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;
}

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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

Quote:

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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?

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I have heard that MSVC++ 2005 express complains whenever you use scanf() or sprintf() or other standard C functions. So yes, it's normal. I didn't know that you could disable it with a #define, though.

Code:

---

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

---

You read in gallons, then go on to process it, assuming it is a valid number; then, after all of the calculations have been performs, and only then, is the expression in the while evaluated and found to be false.

You could fix this with a break statement:

Code:

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if(gallons == -1) break;

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Or you could wrap if(gallons != -1) around the following code.

[edit] A bit late, but oh well. [/edit]

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 );

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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:

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

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