How do i make a program loop until valid choice is given?

Question

My code for C goes as :

/*************************************************************************
******* Program to Calculate either Area or perimeter of given input ****
******* This program was created by Naveen Niraula and is under beta ****/

#include <stdio.h>
#include <conio.h>
#include <windows.h>

int main()
{
/***** Setting title of the console window *****/
SetConsoleTitle("Area or Primeter v 0.1 [beta]");

/***** creating choice to use for switch() and declaring variables to use later *****/
char choice;
int length,breadth,area,perimeter;
printf("\t************************************************************\n");
printf("\t*********** Program to calculate Area or Perimeter *********\n");
printf("\t************************************************************\n\n\n");

/**** getting value of length and breadth from user ****/
printf("\tEnter the Length\n");
printf("\t-> ");
scanf("%d",&length);
printf("\n\tEnter the Breadth");
printf("\n\t-> ");
scanf("%d",&breadth);

/**** Asking for Area or Perimeter choice ****/
printf("\n\tEnter a for Area or P for Perimeter\n");
printf("\t-> ");
scanf("   %c",&choice);
printf("\n");

/***** using switch to calculate Area or Perimeter ******/
switch (choice)
{
/**** for Area ****/
case 'A':
case 'a':
    area = length * breadth;
    printf("\tLength = %d | Breadth = %d | Area = %d\n",length,breadth,area);
break;

/***** for Perimeter *****/
case 'P':
case 'p':
    perimeter = 2 * (length + breadth);
    printf("\n");
    printf("\tYou entered Length = %d | Breadth = %d | 
Perimeter = %d\n",length,breadth,perimeter);
    break;

    /**** if the input is not Valid ****/
default :
    printf("\n");
    printf("\tInvalid Choice");
    printf("\n");
}

/***** pausing the system ****/
getch();
}

My question is how to automatically loop the code if the user enters a invalid choice until he enters the correct choice once either A or P.

And also what is the statement that must be used for clearing screen clrscr() doesnt seem to work.

Answer 1

char choice; // this must be outside the loop, to use it after the loop

bool valid_choice = true; // suppose the choice is correct

do // this starts a do-while construction: here we will return if we need to repeat thing
{
    ask for choice // copy here your code that asks for the user choice

    switch (choice) 
    {

    valid cases // here go your "case 'A'" etc., not to copy it from your question

    default:
        give your warnings // here you give a warning as in your question

        valid_choice = false; // we decide that no, the choice was not valid
        break;
    }

}
while (!valid_choice) // this will return the control to "do" (repeat the whole thing) only if valid_choice is NOT true

// here we come if valid_choice was true

act according to choice