switch statement - ANSI-C

[aaron80v]

Hi,

A simple question: In ANSI-C, how to specify a string as the expression
for switch statement.

eg

switch (mystr) {
case "ABC" : bleh...
break;
case "DEF" : bleh
break'
default: bleh;
break;
}

AFAIK, the expression must produce an integer value. In the case of a
string, what is the work-around?

Aaron

[typedef]

Q: How do I switch on strings?

The simple answers are:
a) don't use switch. use if/else with strcmp
b) make an array of possible strings and use a lookup function to
return the index of your string (or -1) and switch on that.

The most common time I face this sort of thing is parsing config files
for tokens. The approach I take is an enhancement to the simple answer
b) above, using a structure consisting of an enumeration of index
values associated with the string. The enum makes it handy to add new
keys without having to worry about index numbers as opposed to having
case 1:, case 2: to go update. Here's an example for a few strings:

typedef enum
{
eKEY_HOST = 0,
eKEY_PORT, /* implicitly = 1 ... etc */
eKEY_PROTO,
eNUM_KEYS, /* number of valid keys */
eKEY_INVALID
} cfg_key_names;

/* array of structures to map the enum (numerical) values to strings */
struct {
cfg_key_names id;
const char *str;
} cfg_keys[] = {
{ eKEY_HOST, "host" },
{ eKEY_PORT, "port" },
{ eKEY_PROTO, "proto" }
};

/* get the number of entries in the above array automatically */
int n_cfg_keys = sizeof(cfg_keys ) / sizeof(cfg_keys[0]);

/* lookup function */
int parse_cfg_key(c onst char *str)
{
int i;
for (i = 0; i < n_cfg_keys; i++)
if (strcmp(str, cfg_keys[i].str) == 0)
return cfg_keys[i].id;

return eKEY_INVALID;
}

switch (parse_cfg_key( some_str))
{
case eKEY_HOST:
/* ... */
break;
default:
case eKEY_INVALID:
/* NO STRING FOR YOU! */
break;
}

Hope that helps.

[Richard Heathfield]

aa******@yahoo. com.au said:

Hi,

A simple question: In ANSI-C, how to specify a string as the expression
for switch statement.

You can't, but keep reading, because there is a way around this.

eg

switch (mystr) {
case "ABC" : bleh...
break;
case "DEF" : bleh
break'
default: bleh;
break;
}

AFAIK, the expression must produce an integer value. In the case of a
string, what is the work-around?

Associate each string with an integer (a struct works well here), preferably
in a sorted container of some kind (sorted array, or BST, or hash table, or
something of the kind), to make retrieval quicker. Look up the string, find
the associated number, and switch on that instead.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at the above domain, - www.

[Richard Bos]

"typedef" <ty************ @gmail.comwrote :

Q: How do I switch on strings?

The simple answers are:
a) don't use switch. use if/else with strcmp
b) make an array of possible strings and use a lookup function to
return the index of your string (or -1) and switch on that.

c) Use a hash.

Richard

[Giorgio Silvestri]

"typedef" <ty************ @gmail.comha scritto nel messaggio
news:11******** *************@i 12g2000cwa.goog legroups.com...

Q: How do I switch on strings?

The simple answers are:
a) don't use switch. use if/else with strcmp
b) make an array of possible strings and use a lookup function to
return the index of your string (or -1) and switch on that.

The most common time I face this sort of thing is parsing config files
for tokens. The approach I take is an enhancement to the simple answer
b) above, using a structure consisting of an enumeration of index
values associated with the string. The enum makes it handy to add new
keys without having to worry about index numbers as opposed to having
case 1:, case 2: to go update. Here's an example for a few strings:

typedef enum
{
eKEY_HOST = 0,
eKEY_PORT, /* implicitly = 1 ... etc */
eKEY_PROTO,
eNUM_KEYS, /* number of valid keys */
eKEY_INVALID
} cfg_key_names;

/* array of structures to map the enum (numerical) values to strings */
struct {
cfg_key_names id;
const char *str;
} cfg_keys[] = {
{ eKEY_HOST, "host" },
{ eKEY_PORT, "port" },
{ eKEY_PROTO, "proto" }
};

/* get the number of entries in the above array automatically */
int n_cfg_keys = sizeof(cfg_keys ) / sizeof(cfg_keys[0]);

And for checking:

char only_for_check[2 * (sizeof(cfg_key s) / sizeof(cfg_keys[0]) ==
eNUM_KEYS) - 1];

/*
* in this case "n_cfg_keys " is not necessary
*/

>
/* lookup function */
int parse_cfg_key(c onst char *str)
{
int i;
for (i = 0; i < n_cfg_keys; i++)
if (strcmp(str, cfg_keys[i].str) == 0)
return cfg_keys[i].id;

return eKEY_INVALID;
}

switch (parse_cfg_key( some_str))
{
case eKEY_HOST:
/* ... */
break;
default:
case eKEY_INVALID:
/* NO STRING FOR YOU! */
break;
}

If you have found the index, the work is done.
It is possible to add a function field to the
struct to avoid the switch.

x = parse_cfg_key(s ome_str);
if (x != eKEY_INVALID) {
(*cfg_keys[x].fun)(); /* or with arguments */
} else {
/* ... */
}

The "id" field is not necessary if you respect the order:

struct str_and_fun_onl y {
const char *str;
void (*fun)(void);
};

"parse_cfg_ key" can return directly 'i'.

