quick help for weird problem.

[Jeffrey Barrett]

Can someone tell me why I'm having this problem:
When I select drink 5 and deposit too little money, there's a problem
with the 'difference' variable. Try to deposit $.80 instead of the
full $.85 and see what happens if you then keep entering .01 until you
have added the full .85. The program still asks the user to deposit
$0.00. It is telling me for some reason that 0.850000 - 0.850000 =
5.96046e-008.
Any suggestions?

/*
machine.txt:
------------------
Cola
0.75 20
Ruby Red Blast
1.00 10
Lemon Fizz
0.75 8
Grape Soda
0.90 5
Citrus Flip
0.85 0
Habanero Surprise
0.80 11
-------------------
*/

/* ================================================================ */
#include <stdio.h>
#include <string.h>
#include <math.h>
/* ================================================================ */
#define MAXNUMDRINKS 6
#define STRINGSIZE 25
#define noGood 0
/* ================================================================ */

typedef char String [STRINGSIZE];
typedef enum bool { false, true } bool;

typedef struct drinkRec
{
String brand;
float price;
int quantity;
} drinkRec;

typedef drinkRec drinkList [ MAXNUMDRINKS ];

/* ================================================================ */

void ReadAllDrinks ( int* numDrinks, drinkList drinks );
void PrintDrinkMenu (int* menuChoice, int numDrinks, drinkList
drinks);
void GetNewQuantity (int menuChoice, drinkList drinks);
void GetMoney (int menuChoice, drinkList drinks);

/* ================================================================ */

int main ( )
{
drinkList drinks;
int numDrinks, menuChoice;
double profit = 0;
printf ("Welcome to the Soda Machine.\n");
ReadAllDrinks ( &numDrinks, drinks );
do
{
PrintDrinkMenu (&menuChoice, numDrinks, drinks);
if (menuChoice != 0){
GetMoney (menuChoice, drinks);
if (drinks[(menuChoice - 1)].quantity > 0)
profit = profit + .25;
GetNewQuantity (menuChoice, drinks);
}
}
while (menuChoice != 0);
printf("\n%s%.2f\n","Current Run's Profit: $", profit);
printf("\nBYE BYE\n\n");
return ( 0 );

}
/* ================================================================ */

void ReadAllDrinks(int *numDrinks, drinkList drinks)
{
String dummy;
float dummy2 = 0;
char *line;
FILE *inFile;
int count = 0;
if ((inFile = fopen("machine.txt", "r"))) {
while ((line = fgets(drinks[count].brand, STRINGSIZE, inFile))) {
drinks[count].brand[strlen(drinks[count].brand) - 1] = '\0';
fscanf(inFile, "%f%d",
&drinks[count].price,&drinks[count].quantity);
fgets(dummy, STRINGSIZE, inFile);
count++;
}
*numDrinks = count;
fclose(inFile);
}
else printf( "machine.txt not read\n");
}
/* ================================================================ */
void PrintDrinkMenu (int* menuChoice, int numDrinks, drinkList drinks)
{
int count, result;
String dummy;
printf("\n Drink Price Amount Left");
printf("\n ------------- ----- -----------\n");
for (count = 0; count < numDrinks; count++){
printf("%d. %-19s$%4.2f", (count + 1),
drinks[count].brand,drinks[count].price);
if (drinks[count].quantity < 1)
printf(" %8s","SOLD OUT\n");
else
printf(" %8d\n",drinks[count].quantity);
}
printf("\n0. Exit\n");
do
{
printf("\nPlease enter your selection: ");
result = scanf("%d", menuChoice);
if (result == noGood) /* This error checks for any input other
than an integer*/
scanf("%s", &dummy); /* If input is non-integer, force scanf to
stop looping */
}
while ((*menuChoice > numDrinks) || (*menuChoice < 0)|| (result ==
noGood));
}
/* ================================================================ */
void GetNewQuantity (int menuChoice, drinkList drinks)
{
int prevQuantity;
prevQuantity = drinks[(menuChoice - 1)].quantity;
drinks[(menuChoice - 1)].quantity = prevQuantity - 1;
}
/* ================================================================ */
void GetMoney (int menuChoice, drinkList drinks)
{
int result;
String dummy;
float moreMoneyIn, moneyIn, difference;
if (drinks[menuChoice - 1].quantity > 0){
do
{
printf("\n%s%.2f: $","Please deposit $",drinks[menuChoice -
1].price );
result = scanf("%f",&moneyIn);
if (result == noGood) /* This error checks for any input other
than an float*/
scanf("%s", &dummy); /* If input is non-float, force scanf to
stop looping */
}
while (result == noGood);
printf("\n%s$%.2f\n","You put in ",moneyIn);
difference = (drinks[menuChoice - 1].price) - moneyIn;
while (difference != 0){
if (difference > 0){
do
{
printf("%s%.2f: ","Please deposit an additional $",
fabs(difference));
result = scanf("%f", &moreMoneyIn);
if (result == noGood) /* This error checks for any input
other than an float*/
scanf("%s", &dummy); /* If input is non-float, force scanf
to stop looping */
}
while (result == noGood);
printf("\n%s$%.2f.\n","You put in ",moreMoneyIn);
moneyIn = moneyIn + moreMoneyIn;
difference = ((drinks[menuChoice - 1].price) - moneyIn);
}
if (difference < 0){
printf("%s%.2f","Your change is $", fabs(difference));
difference = 0;
}
}
printf("\n\nThank you, money accepted. Enjoy your drink!\n");
}
else printf("\n%s %s\n",drinks[menuChoice - 1].brand, "is sold out.
Please choose another.");
}

