Please help a n00b - several issues with my program

[lok1vip]

PLEASE HELP ASAP!! I have only been learning C for about a month so I am not as familiar with the terms and concepts as I would like to be. I know that there are actually more issues than I have mentioned; particularly with my assignment operators. I know I need to have some variables with asterisks in my function prototypes and calls but I am not sure when, where and why to use the asterisk and ampersand in functions.

This program is supposed to help a bookstore estimate its business.
I am required to: 1. Use a defined constant for the rate of profit.
2. Input should not be case-sensitive, so I have used a tolower() function. If the input is erroneous I am supposed to use an exit() function, which I am not sure I have done correctly.
3. Function to calculate # of books to be ordered, function to calculate profit, and function to output everything.

The things I need help with are:
1. Something is wrong with my logic statements for whether the book is suggested, required, new or old and I am unsure of what it is.

2. How do I use the exit functions correctly so that if someone enters 'Q' when it asks for 'R' or 'S' the program aborts?

3. I get a warning on line 70: "Passing argument 1 of 'profit' makes integer from pointer without a cast". I have very little understanding of what this means and absolutely no idea how to fix it.

4a. What am I supposed to put for the return parameter for my 'output' function? It is supposed to output three print statements.
4b. When calling output() in main, what am I supposed to put inside the parentheses?

Thanks in advance!!!

-LowKey Variation in Production

#include <stdio.h>
#include <stdlib.h>
#define STORE_PROFIT .2

//Function Prototypes
int books(double, char, char);
float profit(int, float);
void output(int num);

int main(void)
{
//Declare Variables
    //ISBN
        char i1;
        char i2;
        char i3;
        char i4;
        char i5;
        char i6;
        char i7;
        char i8;
        char i9;
        char i10;
    //Text Code
        char rs;    //Required or Suggested
        char r;     //Required
        char s;     //Suggested
        char no;    //New or Old
        char n;     //New
        char o;     //Old
    //Other
        double lp;   //List Price
        double ee;   //Expected Enrollment

//Input
    printf("Enter ISBN number: ");
    scanf(" %c %c %c %c %c %c %c %c %c %c", &i1, &i2, &i3, &i4, &i5, &i6, &i7, &i8, &i9, &i10);
    printf("Enter price per copy: ");
    scanf(" %lf", &lp);
    printf("Enter expected class enrollment: ");
    scanf(" %lf", &ee);
    printf("Enter 'R' if required or 'S' if suggested: ");
    scanf(" %c", &rs);
    rs = tolower(rs);
    if (rs = r)
        r = rs;
        else if (rs = s)
                s = rs;
        else exit(0);
    printf("Enter 'N' if new or 'O' if not a new text: ");
    scanf(" %c", &no);
    no = tolower(no);
    if (no = n)
        n = no;
        else if (no = o)
                o = no;
        else exit(0);

//Compute
books (ee, rs, no);
profit (books, STORE_PROFIT);

//Output
output (0);


    return 0;
}//main---------------------------------------------------------------------------------------------------------------------




//Number of Books Function
int books(double expect, char rs, char no)
{
//Pre:  Expected Enrollment and whether r/s and n/o
//Post: Books needed

//Local Declarations
        char r;     //Required
        char s;     //Suggested
        char n;     //New
        char o;     //Old
        int ex;   //Expected Enrollment
        int need; //Books needed

//Logic Statements
    if (rs = r)
        if (no = n)
            need = .9 * ex;
        else if (no = o)
            need = .65 * ex;

    else if (rs = s)
        if (no = n)
            need = .4 * ex;
        else if (no = o)
            need = .2 * ex;


return need;
}//number of books function-------------------------------------------------------------------------------------------------




//Calculate Profit Function
float profit(int needed, float stoprof)
{
//Pre:  List price & Copies needed
//Post: Bookstore's profit

//Local Declarations
    float prof; //Profit
    float sdlp; //Store discount off list price
    int need;   //Books needed

//Compute
    prof = sdlp * need;



return prof;
}//profit function-----------------------------------------------------------------------------------------------------------



//Output Function
void output(int num)
{
//Pre: Data
//Post: Output ISBN, copies needed and profit

//Local Declarations
        char i1;
        char i2;
        char i3;
        char i4;
        char i5;
        char i6;
        char i7;
        char i8;
        char i9;
        char i10;
        int need;
        float prof;


printf("\n\nISBN: %c-%c%c-%c%c%c%c%c%c-%c", i1, i2, i3, i4, i5, i6, i7, i8, i9, i10);
printf("Copies Needed: %d", need);
printf("Profit: $6.2f", prof);


}//output function-----------------------------------------------------------------------------------------------------------

[TheBigH]

3) Try one of the following:

books_needed = books(ee, rs, no);
profit (books_needed, STORE_PROFIT);
/*OR*/
profit ( books(ee, rs, no), STORE_PROFIT);

books(ee, rs, no) is an integer equal to the return value of the function. books with no parantheses gives you the memory location the function is stored at.


4a) "output" is a void function, which means it doesn't return anything. Such functions are ended with a "return;". You seem to be confusing the screen output from the printf statements for the return value; they're not the same thing.