Code skips data entry of char input 1st loop of do while

I thought I would use a do-while because it always loops once anyway
After a numberic value is entered I thought it would prompt for the char entry.
But it is falling all the the way thru; failing and asking a second time giving the correct results.
I would like it to stop on the first go round and get the char input.
What am I doing wrong on this? Am I not understanding the do while process properly?

Code:

 /*
 * Convert Metric Prefix to numeric value
 */

#include <stdio.h>     
#include <math.h>
					
int main(void)

{
	double value, result, exitvalue; // stores numerical value  
	exitvalue=0,  value=0, result=0;   // intitialize variables                      
	char  prefix;  // stores letter value of prefix 
         
	//  ask for and get numeric value 
	printf("Please enter numeric value to convert:");
	scanf("%lf",  &value );

		//test print out of value and power
		printf("the value entered is:%f \n\n", value);
		result = value*pow(10,9);
		printf("the result is:%f \n\n", result);
  	do
	{
	//  ask for prefix  
	printf("Please enter the prefix in the following manner G,M,k,m,u,n,p: \n");
	printf("Please enter the prefix you wish to convert:\n ");
	scanf("%c",  &prefix);

	// evaluate prefix with switch
		switch (prefix)
			{
			case 'G' :
			
				result = value*pow(10,9);
				printf("The conversion is %f\n", result);
				exitvalue=1;
				break;
			
			case 'M' :
			
				result = value*pow(10,6);
				printf("The conversion is %f\n", result);
				exitvalue=1;
				break;
			
			case 'k' :
			
				result = value*pow(10,3);
				printf("The conversion is %f\n", result);
				exitvalue=1;
				break;
			
			case 'm' :
			
				result = value*pow(10,-3);
				printf("The conversion is %f\n", result);
				exitvalue=1;
				break;
			
			case 'u' :
			
				result = value*pow(10,-6);
				printf("The conversion is %f\n", result);
				exitvalue=1;
				break;
			
			case 'n' :
			
				result = value*pow(10,-9);
				printf("The conversion is %f", result);
				exitvalue=1;
				break;
			
			case 'p' :
			
				result = value*pow(10,-12);
				printf("The conversion is %f\n", result);
				exitvalue=1;
				break;
			
			default :
				exitvalue=0;
				printf("Please rerun, Invalid entry.\n");
			}
	}
	while(exitvalue==0);
		
return (0);
}

FAQ.

The newline is still in the standard input buffer from when you pressed enter to send the input.

The issue here is that you are scanning for the %lf value on the lines before and you hitting enter leaves the '\n' in the buffer, which the next scanf call picks up as an entry.

So you'll need to clear the input buffer.

You can use a while loop to do it in-between like this:

Code:

while( getchar() != '\n' );

Do I insert this before the do statement or inside at the top of the do while?

Thanks
Jack

Thanks for the help! I place it above the do statment it worked fine.

I am new to c programming and I am taking my first intro to C class
this was not covered in any of my materials yet.

I appreciate the help.