Need some help doing error checking

Hi everybody,

I am in an introduction to C class and here is a program we had to do. Now, what I am trying to do is expand on it a little more and do some error checking. So, if the user inputs something other than the correct amount or the correct store I want it to loop back up and ask again. Can someone help, I have tried using a for loop but I haven't been able to get it to work correctly.

Code:

---

#include <stdio.h>

// function prototypes
float tax_delmar(float);
float tax_encinitas(float);
float tax_lajolla(float);
float add_total(float, float);

// global variables that can be used by any function
float sales_tax;
float total;


main()

{

        //defining and initializing data type
        float purchase_amt = 0.0;
        char  store = '0';
       
       
        printf("\nWhat is your purchase amount? $");
        //receiving user input for the purchase amount
        scanf("%f", &purchase_amt);
       
        printf("\nWhat store?\n");
        printf("\nPress D for DelMar, E for Encinitas, or L for LaJolla: ");
        scanf("%c", &store);

       
        if(purchase_amt >= 0.0 && purchase_amt <= 1000.0 && store == 'd')
        {
        //sales tax for the Del Mar store rate
        printf("\n\nSales tax for $%.2f at the DelMar store is: $%.2f\n", purchase_amt, tax_delmar(purchase_amt));
        //total price with sales tax included for the Del Mar store
        printf("\nThe total purchase price at the DelMar store is = $%.2f\n\n\n", add_total(sales_tax, purchase_amt));
        }
        else if(purchase_amt >= 0.0 && purchase_amt <= 1000.0 && store == 'e')
        {
        //sales tax for the Encinitas store rate
        printf("\nSales tax for $%.2f at the Encinitas store is: $%.2f\n", purchase_amt, tax_encinitas(purchase_amt));
        //total price with sales tax included for the Encinitas store
        printf("\nThe total purchase price at the Encinitas store is = $%.2f\n\n\n", add_total(sales_tax, purchase_amt));
        }
        else if(purchase_amt >= 0.0 && purchase_amt <= 1000.0 && store == 'l')
        {
        //sales tax for the La Jolla store rate
        printf("\nSales tax for $%.2f at the LaJolla store is: $%.2f\n", purchase_amt, tax_lajolla(purchase_amt));
        //total price with sales tax included for the La Jolla store
        printf("\nThe total purchase price at the LaJolla store is = $%.2f\n\n\n", add_total(sales_tax, purchase_amt));
        }
       
        printf("\nPress return to end program!\n");
       
        //pausing program until user ends program
        getch();
        return;
       
}       

//calculate the sales tax on the purchase amount for the Del Mar store       
float tax_delmar(float purchase_amt)
{
        sales_tax = purchase_amt * 0.0725;
        return sales_tax;
}
//calculate the sales tax on the purchase amount for the Encinitas store
float tax_encinitas(float purchase_amt)
{
        sales_tax = purchase_amt * 0.075;
        return sales_tax;
}
//calculate the sales tax on the purchase amount for the La Jolla store       
float tax_lajolla(float purchase_amt)
{
        sales_tax = purchase_amt * 0.0775;
        return sales_tax;
}
//add up a total for the purchase amount plus the sales tax for that amount
float add_total(float sales_tax, float purchase_amt)
{
        total = sales_tax + purchase_amt;
        return total;
}

---

Your choice for loops are:
for, while, and do

I'll do the easier comparison first, while and do.
A while looks like

Code:

---

while (condition) {
  statement(s);
}

---

So, if condition isn't true to begin with, the loop will never be executed.

A do loop looks like

Code:

---

do {
  statement(s);
} while (condtion);

---

So the statement(s) in the body of a do loop with always be executed at least once. If you want to read in some input and loop until it satisfies some criteria, you may often want a do loop.

The for loop looks something like:

Code:

---

for(statement1, condition, statement2) {
  statement(s);
}

---

and is the same as (stealing from K&R)

Code:

---

statement1;
while (condition) {
  statement(s);
  statement2;
}

---

So, a for is just a special kind of while loop. The context of the problem usually indicated which is better.

Hope that helps some.

Don't forget that scanf will leave stuff in the input buffer if the user enters incorrect information.
Therefore, the easiest approach to this problem is typically to read a string, then convert to appropriate data type and check if the conversion succeeds. If it doesn't, then it's a bad input.
String to float -> strtof.
And for the char, just grab the first character in the string you read.
For reading strings, use fgets.

University of Phoenix assignment? Seems I've seen this before...

Thanks everybody! Yes, it's for UOP.