读取包含命令选项和参数的输入，用C分隔空格

Question

I have user input which consists of 3 or less arguments, separated by white spaces, such as

A Name 20

The first argument 'A' (Add in this case) represents a command option, which determines what to do with the rest of the input. Second argument in this case is the name to be added to a linked list of struct elements, and the last one is an int type value to be stored on the same struct element.

Other command options include eg 'P' for printing all the elements in the list and 'D Name', which deletes the element with the given name form the list.

So far, I have the following code, but it does not seem to work very well. I only included the addElem function, since there seems to be something wrong with this whole thing. I don't know exactly what goes wrong with my code. When I try to add an element by eg 'A Name 20', I get multiple lines of 'Invalid command', and the element does not seem to be found in the list. Is the switch loop not a good way to handle this altogether or am I doing something else very wrong here?

typedef struct element {
   char *name;
   int val;
   struct element *next;
} Elem;

/* Read name dynamically */
char *readName()
{
   int ch, i = 0;
   char *name = malloc(1);

   name[0] = '\0';
   do {
      ch = getchar();
      name = realloc(name, (i + 2));
      name[i] = (char) ch;
      name[i+1] = '\0';
      i++;
   } while (ch != ' ' && ch != '\n');

   return name;
}

/* Add element "name" into linked list */
Elem *addElem(Elem *first)
{
   /* Create new element */
   Elem *new_elem = malloc(sizeof(Elem));

   new_elem->name = readName();
   scanf(" %d", &new_elem->val);
   new_elem->next = NULL;

   /* Add new element to linked list */
   if (first == NULL)
      first = new_elem;
   else {
      Elem *ptr = first;
      while (ptr->next != NULL)
         ptr = ptr->next;
      ptr->next = new_elem;
   }

   return first;
}

int main()
{
   char cmd;
   Elem *first = malloc(sizeof(Elem));

   while (1) {
      scanf("%c", &cmd);
      switch (cmd) {
         case 'A':
            first = addElem(first);
            break;
         case 'D':
            first = delElement(first);
            break;
         case 'P':
            printElems(first);
            break;
         default:
            printf("Invalid command\n");
            break;
      }
   }

   return 0;
}

Answer 1

The first thing your code reads in is an "A". Then it reads in the name up until the next space...which since there is a space after the A, means it stops that after reading just one character. Then it tries reading in a number, which there isn't one as by this point we're at the beginning of the 2nd field still.

So far you've read in just "A " and now you're back to the beginning of the code again and that's why you get loads of "Invalid command" messages.

You'd be a whole lot better off if you read in the entire line into one string and process it as one command rather than reading in bits piecemeal. Here is a simple example of how you might do that.

char cmdline[1024],*pos;
int num;
char name[50], *namepos;

if(fgets(cmdline,1024,stdin))
  {
  switch(*cmdline)
    {
  case 'A':
    pos=cmdline+1;
    while(isspace(*pos)) pos++;
    namepos=name;
    while((*pos)&&(!isspace(*pos)))
      {
      *namepos=*pos;
      namepos++;
      pos++;
      }
    *namepos='\0';
    if(sscanf(pos," %d",&num)==1)
      {
      // Do stuff now that you've processed the line
      }
  break;
    }
  }