问题
I was reading about how to implement a DPDA and found this code in the following Internet address: http://code.zhoubot.com/, This c file implements a simple pushdown automata. The automata will read in a description of their transition function and input, perform its computation on the input, and then print their output.
The input format is like: e01:e0$:000111:a:ad:aeeb$:b0eb0:b10ce:c10ce:ce$de The input is separated by a semicolon “:”, first section is “input alphabet”, second is “stack alphabet”, then “input” and the last whole bunch are transition functions.
/* This C file implements a Deterministic Pushdown Automata
* author: Kevin Zhou
* Computer Science and Electronics
* University of Bristol
*/
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
typedef struct stack {
char content;
struct stack *next;
} Stack;
typedef struct transistion {
char current_state;
char input_symbol;
char pull;
char new_state;
char push;
} Transistion;
/* list of transistion functions */
typedef struct list {
Transistion *content;
struct list *next;
} List;
typedef struct pda {
char *input_alpha;
char *stack_alpha;
char *input;
char start;
char *accept;
List *transistion;
} PDA;
/* create a new empty stack */
Stack *create_stack( void ) {
Stack *s = calloc(1,sizeof(Stack));
if(s==NULL) {
printf("Out of Memory!");
exit(1);
}
return s;
}
/* test if the stack is empty */
int isempty( Stack *s ) {
return (s->next==NULL)? 1:0;
}
Stack *push_stack (Stack *s, char c) {
Stack *new = calloc(1,sizeof(Stack));
if(new ==NULL) {
printf("Out of Memory!");
exit(1);
}
new -> content = c;
new -> next = s;
return new;
}
Stack *pull_stack (Stack *s) {
Stack *head;
if(isempty(s)) {
return '\0';
}
head = s;
s = head -> next;
return s;
}
/*return the top elememt in the stack */
char top (Stack *s) {
return s->content;
}
/* replace a value 'ontop' which on top of the stack with a newvalue 'newvalue'
epsilon represents an empty element*/
Stack *replace(Stack *sta, char ontop, char newvalue, char epsilon) {
if(ontop == epsilon && newvalue == epsilon) return sta;
if(ontop == epsilon && newvalue != epsilon) {
sta = push_stack(sta,newvalue);
return sta;
}
if(ontop != epsilon && newvalue == epsilon) {
if(ontop != top(sta)) return NULL;
sta = pull_stack(sta);
return sta;
}
if(ontop != top(sta)) return NULL;
sta = pull_stack(sta);
sta = push_stack(sta,newvalue);
return sta;
}
/* turn the input string into transistion fields */
Transistion *get_transistion(char *s) {
Transistion *t = calloc(1,sizeof(Transistion));
t->current_state = s[0];
t->input_symbol = s[1];
t->pull = s[2];
t->new_state = s[3];
t->push = s[4];
return t;
}
/* turn the string into transitions and add into list */
List *insert_list( List *l, char *elem ) {
List *t = calloc(1,sizeof(List));
List *head = l;
while(l->next!=NULL)
l = l->next;
t->content = get_transistion(elem);
t->next = NULL;
l->next = t;
return head;
}
/* insert a transistion into a list */
List *insert_list_transistion( List *l, Transistion *tr) {
List *t = calloc(1,sizeof(List));
List *head = l;
while(l->next!=NULL)
l = l->next;
t->content = tr;
t->next = NULL;
l->next = t;
return head;
}
/*test if the char c is in the string s */
int contains ( char c, char *s ) {
int i=0;
while(1) {
if(c== s[i]) return 1;
if(s[i] == '\0') return 0;
i++;
}
}
/* test if the input is a valid input */
int is_valid_input( char *input_alpha, char *input ) {
int i=0;
char c;
while(1) {
c = input[i];
if(c == '\0') break;
if(!contains(c,input_alpha)) return 0;
i++;
}
return 1;
}
/* test if the input is a valid transistion */
int is_valid_transistion ( List *l, PDA *m) {
Transistion *t;
while(1) {
if(l==NULL) break;
t = l->content;
if(!contains(t->input_symbol,m->input_alpha)) return 0;
if(!contains(t->pull,m->stack_alpha)) return 0;
if(!contains(t->push,m->stack_alpha)) return 0;
l = l->next;
}
return 1;
}
