How to make this program ask for input again if invalid input is entered (in C programming)? [duplicate]

https://stackoverflow.com/questions/42464437/how-to-make-this-program-ask-for-input-again-if-invalid-input-is-entered-in-c-p?fbclid=IwAR1BB7wkcELX0D_GkWAOVPe7m3of-gWMo60PztXTV1YqFD6jijQy1oYkR70

Loop Infinite

#include <stdio.h>
#include <ctype.h>

int main(void)
{
    char ch;
    float num1,num2,answer;
    printf("Enter the operation of your choice:\n");
    printf("a. add      s. subtract\n");
    printf("m. multiply q. divide\n");
    printf("q. quit\n");
    while ((ch = getchar())!='q')
    {
        ch=tolower(ch);
        //Ignore whitespace
        if (ch=='\n')
            continue;
        else
        {
            switch(ch)
            {
                //Addition part
                case 'a':
                    //First number
                    printf("Enter first number: ");
                    //Check to see if input is a number
                while (scanf("%f",&num1)==0)
                {
                    printf("Invalid input. Please enter a number, such as 2.5, -1.78E8, or 3: ");
                    scanf("%*s");
                }
                //Second number
                printf("Enter second number: ");
                while (scanf("%f",&num2)==0)
                {
                    printf("Invalid input. Please enter a number, such as 2.5, -1.78E8, or 3: ");
                    scanf("%*s");
                }
                //Do math for respective operation
                answer = num1 + num2;
                //Print out result
                printf("%.3f + %.3f = %.3f\n", num1,num2,answer);
                break;
            //Subtraction part
            case 's':
                printf("Enter first number: ");
                while (scanf("%f",&num1)==0)
                {
                    printf("Invalid input. Please enter a number, such as 2.5, -1.78E8, or 3: ");
                    scanf("%*s");
                }
                printf("Enter second number: ");
                while (scanf("%f",&num2)==0)
                {
                    printf("Invalid input. Please enter a number, such as 2.5, -1.78E8, or 3: ");
                    scanf("%*s");
                }
                answer = num1 - num2;
                printf("%.3f - %.3f = %.3f\n", num1,num2,answer);
                break;
            //Multiplication part
            case 'm':
                printf("Enter first number: ");
                while (scanf("%f",&num1)==0)
                {
                    printf("Invalid input. Please enter a number, such as 2.5, -1.78E8, or 3: ");
                    scanf("%*s");
                }
                printf("Enter second number: ");
                while (scanf("%f",&num2)==0)
                {
                    printf("Invalid input. Please enter a number, such as 2.5, -1.78E8, or 3: ");
                    scanf("%*s");
                }
                answer = num1 * num2;
                printf("%.3f * %.3f = %.3f\n", num1,num2,answer);
                break;
            //Division part
            case 'd':
                printf("Enter first number: ");
                while (scanf("%f",&num1)==0)
                {
                    printf("Invalid input. Please enter a number, such as 2.5, -1.78E8, or 3: ");
                    scanf("%*s");
                }
                printf("Enter second number: ");
                while (scanf("%f",&num2)==0)
                {
                    printf("Invalid input. Please enter a number, such as 2.5, -1.78E8, or 3: ");
                    scanf("%*s");
                }
                //Check for if number is a zero
                while (num2==0)
                {
                    printf("Please enter a non-zero number, such as 2.5, -1.78E8, or 3: ");
                    while (scanf("%f",&num2)==0)
                    {
                        printf("Invalid input. Please enter a number, such as 2.5, -1.78E8, or 3: ");
                        scanf("%*s");
                    }
                }
                answer = num1 / num2;
                printf("%.3f / %.3f = %.3f\n", num1,num2,answer);
                break;
            //For if a non-valid operation is entered
            default:
                printf("That is not a valid operation.\n");
                break;
        }
    }
    printf("Enter the operation of your choice:\n");
    printf("a. add      s. subtract\n");
    printf("m. multiply q. divide\n");
    printf("q. quit\n");
}
printf("Bye.\n");
return 0;}