I'm trying to write a program that will accept input from the user as sales, then it will total the sales and display both the total and the sales tax (total multiplied by .0825

#include <stdio.h>
#include <stdlib.h>
#define SIZE 9
int main(void)
{
    /*initialize */
 int a[SIZE]= {0};
 int  total, sales, tax;
/*input data */
 printf ( "Enter sales (enter -1 to end):$ ");
        scanf ("%d", &sales );
 while ( sales != -1 ){
       total = sum_array(a,SIZE) ;
       tax = total * .0825;
 printf ("Enter sales in a week(enter -1 to end):$ ");
        scanf ( "%d", &sales );
 }
 printf("total =  %d\n", sales);
 printf("sales tax= %d\n", total);
  system("PAUSE");	
  
}
int sum_array(int a[], int num_elements)
{
   int i, sum=0;
   for (i=0; i<num_elements; i++)
   {
	 sum = sum + a[i];
   }
   return(sum);
}

output

Enter sales (enter -1 to end):$ 100
Enter sales in a week(enter -1 to end):$ 200
Enter sales in a week(enter -1 to end):$ 200
Enter sales in a week(enter -1 to end):$ -1
total = -1
sales tax= 0
Press any key to continue . . .

actualy its just part of a like 6 part program and these two parts im having trouble with, im combining them all into one, the other 4 i have done and working

and obviously i've put work into it, its not like im one of those people just asking for a free answer

Your use of int is going to be a problem first. Rounding comes into play here. You should do your multiplications in double or float.

It appears you may also want to store each of the sales input into the array a. Hence you should first get all of the sales, assign each input to a. You also need to make sure that no more that 9 inputs are entered, so you should keep a count of each input and terminate input when either salses = -1 of input_count >= 9. Reason is if more than 9 is entered, then you will overflow the array a and cause a bad memory access. ie: program crashesm if user enters 10 or 11 inputs.

other issues also with summing up the sales.
Have a loop that first gets all inputs:

float a;
int sales_count = 0;
while(sales != -1 && sales_count < 9) {
printf("\tInput sale item %d", sales_count);
scanf("%f", &sales);
a[sales_count] = sales;
}

// now you can call the average function and average array a, etc.

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