K&R 3-3 expand function

Write a function expand(s1,s2) that expands shorthand notations like a-z in the string s1 into the equivalent complete list abc...xyz in s2 . Allow for letters of either case and digits, and be prepared to handle cases like a-b-c and a-z0-9 and -a-z.

Could someone please give me some constructive criticism in regards to my code? I have looked at the way other people did it as well as the solution at the end of the book which was nothing like what I managed to come up with.

Code:

void expand (char s1[], char s2[]){
 
  int i = j = 0, q;
 
 
  while(s1[i] != '\0')
  {
    if(( i == 0 || s1[i+1] == '\0') && s1[i] == '-') //1st or last char is a "-"
      {  
          s2[j++] = s1[i];
       else if(s1[i] == '-')
       {
            if(s1[i-1] >= '0' && s1[i-1] <= '9')  // check if the left char is a 0 or a 9 eg. 0-9, 9-0
            {
                if(s1[i+1] >= '0' && s1[i+1] <= '9')  //check the same for the right char
                {
                   if(s1[i-1] <= s1[i+1])
                   {
                     for(q = s1[i-1]; q <= s1[i+1]; q++)
                       s2[j++] = q;
                   }
                
                }

                else if(s1[i-1] == islower() || s1[i-1] == isupper() // check if characters are digits
                     || s1[i+1] == islower || s1[i+1] == isupper())
                printf("Mismatching arguements unable to expand.");
                else s2[j++] = s1[i];   
            } 
            elseif(s1[i-1] == islower() || s1[i-1] == islower())
            {
                if(s1[i+1] == islower() || s1[i+1] == islower())
                {
                   if(s1[i-1] <= s1[i+1])
                   { 
                      for(q = s[i-1];q = s[i+1]; q++)
                          s2[j++] = q;
                   }
                }
                else if(s1[i-1] == isdigit() || s1[i-1] == isupper() // check if characters are lowercase
                     || s1[i+1] == isdigit || s1[i+1] == isupper())
                printf("Mismatching arguements unable to expand.");
                else s2[j++] = s1[i];
            }
            elseif(s1[i-1] == isupper() || s1[i-1] == isupper())
            {
                if(s1[i+1] == isupper() || s1[i+1] == isupper())    
                { 
                   if(s1[i-1] <= s1[i+1])
                   {
                     for(q = s[i-1];q = s[i+1]; q++)
                         s2[j++] = q;
                   }
                }
                else if(s1[i-1] == isdigit() || s1[i-1] == islower() //check if characters are uppercase
                     || s1[i+1] == isdigit || s1[i+1] == islower())
                printf("Mismatching arguements unable to expand.");
                else s2[j++] = s1[i];
            }
            else s2[j++] = s1[i];
      }
      else if( s1[i+1] == '-') // check if there is a '-' at the end
      {
           if(s1[i] >= isdigit() || s1[i] >=isupper() || s1[i] >= islower())
           ;
           else s2[j++] = s1[i]
      }
      else if(s1[i-1] !=  '-' && s1[i+1] != '-') //check if there is a '-' at the start
           s2[j++] = s1[i];
   
      i++;
      }
   s2[j] == '\0';
}

Code:

if(( i == 0 || s1[i+1] == '\0') && s1[i] == '-') //1st or last char is a "-"
      {  
          s2[j++] = s1[i];
      } /* Missing brace */                      

      else if(s1[i] == '-')
      {

Do you have code that compiles?

As click has mentioned, you're missing braces. And your indentation is presumably incorrect. if/else if's should line up like this:

Code:

if (...) {
}
else if (...) {
}
else if (...) {
}
else {
}

Line 3 should be
int i = 0, j = 0, q;

Line 30 (or is it 31?) should have a space between the if and the else. Same with the other elseif's.

Line 74 should be
s2[j] = '\0';