While loop

[f4wardMomentum]

I was wondering why in my determine_teacher function the printf call executes twice every time I enter an invalid character?

  #include<stdio.h> /* for printf and scanf */
 
 /* Macro constants */
 #define UNDER_HUN_DISC .10
 #define OVER_HUN_DISC .12
 #define TAX_RATE .05
 
 /* function prototypes */
 int determine_teacher(char user_response); /* prompt user for total and if teacher */
 
 double calculate_discount(double sale_total); /* if teacher, calculate the discount */
 
 /* program entry point */
 int main(void)
 {
 	/* variable dclarations */
 	char teacher_response; /* input, response from user if teacher */
 	double purchase_total; /* input, stores sales total */
 	
 	int teacher; /* store customer status 0(false) not teacher 1(true) teacher */
 	double discount_amount; /* if customer is a teacher stores the discount amount */
 	double discounted_total; /* purchase total minus discount amount */
 	double tax_total; /* stores the total of the sales tax */
 	double trans_total; /* transaction total */
 	
 	/* prompt user for input */
 	printf("Is customer a teacher(Y/N)?");
 	scanf("%c", &teacher_response);

 	teacher = determine_teacher(teacher_response);
 	
 	if(teacher)
 	{
 		printf("Teacher");
 	}
 	else
 	{
 		printf("Not Teacher");
 	}
 	
 	
 	
 }/* end of main() */
 
 /* function that prompts user for input and return if teacher or not */
 int determine_teacher(char input_response)
 {
 	 while(!(input_response == 'Y' || input_response == 'N' || input_response == 'n' || input_response == 'y'))
 	 {
 	 	printf("invalid");
 	 	scanf("%c", &input_response);
 	 }
 	 switch(input_response)
 	 {
 	 	case 'Y':
 	 	case 'y':
 	 		return 1;
 	 	case 'N':
 	 	case 'n':
 	 		return 0;
 	 }
			 	
 }/* end of determine_teacher() */

[Branflakes91093]

It looks like when execution reaches line 51, scanf reads the newline character from the previous input.

You could either clear the input buffer before proceeding: while ((ch = getchar()) != '\n' && ch != EOF);

Or, add a space before the %c in the scanf call on line 51. That will make scanf ignore all leading whitespace (including any newline characters). I'm unsure if it will skip carriage return '\r', though.