In need of help for a string generator based on a grammar

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hi guys i am trying to make a string generator that generates a random string based upon a given grammar for my uni
i am using rand to choose between the choices given from the gramamar
but i end up with wrong outputs and i dont know why
my grammar is this
<Ε>::=(<Υ>)
<Υ>::=<Α><Β>
<Α>::=ν|<Ε>
<Β>::=-<Υ>|+<Υ>|ε

C++

#include <iostream>
using namespace std;
#include <stdlib.h> 
#include <cstring>
#include <stdio.h>
string st="-->";

string symbo(string s )
{
    label0:
        //int counter=0;
        s=s+"(";
        
        label1:
            int f = rand() % 2; 
            if(f==1)
            {  
                cout<<"the string right now is "<<s<<" and i am in the A-->v \n";
                s=s+"v";
            }
            if(f==0)
            {
                cout<<"the string right now is "<<s<<" and i am in the A--><E> \n";
                //counter++;
                goto label0;
            }
        cout<<"currently the string is "<< s << " and i am out of A \n";
        int g = rand() % 3;
        label2:
            
            cout<<"i am in the B \n";
            if(g==2)
            {
                cout<<"i am in the B--> -<Y> and the string is "<<s<<"\n";
            
                s=s+"-";
                goto label1;
            }
            if(g==1)
            {
                cout<<"i am in the B--> +<Y> and the string is "<<s<<"\n";
                s=s+"+";
                goto label1;
            }
            if(g==0)
            {
                
                cout<<"i am in the B--> e and the string is "<<s<<"\n";
                
            }
        cout<<"i am out of the B and the string is "<<s<<"\n";
        s=s+")";    
    return s;
}

int main()
{
    cout<<"producing a randomly generated string based on the given language and its rules \n";
    
    cout<<"the string is "<< symbo(st)<<"\n";

}

it outpouts constantly this (in the end) even after running the programm like 10 times

the string is -->(v-((v+((((v-v)

Thanks alot for any answers

What I have tried:

i have tried changing the rand numbers i give in each if statements condition other than that i am pretty much exhausted.

Posted 20-Jan-21 13:19pm

RastamanVibrations

Updated 20-Jan-21 20:46pm

v2

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Comments

Christian Graus 20-Jan-21 20:27pm

Use switch statements. Use srand to seed your generator to get different results

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CPallini · Accepted Answer · 2021-01-20T20:46:00

This task is typically accomplished using recursive function calls.
Moreover, you are getting always the same result because the random generator starts always with the same sequence: You do need to call srand in order to provide a different starting point (see srand(3): pseudo-random number generator - Linux man page[^]).

Try the program listed below.
(Note, such a code gives more chances to the empty production, in rule B in order to give the program itself chances, pardon the pun, to terminate).

C

#include <stdio.h>
#include <time.h>
#include <stdlib.h>

/*
<Ε>::=(<Υ>)
<Υ>::=<Α><Β>
<Α>::=ν|<Ε>
<Β>::=-<Υ>|+<Υ>|ε
*/


void produce_E();
void produce_Y();
void produce_A();
void produce_B();


int main()
{
  time_t t;
  srand((unsigned) time(&t));

  produce_E();

  return 0;
}


void produce_E()
{
  putchar('(');
  produce_Y();
  putchar(')');
}

void produce_Y()
{
  produce_A();
  produce_B();
}

void produce_A()
{
  switch( rand() % 2) // 2 random choosen alternatives
  {
  case 0:
    putchar('v');
  break;
  case 1:
    produce_E();
  break;
  }
}

void produce_B()
{
  switch ( rand() % 5) // 3 random choosen (unbalanced) alternatives
  {
  case 0:
    putchar('-');
    produce_Y();
  break;
  case 1:
    putchar('+');
    produce_Y();
  break;
  default: // 2,3,4
    // empty
  break;
  }
}