/* create a pushdown automata */
PDA *createPDA (char *input) {
PDA *m = calloc(1,sizeof(PDA));
List *tr = calloc(1,sizeof(List));
char *buffer;
char *epsilon = calloc(1,sizeof(char));
/*read input alphabet of PDA*/
buffer = strtok(input,":");
if(buffer == NULL) {
printf("Error in reading input alphabet!\n");
exit(1);
}
m->input_alpha = buffer;
epsilon[0] = m->input_alpha[0];
/*read stack alphabet*/
buffer = strtok(NULL,":");
if(buffer == NULL) {
printf("Error in reading stack alphabet!\n");
exit(1);
}
m->stack_alpha = buffer;
/*read input sequence*/
buffer = strtok(NULL,":");
if(buffer == NULL) {
printf("Error in reading input sequence!\n");
exit(1);
}
if(!is_valid_input(m->input_alpha,buffer)) {
printf("Error! Input contains some invalid characters that don't match the input alphabet!\n");
exit(1);
}
m->input = buffer;
/*read start state*/
buffer = strtok(NULL,":");
if(buffer == NULL) {
printf("Invalid string!\n");
exit(1);
}
m->start = buffer[0];
/*read accept state*/
buffer = strtok(NULL,":");
if(buffer == NULL) {
printf("Invalid string!\n");
exit(1);
}
m->accept = buffer;
/*read transistion function*/
while(1) {
buffer = strtok(NULL,":");
if(buffer == NULL) break;
tr = insert_list(tr,buffer);
}
if(!is_valid_transistion(tr->next,m)) {
printf("Error! Invalid transistion functions!\n");
exit(1);
}
m->transistion = tr->next;
return m;
}
/*print a stack */
void print_stack2(Stack *s) {
if(s==NULL) {
return;
}
print_stack2(s->next);
printf("%c",s->content);
}
void print_stack(Stack *s) {
print_stack2(s);
printf("\n");
}
/* find a proper transition function for the current state */
Transistion *find_transistion(List *list,char input,char current,char e) {
Transistion *t;
while(1) {
if(list==NULL) return NULL;
t = list -> content;
if(t->current_state == current && t->input_symbol == input)
return t;
if(t->current_state == current && t->input_symbol == e)
return t;
list = list->next;
}
}
int isAccept(char current, char* accept) {
int i=0;
while(1) {
if(accept[i]=='\0') return 0;
if(accept[i]==current) return 1;
i++;
}
}
/*simulate the Pushdown automata */
void simulate(PDA *m) {
/* first symbol in input symbol used to represent the usual */
const char epsilon = m->input_alpha[0];
char current_state = m->start;
char input;
int i=0;
Stack *sta = create_stack();
Transistion *current_transistion;
Stack *backup;
while(1) {
/*get input*/
input = m->input[i];
if(input == '\0'&&isAccept(current_state,m->accept)) {
printf("Accept\n");
print_stack(sta);
break;
}
/*get transistion function*/
current_transistion = find_transistion(m->transistion,input,current_state,epsilon);
if(current_transistion==NULL) {
printf("Reject\n");
print_stack(sta);
break;
}
current_state = current_transistion->new_state;
backup = sta;
sta = replace(sta, current_transistion->pull, current_transistion->push,epsilon);
if(sta == NULL) {
printf("Reject\n");
print_stack(backup);
break;
}
if(current_transistion->input_symbol != epsilon&¤t_transistion->input_symbol != '\0')
i++;
}
}
void print(PDA *m) {
printf("input alphabet:%s\n",m->input_alpha);
printf("stack alphabet:%s\n",m->stack_alpha);
printf("input sequence:%s\n",m->input);
printf("start state:%c\n",m->start);
printf("accept state:%s\n",m->accept);
}
int main(void) {
char s[300];
PDA *p;
scanf("%s",s);
p = createPDA(s);
simulate(p);
return 0;
}
When trying to compile, the compiler tells me the following error:
Line 41: "error: invalid conversion from 'void *' to 'Stack *'
how I can fix this error, since I'm trying to understand code?
回答1:
You are likely using a C++ compiler instead of a C one. In C it's not required (it's actually discouraged) to cast void *. In C++ it's mandatory.
Incidentally, this C FAQ answers your problem. This one explains why casting void * can be problematic.
来源：https://stackoverflow.com/questions/6720460/error-compiling-an-deterministic-pushdown-automaton-in-c-dpda