {
      start = pch-query;
      pch=strchr(pch+1,'?');
      end = pch-query;
    }
    // ...or return
    if(!start || !end)
    {
      return 1;
    }
    
    // copy everything before first '?', concat the replacement value
    strncpy(newquery,query,start);
    strcat(newquery,repl_val);
    
    // concat everything after the second '?', one char at a time
    for(count=end; count<strlen(query); count++)
    {
        strcat(newquery,query[count]);
    }
    
    printf("new query: %s", newquery);
    
    return 0;
}

The last for() loop wont compile, and even if i comment it out, all that prints is therepl_val surrounded by garbage. Am I just going about this all wrong?

[Dave Sinkula]

Perhaps this may help a little: Strings: Search and Replace .

[Salem]

char* query = "SELECT %s FROM Table";
    char newquery[100];
    char* repl_val = "VariableFieldName";
    sprintf( newquery, query, repl_value );

Any particular reason why you can't do this?

[proteusx]

well, we've had to revise our design AGAIN. now, what i need to do is strip out every alphanumeric string (0-9,a-z,_) delimited by question marks, then grab an environment variable of the same name. at the same time, we want to silently pass over anything that sint a solid alphanumeric string.

ie, in the following query, i need to replace ?sql_fild1? with the environment variable ${sql_field1} , and so on for sql_field2 and sort_field. We don't want to even touch the question mark for QnAField, however.

"SELECT ?sql_field1?, ?sql_field2? FROM Table WHERE QnAField = "What is the question?" ORDER BY ?sort_field? ASC"

// should become:
"SELECT PassedField, AnotherPassedField FROM Table WHERE QnAField = "What is the question?" ORDER BY NameField ASC"

ok, ive managed to put together somthing that *finds* the placeholders (and prints them to stdout for debug purposes), im just not sure how to replace them inline with a getenv() call:

#include <stdio.h>
#include <string.h>

int do_sql_placeholders(char* source)
{
  int in_placeholder = 0; // indicate if we are inside a possible placeholder
  int pos = 0;
 
  char placeholder[256]; // to store placeholder name
  int placepos = 0;

  for (pos=0; pos < (signed int)strlen(source); pos++)
  {
    if (in_placeholder)
    {
      // if continue placeholder
      if (
          (source[pos] >= '0' && source[pos] <= '9') ||
          (source[pos] >= 'A' && source[pos] <= 'Z') ||
          (source[pos] >= 'a' && source[pos] <= 'z') ||
          source[pos] == '_'
        )
      {
        placeholder[placepos] = source[pos];
        placepos++;
      }
      // else, end place holder
      else
      {
        // valid terminated placeholder, interpolate
        if (source[pos] == '?')
        {
          placeholder[placepos] = '\0';
          /*
            instead of printing this out as a match, i need to call getenv(placeholder)
            and replace the placeholder in source with the output of getenv
          */
          printf("matched placeholder '%s' at position %d\n", placeholder, pos);
        }
        // cleanup
        in_placeholder = 0;
      
        placeholder[0] = '\0';
        placepos=0;
      }
    }
    else
    {
      if (source[pos] == '?') // entering a potential placeholder
      {
        in_placeholder = 1;
        placepos = 0;
      }
    }
  }
  return 0;
}

int main()
{
 char query[100] = "SELECT ?sql_field1?, ?sql field2? FROM Table WHERE QnAField = 'What is the question?' ORDER BY ?sort_field? ASC";

 do_sql_placeholders(query);
 printf("query: %s\n", query);

 return 0;
}

[dwks]

(source[pos] >= '0' && source[pos] <= '9')

Look into isdigit() in . And isalpha() or isupper()+islower() for the other lines nearby.

Have you every used getenv() before? . . . if not, try reading a man page or searching google for it.

[proteusx]

i've managed to get something working using Dave's replace function .

its shaky at best, but it does work for the time being. thanks for the advice. and i'll look into isalpha() and isdigit() as well.