Generating strings based on a given grammar sometimes results in non stop printing - c++

Question

#include <iostream>
#include <time.h>
#include <list>
using namespace std;
void PareY();
void PareA();
void PareB();
void PareE();
static list<string> s;
void showlist(list<string> g)
{
list<string>::iterator it;
for (it = g.begin(); it != g.end(); ++it)
cout << ' ' << *it;
cout << '\n';
}
int main() {
time_t t;
srand((unsigned) time(&t));
cout << "<E>::=(<Y>)" << endl;
cout << "<Y>::=<A><B>" << endl;
cout << "<A>::=ν|<Ε>" << endl;
cout << "<Β>::=-<Υ>|+<Υ>|ε" << endl;
PareE();
cout << "---------------" << endl;
showlist(s);
return 0;
}
void PareE(){
cout << "E -> (Y)" << endl;
s.push_back("(");
PareY();
s.push_back(")");
}
void PareY(){
cout << "Y -> AB" << endl;
PareA();
PareB();
}
void PareA(){
if((rand() % 2 ) == 0){
cout << "A -> ν" << endl;
s.push_back("ν");
}else{
cout << "A -> E" << endl;
PareE();
}
}
void PareB(){
if((rand() % 3) == 0){
cout << "B -> -Y" << endl;
s.push_back("-");
PareY();
}
else if((rand() % 3) == 1){
cout << "B -> +Y" << endl;
s.push_back("+");
PareY();
}else{
cout << "B -> ε" << endl;
}
}
I have to create strings based on the grammar seen in the couts in the main function.
When i run tests, it sometimes prints stuff nonstop (it only stops if i hit the stop) why is that?
I have to show the steps as well e.x E -> (Y) -> (AB) -> (νB) -> (ν-Y) -> (ν-(ΑΒ)) and so on instead of what i've done (i'm just showing the production rule) and i don't know how to do that. I'm using a list there just for the outcome but should i use a list as well for the production? I don't know how else to do it.
<E>::=(<Y>)
<Y>::=<A><B>
<A>::=ν|<Ε>
<Β>::=-<Υ>|+<Υ>|ε
E -> (Y)
Y -> AB
A -> ν
B -> ε
---------------
( ν )
That's an example. It should be something like E -> (Y) -> (AB) -> (νΒ) -> (ν). The only thing i can think of is having a list storing the current string..

Answer 1

The grammar has recursions. That is the reason, why it may continue long.
For example can call again recursivly. And so on and so on. Depending on the random number. In it will quite often happen that it runs until stack overflow :-) The grammar is rather explosiv...
And if we show the states in a recursive decent way, we can just output the state in the nonterminal function. But I am not sure if that is what you want. I put the characters of the string in a vector. But, whatver container is ok.
The only terminals that you have are 'v', '+' and '-'.
#include <iostream>
#include <time.h>
#include <vector>
using namespace std;
void PareY();
void PareA();
void PareB();
void PareE();
vector<vector<char>> strings{};;
int main() {
strings.push_back({});
time_t t;
srand((unsigned)time(&t));
cout << "<E>::=(<Y>)" << endl;
cout << "<Y>::=<A><B>" << endl;
cout << "<A>::=ν|<Ε>" << endl;
cout << "<Β>::=-<Υ>|+<Υ>|epsilon" << endl;
cout << "\n\n---------------\n\nStart ";
PareE();
cout << "---------------\n\nStrings:\n\n" << endl;
for (const auto& s : strings) {
for (const char c : s) std::cout << c;
std::cout << '\n';
}
return 0;
}
void PareE() {
cout << " -> (E)";
PareY();
}
void PareY() {
cout << " -> (Y)";
PareA();
PareB();
}
void PareYPlus() {
cout << " -> +(Y)";
PareA();
PareB();
}
void PareYMinus() {
cout << " -> -(Y)";
PareA();
PareB();
}
void PareA() {
cout << " -> (A)";
if ((rand() % 2) == 0) {
cout << " -> v";
strings.back().push_back('v');
}
else {
PareE();
}
}
Potential output:
<E>::=(<Y>)
<Y>::=<A><B>
<A>::=ν|<Ε>
<Β>::=-<Υ>|+<Υ>|epsilon
---------------
Start -> (E) -> (Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> v -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> epsilon -> (B) -> +(Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> v -> (B) -> epsilon -> (B) -> epsilon -> (B) -> +(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> v -> (B) -> epsilon -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> v -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> epsilon -> (B) -> epsilon -> (B) -> epsilon -> (B) -> epsilon -> (B) -> +(Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> epsilon -> (B) -> +(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> epsilon -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> v -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> epsilon -> (B) -> epsilon -> (B) -> epsilon -> (B) -> epsilon -> (B) -> epsilon -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> epsilon -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> v -> (B) -> epsilon -> (B) -> epsilon -> (B) -> +(Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> epsilon -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> v -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> v -> (B) -> epsilon -> (B) -> epsilon -> (B) -> epsilon -> (B) -> +(Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> v -> (B) -> epsilon -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> epsilon -> (B) -> epsilon -> (B) -> +(Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> epsilon -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> v -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> v -> (B) -> epsilon -> (B) -> epsilon -> (B) -> +(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> epsilon -> (B) -> epsilon -> (B) -> epsilon -> (B) -> +(Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> epsilon -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> v -> (B) -> epsilon -> (B) -> +(Y) -> (A) -> v -> (B) -> epsilon -> (B) -> +(Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> v -> (B) -> epsilon -> (B) -> epsilon -> (B) -> epsilon -> (B) -> epsilon -> (B) -> epsilon -> (B) -> +(Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> epsilon -> (B) -> +(Y) -> (A) -> v -> (B) -> epsilon -> (B) -> +(Y) -> (A) -> v -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> v -> (B) -> epsilon -> (B) -> epsilon -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> v -> (B) -> epsilon -> (B) -> epsilon -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> (E) -> (Y) -> (A) -> (E) -> (Y) -> (A) -> v -> (B) -> epsilon -> (B) -> epsilon -> (B) -> -(Y) -> (A) -> v -> (B) -> -(Y) -> (A) -> v -> (B) -> +(Y) -> (A) -> v -> (B) -> epsilon -> (B) -> epsilon---------------
Strings:
v-v-v+v
-v
-v-v+v+v
+v+v+v
+v-v
-v-v-v+v
-v-v
-v-v+v
+v-v-v+v
+v
-v-v-v
-v-v
-v-v+v+v-v+v+v-v