Why "A"="" Is Not the Same as ""="A"

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The code that follows is an example of how a string comparison routine might be written in the C programming language. Although this is not the exact code used by the architects of Visual FoxPro, it has been written in order to explain why a comparison between a string to an empty or null string might result in a true or false, depending on which side of the equal sign the null string appears.

#include <stdio.h>
#include <string.h>
int mycompare( char *, char * );        /* declare compare function  */ 

main()
   {
   char a1[]="Test", a2[]="";   /* data to compare */ 

   /* Call the compare function    */ 
   printf("%c\n", mycompare(a1,a2)?'T':'F' );

   /* Call the compare function, switching the addresses of the data */ 
   printf("%c\n", mycompare(a2,a1)?'T':'F' );
   }  /* end of main program */ 

   mycompare( char * c1, char * c2)
   {
   /* This is an example of C source code to compare two 'strings'   */ 
   /* in memory. The starting addresses of the two strings already   */ 
   /* in memory is passed to this function and assigned to local     */ 
   /* variables designated by "star" (meaning 'pointer to' or        */ 
   /* 'address of') c1 and "star" c2                                 */ 

   /* Create a local variable to hold the length of the string, loop */ 
   /* counter, and the result                                        */ 
   int iLen, iCounter, iResult;

   /* get the size of the string being compared to the primary string*/ 
   iLen = strlen( c2 );

   /* initially set the result to true (non-zero)                    */ 
   iResult = 1;

   for ( iCounter=0; iCounter < iLen; iCounter++)
   /* A standard C 'for' loop. The first element of any C array is    */ 
   /* element zero, not element one, so you start the counter at zero.*/ 
   /* Now, the array in memory actually contains the 'string' plus    */ 
   /* an additional ASCII zero terminating character. You must        */ 
   /* subtract one from the length to get the actual length of the    */ 
   /* string being compared, thus iLen-1. Even so, because the loop   */ 
   /* counter begins at zero and the strlen function returns the      */ 
   /* actual length of the string, you loop as long as the counter    */ 
   /* is less than the length.                                        */ 
      {
      if ( *(c1+iCounter) != *(c2+iCounter) )
         if (iResult)
            iResult = 0;
      }

   /* This code goes through and compares both strings character     */ 
   /* by character until the end of string 2, pointed to by          */ 
   /* *(c2+iCounter, is reached. During this time, if the characters */ 
   /* do not match and the result is not already false, the result   */ 
   /* is set to false.                                               */ 

   /* But what happens if iLen-1 is already zero because the string  */ 
   /* is empty?  You never enter the loop and iResult remains set    */ 
   /* to 1 (true). This is returned by the function, thus            */ 
   /* "Test"="" is true. However, when you reverse the data, iLen-1  */ 
   /* contains the length of the string being compared. You enter    */ 
   /* the For loop and iResult is set to false when comparing the    */ 
   /* first character.                                               */ 

   return iResult;
   }  /* end of mycompare() function */