Thread: Problem with sentinel values

I am working independently through Deitel's How to Program C and I am having some difficulty with sentinel values. The problem is that the sentinel value doesn't stop the program, but instead uses the sentinel value( in this case -1) in the calculation. This leads to averages that are negative or far from the correct value. Please help. Here is what I have so far, if it helps.

also - I am using visual c++ 2005 express addition, and i am constantly having to insert the first piece of code (#define _CRT_SECURE_NO_DEPRECATE)
because visual c++ wants me to use scanf_s instead of scanf. Is this normal?

Code:

#define _CRT_SECURE_NO_DEPRECATE
#include <stdio.h>

int main( void )
{
	int miles, gallons, totalmiles, totalgallons;
	float mpg, totalmpg;

	totalmiles = 0, totalgallons = 0;

	printf( "Enter the gallons used (-1 to end): " );
	scanf( "%d", &gallons );

	totalgallons = totalgallons + gallons;

	printf( "Enter the miles driven: " );
	scanf( "%d", &miles );

	totalmiles = totalmiles + miles;

	mpg = ( float )miles / gallons;
	
	printf( "The miles/gallon for this tank was %.6f\n", mpg );

	while ( gallons != -1 ) {
		printf( "Enter the gallons used (-1 to end): " );
		scanf( "%d", &gallons );
		
		totalgallons = totalgallons + gallons;
		
		printf( "Enter the miles driven: " );
		scanf( "%d", &miles );

		totalmiles = totalmiles + miles;

		mpg = miles / gallons;
		printf( "The miles / gallon for this tank was %.6f\n", mpg );
	}
		
	if ( totalmiles != 0 ) {
		totalmpg = ( float )totalmiles / totalgallons;

		printf( "The overall average miles/gallon was %.6f\n", totalmpg );
	}
	else {
		printf( "No gallons were entered\n" );
	}

	return 0;
}

Yes, of course it uses the -1 in the calculation (and it asks for a number of miles too, right?)

You need to add a check to see if the gallons is -1 and only do the rest of the loop if it is. A while-loop doesn't test the value at every instance the value is changed, only when it reaches the top of the loop.

--
Mats

Code:

	printf( "Enter the gallons used (-1 to end): " );
	scanf( "%d", &gallons );

	totalgallons = totalgallons + gallons;

	printf( "Enter the miles driven: " );
	scanf( "%d", &miles );

	totalmiles = totalmiles + miles;

	mpg = ( float )miles / gallons;
	
	printf( "The miles/gallon for this tank was %.6f\n", mpg );

It looks like you've duplicated this code twice, once outside the loop and once inside. If you get rid of the instance outside the loop, you could then add a check for

Code:

if (gallons < 0) {
    printf ("exiting\n"); 
    break; 
}

just after scanf. You should avoid code duplication wherever possible!