Chapter 8

Notice that this call is similar to av_fetch(), and similarly returns an SV** that may 

be NULL if the requested key does not exist. Just like av_exists(), hv_exists() pro 

vides a simple way to avoid dealing with SV**s:

SV *sv;

// check that $hv{foo} exists

if (hv_exists(hv, "foo", strlen("foo"))) {

   // safely trust hv_fetch to return non NULL

   sv = *(hv_fetch(hv, "foo", strlen("foo"), 0));

} else {

   croak("fetch failed: hv does not contain value for key foo");

}

Aside from reading a specific key, the other common way to read from a hash 

is to iterate through its keys and values. This is done using the hv_iter functions. 

For example, here's a function that prints out the keys and values in a hash:

void print_hash (HV *hv) {

   SV *sv;

   I32 i, count;

   char *key_string;

   STRLEN len;

   // initialize the iteration

   count = hv_iterinit(hv);

   // loop over key/value pairs

   for (i = 1; i <= count; i++) {

      sv = hv_iternextsv(hv, &key_string, (I32*) &len);

      printf("%s : %s\n", key_string, SvPV(sv, len));

    }

}

The preceding function uses two new Perl API calls, hv_iterinit() and 

hv_iternextsv(). The first initializes the iteration and returns the number 

of key value pairs in the hash:

count = hv_iterinit(hv);

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