int main()
{
   
  int chType;
  bool flag = true;
   
  printf("Choose the task:\n");
  printf("Press 1 for the equation decision\n");
  printf("Press 2 for calculation the sum of numbers\n");
  printf("Press 3 for factorial calculation (rec)\n");
  printf("Press 4 for factorial calculation (cyc)\n");
  printf("Press 5 for exit\n");
  while(flag)
  {
  try {
       
      if (scanf ("%d", &chType)==0) throw chType;
      }
      catch (...)
   {
           printf ("Warning! Wrong number!\n");
   }
  }
    
   switch(chType)
   {
   case 1:
    {
     MainEquation();
     break;
    }
   case 2:
    {
     MainSum();
     break;
    }
   case 3:
    {
     MainFactorialRec();
     break;
    }
   case 4:
    {
     MainFactorialCyc();
     break;
    }
   case 5:
    {
     break;
    }
   }
  getch();
  return 0;
}

char ch;     
ch = getchar();

switch(ch)
{
    case '1':
       ...
       break;
    case '2':
       ...
       break;
}

#include <stdio.h>
#include <conio.h>
#include <math.h>
#include <string.h>

int factorial (int n);
void MainEquation();
void MainSum();
void MainEquation();
void MainFactorialCyc();

float ask (char *ParameterName) //Запрос параметров дял квадратного уравнения
{
  float fResult;
  printf("Input %s value:\n", ParameterName);
  if (scanf("%f", &fResult)<=0)  
  {
   printf ("Warning! Wrong number!");  
   return 0;
  }
  else  
  return fResult;
}

void Calculate (float a, float b, float c) //Вычисление его корней

{
  float discrim, fRadical, sRadical;
  discrim=(b*b)-(4*a*c);
    
   if (discrim<0)
   {
    printf("Radicals are not present\n");
   }
   else
   {
    if (discrim==0)
    {
     fRadical=(-1*b)/(2*a);
     printf("Unicue radical = %f\n", fRadical);  
    }
    else
    {
     fRadical=((-1*b)-sqrt(discrim))/(2*a);
     sRadical=((-1*b)+sqrt(discrim))/(2*a);
     printf("1st radical = %f\n", fRadical);
     printf("2nd radical = %f\n", sRadical);
      
    }
   }
}
void MainEquation() //Квадратное уравнение с 2 функциями
{
  float a,b,c;
  int change;

  do
  {
   a=ask("first");
   if (a==0){break;}
   b=ask("second");
   if (b==0){break;}
   c=ask("third");
   if (c==0){break;}
   Calculate(a,b,c);  
   printf ("The decision is ended, you wish to solve one more equation?\n");
   printf("1 - for continue, 2 - for exit\n");
   if (scanf("%d", &change)<=0)  
   {
    printf ("Warning! Wrong number!");  
    break;
   }
   else  
    {
     while(change!=1 && change!=2)
     {
      printf("Not distinguished command\n");
      printf("Try once again\n");
      if (scanf("%d", &change)<=0)  
      {
       printf ("Warning! Wrong number!");  
       break;
      }  
     }
    }
   if (scanf("%d", &change)<=0)  
   {
    break;
   }  
  }while (change!=2);
}

void MainSum() //подсчет суммы чисел
{
  int i,n, Sum=0,change;
  do
  {
   printf("Input your number\n");
   if (scanf("%d", &n)<=0)  
   {
    printf ("Warning! Wrong number!\n");  
    break;
   }
   if(n<0)
   {
    printf("Try once again\n");
   }
   else
   {
    for (i=0; i<=n; i++)
    {
     Sum+=i;
    }
   }
   printf("The sum of all numbers = %d\n", Sum);
   printf ("The decision is ended, you wish to solve one more equation?\n");
   printf("1 - for continue, 2 - for exit\n");
   if (scanf("%d", &change)<=0)  
   {
    printf ("Warning! Wrong number!");  
    break;
   }
   else  
    {
     while(change!=1 && change!=2)
     {
      printf("Not distinguished command\n");
      printf("Try once again\n");
      if (scanf("%d", &change)<=0)  
      {
       printf ("Warning! Wrong number!");  
       break;
      }  
     }
    }
   if (scanf("%d", &change)<=0)  
   {
    break;
   }  
  }while (change!=2);
}

int factorial(int n)
{
   if (n<0)  
   {
    printf("Kill yourself, stupid user!\n");
    return -1;
   }
    return (n>1) ? (n*factorial(n-1)) : 1; //сравнивается с 0
}

void MainFactorialRec()//Рекурсивный факториал
{
  int n, Factorial,change;
  do
  {
   do
   {
    printf("Input your number\n");
    if (scanf("%d", &n)<=0)  
    {
     printf ("Warning! Wrong number!\n");  
     break;
    }
    Factorial = factorial (n);
    if (Factorial == -1)
    {
     printf("Try once again\n");
    }
   }while(Factorial==-1);
   if (scanf("%d", &n)<=0)  
   {  
    break;
   }
   printf("Calculation is ended\n");
   printf("Factorial = %d\n", Factorial);
   printf ("The decision is ended, you wish to solve one more equation?\n");
   printf("1 - for continue, 2 - for exit\n");
   if (scanf("%d", &change)<=0)  
   {
    printf ("Warning! Wrong number!");  
    break;
   }
   else  
    {
     while(change!=1 && change!=2)
     {
      printf("Not distinguished command\n");
      printf("Try once again\n");
      if (scanf("%d", &change)<=0)  
      {
       printf ("Warning! Wrong number!");  
       break;
      }  
     }
    }
   if (scanf("%d", &change)<=0)  
   {
    break;
   }  
  }while (change!=2);

}

void MainFactorialCyc()
{
  int i,n, Factorial=1,change;
  do
  {
   printf("Input your number\n");
   scanf("%d", &n);
   for (i=1; i<=n; i++)
   {
    Factorial=Factorial*i;
   }
   printf("Calculation is ended\n");
   printf("Factorial = %d\n", Factorial);
   printf ("The decision is ended, you wish to solve one more equation?\n");
   printf("1 - for continue, 2 - for exit\n");
   if (scanf("%d", &change)<=0)  
   {
    printf ("Warning! Wrong number!");  
    break;
   }
   else  
    {
     while(change!=1 && change!=2)
     {
      printf("Not distinguished command\n");
      printf("Try once again\n");
      if (scanf("%d", &change)<=0)  
      {
       printf ("Warning! Wrong number!");  
       break;
      }  
     }
    }
   if (scanf("%d", &change)<=0)  
   {
    break;
   }  
  }while (change!=2);
}

int main()
{
   
  char chType;
  bool flag = true;
   
  printf("Choose the task:\n");
  printf("Press 1 for the equation decision\n");
  printf("Press 2 for calculation the sum of numbers\n");
  printf("Press 3 for factorial calculation (rec)\n");
  printf("Press 4 for factorial calculation (cyc)\n");
  printf("Press 5 for exit\n");
  while(flag)
  {
   try  
   {
    chType=getch();
    if (chType>1) throw chType;
    switch(chType)
    {
    case '1':
     {
      MainEquation();
      break;
     }
    case '2':
     {
      MainSum();
      break;
     }
    case '3':
     {
      MainFactorialRec();
      break;
     }
    case '4':
     {
      MainFactorialCyc();
      break;
     }
    case '5':
     {
       
      break;
     }
    }
   }
   catch (...)
   {
    printf ("Warning! Wrong number!\n");
   }
  }
  getch();
  return 0;
}