My question about the question itself of K&R C book exercise 6.2

Question

When I was finishing exercise 6.2, this is what it says:

Exercise 6-2. Write a program that reads a C program and prints in alphabetical order each group of variable names that are identical in the first 6 characters, but different somewhere thereafter . Don’t count words within strings and comments. Make 6 a parameter that can be set from the command line.

Unfortunately I was NOT able to understand something like this

prints in alphabetical order each group of variable names that are identical in the first 6 characters

Could someone explain it a bit to me? Given the c program I am about to read is the following:

int main(int argc, char *argv[]){
   char line[MAXLINE];
   long lineno = 0;
   int c, except = 0, number = 0, found = 0;
   while (--argc > 0 && (*++argv)[0] == '-'){
       while (c = *++argv[0]){
           switch(c){
               case 'x':
                  except = 1;
                  break;
               case 'n':
                  number = 1;
                  break;
               default:
                  printf("find: illegal option %c\n", c);
                  argc = 0;
                  found = -1;
                  break;
           }
       }
   }
   if (argc != 1)
           printf("Usage: find -x -n pattern\n");
   else
       while(my_getline(line,MAXLINE) > 0){
           lineno++;
           if ((strstr(line, *argv) != NULL) != except){
               if (number)
                   printf("%ld", lineno);
               printf("%s", line);
               found++;
           }
       }
   return found;
}

Hope the C program to be read is not complex.

Answer 1

prints in alphabetical order each group of variable names that are identical in the first 6 characters

Could someone explain it a bit to me?

My understanding is that you need to parse the C-code and identify all variable names. Then you need to identify names starting with the same 6 characters and put them groups.

Simple example:

int main(void)
{
    int bbbbbb2 = 42;
    int aaaaaa2 = 41;
    int bbbbbb1 = 43;
    int aaaaaa1 = 44;
    printf("%d\n", aaaaaa2*aaaaaa1  + bbbbbb2/bbbbbb1);
    return 0;
}

This program has 4 variables:

    bbbbbb2 aaaaaa2 bbbbbb1 aaaaaa1

There are two groups:

Group bbbbbb: bbbbbb2 bbbbbb1 
Group aaaaaa: aaaaaa2 aaaaaa1

so your sorted output must be something like

Group aaaaaa
------------
    aaaaaa1
    aaaaaa2

Group bbbbbb
------------
    bbbbbb1
    bbbbbb2 

To implement this I would probably start with these steps:

1. Parse file and identify all variable names. When a variable is found, add the name to a dynamic array of strings.
2. Use qsort to sort the array
3. Remove duplicate names from the array (maybe create a new array with the unique names)
4. Iterate the array of unique variable names and print the groups