Switch statement using string on an array

Question

#include

int main(){

    char name[20];

    printf(\"enter a name \");
    scanf(\"%s\",name);
    switch(name[20]){
        case \"kevin\" : 
             


        

Answer 1

If you are after performing specific actions for specific strings this implies you know the strings in advance. This in turn implies their number is limited, is countable, like for example a set of N commands:

const char * commands[] = {
 "command-1",
 "command-2",
 ...
 "command-N"

}

To address those commands inside the array above from your code using a swtich you need to know their index, which is error prone. So number them, give them an ID:

enum Command_id {
  NO_COMMAND,
  COMMAND_1,
  COMMAND_2,
  //...
  COMMAND_N,
};

Now put the two above together using a struct:

struct Command_info {
  const char * command;
  enum Command_id id;
} command_infos[] = {
  {"", NO_COMMAND},
  {"command-1", COMMAND_1},
  {"command-2", COMMAND_2},
  // ...
  {"command-N", COMMAND_N},
};

Now you have nice mapping of strings and their related IDs. To be able to map from string to ID during runtime the mapping above needs to be searched. To do this in a efficient manner you want to us binary search. The C library proveids bsearch() for this. The only prerequsite is that the array to be searched need to sorted.

To sort use qsort() also proveid by the C library. For qsort() to work we you need a comparsion function:

int cmp_command_infos(const void * pvCI1, const void* pvCI2)
{
  const struct Command_info * pCI1 = pvCI1;
  const struct Command_info * pCI2 = pvCI2;

  return strcmp(pCI1->command, pCI2->command);
}

Call qsort() like this

qsort(command_infos, sizeof command_infos / sizeof *command_infos, sizeof *command_infos, cmp_command_infos);

Now as the array is sorted one can look it up using bsearch(). For "COMMAND-2" this would look like this:

    ... = bsearch(&(struct Command_info){"COMMAND-2", NO_COMMAND}, command_infos, sizeof command_infos / sizeof *command_infos, sizeof *command_infos, cmp_command_infos);

Putting all this together could result in:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>


enum Command_id {
  NO_COMMAND,
  COMMAND_1,
  COMMAND_2,
  //...
  COMMAND_N,
};

struct Command_info {
  const char * command;
  enum Command_id id;
} command_infos[] = {
  {"", NO_COMMAND},
  {"command-1", COMMAND_1},
  {"command-2", COMMAND_2},
  // ...
  {"command-N", COMMAND_N},
};


int cmp_command_infos(const void * pvCI1, const void* pvCI2)
{
  const struct Command_info * pCI1 = pvCI1;
  const struct Command_info * pCI2 = pvCI2;

  return strcmp(pCI1->command, pCI2->command);
}


int main(int argc, char ** argv)
{
  qsort(command_infos, sizeof command_infos / sizeof *command_infos, sizeof *command_infos, cmp_command_infos);


  {
    enum Command_id command_id = NO_COMMAND;
    struct Command_info * pCI = bsearch(&(struct Command_info){argv[1], NO_COMMAND}, command_infos, sizeof command_infos / sizeof *command_infos, sizeof *command_infos, cmp_command_infos);

    if (NULL == pCI)
    {
      printf("Command = '%s' is unknown\n", argv[1]);
    }
    else
    {
      printf("Command = '%s' --> ID = %d\n", pCI->command, pCI->id);


      switch(command_id)
      {
        case COMMAND_1:
          /* perform action on COMMAND 1 here */
          break;

        case COMMAND_2:
          /* perform action on COMMAND 1 here */
          break;

        default:
          /* unknow command, do nothing */
          break;
      }
    }
  }
}

Call it like:

./a.out command-1

giving:

Command = 'command-1' --> ID = 1

or:

./a.out command-bla

giving:

Command = 'command-bla' is unknown

or even

./a.out ""

giving:

Command = '' --> ID = 0

Answer 2

Switch statements in C aren't smart like one's found in other languages (such as Java 7 or Go) you cannot switch on a string (Nor can you compare strings with ==). Switch can only operate on integral types (int, char, etc).

In your code you call switch with: switch(name[20]). That means switch(*(name + 20)). In other words switch on the 21st char in name (because name[0] is the first). As name only has 20 chars you are accessing whatever memory is after name. (which could do unpredictable things)

Also the string "kevin" is compiled to a char[N] (where N is strlen("kevin") + 1) which contains the string. When you do case "kevin". It will only work if name is in the exact same piece of memory storing the string. So even if I copied kevin into name. It still would not match as it is stored in a different piece of memory.

To do what you seem to be trying you would do this:

#include <string.h>
...
    if (strcmp(name, "kevin") == 0) {
        ...
    }

String compare (strcmp) returns different values based on the difference in the strings. Eg:

int ord = strcmp(str1, str2);
if (ord < 0)  
    printf("str1 is before str2 alphabetically\n");
else if (ord == 0) 
    printf("str1 is the same as str2\n");
else if (ord > 0)  
    printf("str1 is after str2 alphabetically\n");

Side note: Dont use scanf("%s", name) in that form. It creates a common security problem use fgets like this: (there is a safe way to use scanf too)

#define MAX_LEN 20
int main() { 
    name[MAX_LEN]; 
    fgets(name, MAX_LEN, stdin);
    ...