Getting list elements from Prolog in a C interface -

i try implement c interface using prolog script based on gnu prolog. problem single elements of nested prolog list.

actually c code looks like

... int func; plterm arg[10]; plterm *sol_gb; plbool res; int nmb;  char *strhead; char *strtail; pllong nummero; plterm pl_nummero;  pl_start_prolog(argc, argv);   pl_query_begin(pl_true);  arg[0] = pl_mk_string(strrname); arg[1] = pl_mk_variable(); arg[2] = pl_mk_variable(); arg[3] = pl_mk_string("true");  res = pl_query_call(func, 4, arg);  sol_gb = pl_rd_list(arg[2]); nmb = pl_list_length(sol_gb[0]);  strhead = pl_write_to_string(sol_gb[0]);       printf("strhead = %s\n",strhead); strtail = pl_write_to_string(sol_gb[1]);       printf("strtail = %s\n",strtail); ... 

the prolog list returned in arg[2] looks like

[ [ spezial bolognese,      [2, ,zwiebeln,300,gramm,hackfleisch,10, ,tomaten,     100,ml,sahne,500,gramm,spaghetti]    ],   [ spaghetti bolognese,     [2, ,zwiebeln gehackt,300,gramm,hackfleisch,10, ,fleischtomaten,      100,ml,sahne,500,gramm,spaghetti]   ] ] 

the output of conversion string is

strhead = [spezial bolognese,[2, ,zwiebeln gehackt,300,gramm,hackfleisch,           10, ,fleischtomaten,100,ml,sahne,500,gramm,spaghetti]]  strtail = [[spaghetti bolognese,[2, ,zwiebeln gehackt,300,gramm,hackfleisch,           10, ,fleischtomaten,100,ml,sahne,500,gramm,spaghetti]]] 

so assume, "nearly there" have re-activate c knowledge not solution how enter in next level of list each element string ("spezial bolognese", next step: "2", "zwiebeln" etc.).

how can step through prolog list in c?

i happy every hint, thank again!

to content of list c-code can use 2 kind of functions.

first possibility (simple since list seen flat object requires more memory , needs proper list, i.e. not work lists not terminated [])

int pl_rd_proper_list_check(plterm the_prolog_list, plterm *the_array_receiving_arguments); 

this functions receives array (it ensure large enough), stores each element of list in array , returns total number of elements. example:

plterm list = ...some prolog list... int nelem = pl_list_length(list); plterm *elem = (plterm *) calloc(nelem, sizeof(plterm)); pl_rd_proper_list_check(list, elem); int i; for(i = 0; < nelem; i++) {     // here there argument in elem[i], let's print    pl_write(elem[i]); } 

second possibility (more general see list chained list, each cell contains head , tail (a list))

plterm *pl_rd_list(plterm the_prolog_list); 

this function returns array of 2 elements corresponding head , tail of received list. function should called on each element of list; either know number of elements or test end of list (e.g. waiting end of list atom []). here code doing (it expected inside above loop since know second argument of list nested list.

plterm list = ... prolog list...; while(!pl_un_atom(pl_atom_nil(), list)) {    plterm *lst_arg = pl_rd_list(list); // [0] = head element, [1] = tail list    // here there argument in lst_arg[0], let's print    pl_write(lst_arg[0]);    list = lst_arg[1]; } 

in example, first list looks like:

[ 'spezial bolognese',      [2,' ','zwiebeln',      300,'gramm','hackfleisch',      10,' ','tomaten',      100,'ml','sahne',      500,'gramm','spaghetti']  ] 

so second element nested list. following code uses first method above list (which has 2 elements), , second method nested list:

nelem = pl_list_length(sol_gb[0]); plterm *elem = (plterm *) calloc(nelem, sizeof(plterm)); pl_rd_proper_list_check(sol_gb[0], elem); int i; for(i = 0; < nmb; i++) {     if (i != 1) {        pl_write(elem[i]);       printf("\n");    } else {               // know list       printf("(");       plterm list = elem[i];       while(!pl_un_atom(pl_atom_nil(), list)) {           plterm *lst_arg = pl_rd_list(list); // [0] = head element, [1] = tail list           printf(" ");           pl_write(lst_arg[0]);           list = lst_arg[1];       }       printf(" )\n");    } } 

here should output

spezial bolognese ( 2   zwiebeln 300 gramm hackfleisch 10   tomaten 100 ml sahne 500 gramm spaghetti )