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compare 2 strings

 
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Christophe



Joined: 10 May 2005
Posts: 323
Location: Belgium

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compare 2 strings
PostPosted: Thu Sep 14, 2006 8:44 am     Reply with quote
What would be the fastest and shortest way to compare 2 strings?


Code:
const int8 Command_Strings [2][5] = {
    "COM1"
    "COM2"
}; 



and


Code:
char Temp_Array [4];



Now when receiving characters / commando's from another processor, I need to read them.

How would yo do:


Code:
if ( char Temp_Array [4] == "COM1" )
     return COM1_COMMAND_READ;



Now I'm using:


Code:
// Commando gaan vergelijken met de geprogrammeerde commando's
// Schrijf Braille
   strcpy (command,BRAILLE_COMMANDO);
   if ( !strncmp (temp_array,command,4) )
      return SCHRIJF_NAAR_LEESREGEL;
// Na 30s idlen gaat de E3 in suspend
   strcpy (command,E3_SUSPEND);
   if ( strncmp (temp_array,command,4) == 0 )
   return E3_IN_SUSPEND;
// Batterij commando?
   strcpy (command,BATTERIJ_COMMANDO);
   if ( strncmp (temp_array,command,4) == 0 )
      return VBAT_METEN;



So actually the same code (= waste of rom?) again.

How can this be done with the fewest rom and the fastest way? My gues is that this smells like pointers, however you cannot create pointers to constants I read in the manual..
asmallri



Joined: 12 Aug 2004
Posts: 1634
Location: Perth, Australia

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PostPosted: Thu Sep 14, 2006 9:39 am     Reply with quote
strcmp()

sorry - fingers on the keyboard before the brain was engaged.
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Last edited by asmallri on Thu Sep 14, 2006 10:04 am; edited 1 time in total
Ttelmah
Guest
PostPosted: Thu Sep 14, 2006 10:01 am     Reply with quote
He is already using strncmp.
The problem is that the internal compare functions, do not accept constant strings. Therefore he has the overhead of copying the strings to RAM, and then comparing.
There are a number of possible answers. If you just declare the strings as normal, not constant strings:
Code:

int8 Command_Strings [2][5] = {
    "COM1"
    "COM2"
};

Then the compiler will copy them from ROM into RAM, _once_ during the initialisation (if you put them outside any function) . Strcmp, or strncmp, can then be directly used. The downside is the amount of RAM used (though small for just these two strings).
The alternative is to remember that there is nothing 'special' about strcmp, except the fact that it is already written for you. You can perfectly well compare the characters, with the character constants yourself, on a character by character basis, without having to copy them to RAM. Even better, for two strings that contain the same characters, why not just look for the single string, and then look for the extra charater. So something like:
Code:

char COM_STR[4]="COM";
#define COM1 (1)
#define COM2 (2)
#define NO_MATCH (3)

int8 look_for_com(char * string_to_test) {
   if (strncmp(string_to_test,COM_STR,3)==0) {
      //Here 'string_to_test', contains 'COM'
      if (string_to_test[3]=='1') return COM1;
      if (string_to_test[3]=='2') return COM2;
   }
   return NO_MATCH;
}


Best Wishes
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