What is the correct initialization of a variable and subsequent order of a function in a sentinel-controlled loop?

Question

I am new to C++, I wrote this code for practice and can't seem to understand why my code still loops after I input the sentinel value.

Sample output: Please enter your sales amount or -1 to exit: 25100 Your salary for the month is:$31110.00 Please enter your sales amount or -1 to exit: -1 Your salary for the month is:$3499.00 –

Here is my code:

#include <stdio.h>
#include <math.h>
#define MIN 25000
#define COMP .10
#define BASPAY 3500
//Function Prototypes
double MonthSales(void);
double CalcPay(double sales);

//Begin Main
int main(void)
{
//declare variables
    double sales = 0;
// Begin Program with sentinel value for exit
    while (sales != -1) 
    {
        sales = MonthSales();
        printf("Your salary for the month is:$%.2lf\n", CalcPay(sales));
//The line above still runs after I input -1 :S.
    }
}
//Function for sales
double MonthSales(void) 
{
    double sales;
    printf("Please enter your sales amount or -1 to exit:\n");
    scanf("%lf", &sales);
    return sales;
}

//Function for total Pay
double CalcPay(double sales) 
{
    double pay;
    double commission;
    if (sales > MIN) 
    {
        commission = sales * COMP;
        pay = commission + sales + BASPAY;
    } 
    else 
    {
        pay = sales + BASPAY;
    }
    return pay;
}

Answer 1

In this loop:

while (sales != -1) 
    {
        sales = MonthSales();
        printf("Your salary for the month is:$%.2lf\n", CalcPay(sales));
    }

the control expression sales != -1 is evaluated before each iteration of the loop. It is not evaluated continuously or at points inside the iterations. The loop does not automatically stop the moment sales != -1 becomes false.

Instead, you must test the value of sales at the point where you want the control flow to change. For example, you can do this:

while (1)
{
    sales = MonthSales();
    if (sales == -1)
        break;
    printf("Your salary for the month is:$%.2lf\n", CalcPay(sales));
}