Switch statement using string on an array - c

Question

#include<stdio.h>
int main(){
char name[20];
printf("enter a name ");
scanf("%s",name);
switch(name[20]){
case "kevin" :
printf("hello");
break;
}
printf("%s",name);
getch();
}
It seems it will not work. Is this possible? I mean is there any way we can make a switch statement of a string. How to solve the problem, actually?

Answer 1

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() {
char name[MAX_LEN];
fgets(name, MAX_LEN, stdin);
...

Answer 2

Switch statements work on int values (or enum), but not on char arrays.
You could do
if (strcmp(name, "kevin")==0) {
printf("hello");
}
else if (strcmp(name, "Laura")==0) {
printf("Allo");
}
else if (strcmp(name, "Mike")==0) {
printf("Good day");
}
else {
printf("Help!");
}

Answer 3

There are plenty of ways to go about this! For example, use a...
3-letter hash
#include <stdio.h>
int main(){
char name[20];
printf("enter a name ");
scanf("%s",name);
switch((int)*name * (int)*(name+1) * (int)*(name+2)){
case (1275226) : // "kevin"
printf("hello %s.\n", name);
break;
case (1293980) : // "astro"
printf("welcome %s.\n", name);
break;
}
printf("%d",(int)*name * (int)*(name+1) * (int)*(name+2));
}

Answer 4

No, you cannot use the switch statement in C with the value of a string or character array. The closest alternative is to use some sort of data structure mapping strings to function pointers. The function pointer could be called after a string is used to look it up.

Answer 5

since the name is declared as a char type ,it would be better if you use "%c" instead of using "%s" inside the scanf() method.

Answer 6

You can use "hash-string.h" library that converts strings into hash code integer.
Create a header file and paste this code:
http://www.opensource.apple.com/source/gcc/gcc-5484/intl/hash-string.h
#include <stdio.h>
#include <stdlib.h>
#include "hash-string.h"
int main(){
char name[20];
printf("Enter a name: ");
scanf("%s",name);
unsigned long nameInt = hash_string(name);
switch(nameInt){
case 7458046 /* "kevin" */: { printf("Hello %s", name); break; }
default: { printf("You are not kevin"); }
}
printf("\n");
return 0;
}

Answer 7

Remember the rules while using switch statements.
Switch constraints
1. The controlling expression of a switch statement must have "integer type".
2. The expression of each case label shall be an integer constant expression and no two of
the case constant expressions in the same switch statement shall have the same value
after conversion. There may be at most one default label in a switch statement.
3. Any enclosed switch statement may have a default label or case constant expressions with values that duplicate case constant expressions in the enclosing switch statement.

Answer 8

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