--
Giorgio Silvestri
DSP/Embedded/Real Time OS Software Engineer

[John Bode]

Richard Bos wrote:

"typedef" <ty************ @gmail.comwrote :

Q: How do I switch on strings?

The simple answers are:
a) don't use switch. use if/else with strcmp
b) make an array of possible strings and use a lookup function to
return the index of your string (or -1) and switch on that.

c) Use a hash.

Richard

That inspired an entry on The Daily WTF. I wouldn't recommend it
myself.

[John Bode]

aa******@yahoo. com.au wrote:

Hi,

A simple question: In ANSI-C, how to specify a string as the expression
for switch statement.

eg

switch (mystr) {
case "ABC" : bleh...
break;
case "DEF" : bleh
break'
default: bleh;
break;
}

AFAIK, the expression must produce an integer value. In the case of a
string, what is the work-around?

After 16 years, I've become an advocate for simplicity. Any workaround
that allows you to use a switch is just going to bog you down in extra
work and complexity, so I'd advocate ditching the switch statement
entirely and just going with

if (!strcmp(mystr, "ABC"))
{
/* do stuff */
}
else if (!strcmp(mystr, "DEF"))
{
/* do stuff */
}
...
else
{
/* do default */
}

[Richard Heathfield]

John Bode said:

>
aa******@yahoo. com.au wrote:

>Hi,

A simple question: In ANSI-C, how to specify a string as the expression
for switch statement.

eg

switch (mystr) {
case "ABC" : bleh...
break;
case "DEF" : bleh
break'
default: bleh;
break;
}

AFAIK, the expression must produce an integer value. In the case of a
string, what is the work-around?

After 16 years, I've become an advocate for simplicity. Any workaround
that allows you to use a switch is just going to bog you down in extra
work and complexity, so I'd advocate ditching the switch statement
entirely and just going with

if (!strcmp(mystr, "ABC"))
{
/* do stuff */
}
else if (!strcmp(mystr, "DEF"))
{
/* do stuff */
}
...
else
{
/* do default */
}

That's a perfectly reasonable alternative, but what if you have a great many
cases? It's not a very scalable solution. You can, of course, use the
result of strcmp:

diff = strcmp(mystr, "MNO");
if(diff == 0)
{
/* whatever */
}
else if(diff < 0)
{
/* handle cases "AAA" to "MNN" */
diff = strcmp(mystr, "HIJ");
if(diff == 0) etc etc
}
else
{
/* handle cases "MNP" to "ZZZ" */
diff = strcmp(mystr, "RST");
if(diff == 0) etc etc
}

but really, if you have so many cases as to make this worth doing, you're
better off with a bunch of string/functionptr structs in a sorted array, a
tree, or a hashtable.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at the above domain, - www.

[David T. Ashley]

<aa******@yahoo .com.auwrote in message
news:11******** **************@ 42g2000cwt.goog legroups.com...

Hi,

A simple question: In ANSI-C, how to specify a string as the expression
for switch statement.

eg

switch (mystr) {
case "ABC" : bleh...
break;
case "DEF" : bleh
break'
default: bleh;
break;
}

AFAIK, the expression must produce an integer value. In the case of a
string, what is the work-around?

You can't. Other posters have made great suggestions. I'll make more.

The disadvantage of else-if is of course that the execution time to match
one of the last clauses will be longest of all. Depends on how many you
have. I have a couple suggestions, all of which will get me boo'd on this
group.

#1
---
Make the compiler work for a living:

switch(mystr[0])
{
case 'A' :
{
switch (mystr[1])
{
//And nest arbitrarily deeply.
#2
---
Standard BSEARCH, with the table of strings ordered in the source code. You
can include a function pointer as part of the table. That, at least is
O(log N) rather than O(N).

http://en.wikipedia.org/wiki/Bsearch

---------

The suggestions by the other posters involving hashing were of course very
good ... but BSEARCH seems easier and cleaner. For one thing, no hash
collisions to worrry about.

Dave.

[=?utf-8?B?SGFyYWxkIHZhbiBExLNr?=]

David T. Ashley wrote:

<aa******@yahoo .com.auwrote in message
news:11******** **************@ 42g2000cwt.goog legroups.com...

Hi,

A simple question: In ANSI-C, how to specify a string as the expression
for switch statement.

You can't. Other posters have made great suggestions. I'll make more.

[...]

The suggestions by the other posters involving hashing were of course very
good ... but BSEARCH seems easier and cleaner. For one thing, no hash
collisions to worrry about.

If you use a hash function generator such as gperf, you have no hash
collisions to worry about either.