Btukfyl

In this code I have several querys for user input. If there is an invalid input like 'r' instead of 4, I want my programm to say "invalid input" and ask for another user input. I tried a lot but I couldn't get it to work.
I commented the problematic locations in the code. Thanks for help.

#include <stdio.h>



int main()

{ 

double Operand1;

double Operand2;

int Menuchoice; 

int Input;

char Dummy;

double Result;



do

{



  printf("Simple Calculatorn");

  printf("========================n");

  printf("n");

  printf("1. Additionn");

  printf("2. Subractionn");

  printf("3. Multiplicationn");

  printf("4. Divisionn");

  printf("9. Quitn");





  Input = scanf("%i", &Menuchoice);  // At this point I want to check if there is a valid input and 

  do scanf("%c", &Dummy);            //  if not the programm should ask again

  while (Dummy != 'n');



  if(Input)

  {

     switch(Menuchoice)

     {

        case 1: printf("Type in the first operand:n");

                scanf("%lf", &Operand1)                     // Here I want to validate the input         

                printf("Type in the second operand:n");    // again and the programm should also ask 

                scanf("%lf", &Operand2)                     // again if it was invalid

                printf("%lf + %lf = %lfn", Operand1, Operand2, Result);

                break;

        case 2: 

        case 3: 

        case 4: 

        default: printf("No valid input!n");

                break;

     }

  }



}while (Menuchoice != 9);



 return 0;

}

Manual page of scanf:

On success, these functions return the number of input items successfully matched and assigned; this can be fewer than provided for, or even zero, in the event of an early matching failure.

So here is a sample which could lead you to solve your problem:

#include <stdio.h>



int main (int argc, char* argv)

{

  double o;

  int res;



  // To illustrate, I chose to set up an infinite loop.

  // If the input is correct, we'll "break" it

  while(1)

    {

      printf("Enter a double: ");

      res = scanf("%lf",&o);



      // Success = 1 read input

      if (res == 1)

      {

        printf("Yahoo, got it right: %fn",o);

        break; // We exit the loop

      }



      // Ah, we failed

      printf("Please retry.n");

      // popping the CR character to avoid it to be got by the next scanf()

      getchar();



      // Here we go for another loop.

    }



    // Good, we got our double.

    printf("Hey, sounds like we got outside this infinite loop.n");

}

Example:

user@so:~$ ./a.out 

Enter a double: r

Please retry.

Enter a double: f

Please retry.

Enter a double: 6.543

Yahoo, got it right: 6.543000

Keep in mind this check is not perfect. For example, "frg6sgg" will success and be displayed as 6.0000000 by printf().