 

[Malcolm]

Can someone tell me why I'm having this problem:
When I select drink 5 and deposit too little money, there's a problem
with the 'difference' variable. Try to deposit $.80 instead of the
full $.85 and see what happens if you then keep entering .01 until you
have added the full .85. The program still asks the user to deposit
$0.00. It is telling me for some reason that 0.850000 - 0.850000 =
5.96046e-008.
Any suggestions?

The amount 0.01 has no exact binary representation. In effect you have a
small random error whenever you calculate a multiple of such an amount,
hence the tiny difference in your subtraction.

The solution is to store the monetary value as an amount of pennies / cents,
and then just convert to dollars immediately before output to the user.

 

[Kelsey Bjarnason]

[snips]

Can someone tell me why I'm having this problem:
When I select drink 5 and deposit too little money, there's a problem
with the 'difference' variable. Try to deposit $.80 instead of the
full $.85 and see what happens if you then keep entering .01 until you
have added the full .85. The program still asks the user to deposit
$0.00. It is telling me for some reason that 0.850000 - 0.850000 =
5.96046e-008.
Any suggestions?

Yeah; don't use floats/doubles. What you're seeing, apparently, is the
following: instead of storing 0.85 exactly (in at least one case) it's
storing a value something like 0.8500000059604. Since the two aren't the
same, any comparison that assumes they should be will fail.

Simplest answer: use integer types and scale on I/O. That is, instead of
storing 0.85 dollars, store 85 cents and, if you need to, display it as
0.85 dollars.

 

[Ralmin]

Can someone tell me why I'm having this problem:
When I select drink 5 and deposit too little money, there's a problem
with the 'difference' variable. Try to deposit $.80 instead of the
full $.85 and see what happens if you then keep entering .01 until you
have added the full .85. The program still asks the user to deposit
$0.00. It is telling me for some reason that 0.850000 - 0.850000 =
5.96046e-008.
Any suggestions?

Yeah, no need to even read your program. The problem is that, of the numbers
0.80, 0.85 and 0.01, none of them can be represented exactly in a binary
floating point system. The solution to your problem is to represent all
values as a whole number of cents. Don't bother with floating point, just
use an int.

Then you can use a printf format like this:
printf("$%d.%02d", price / 100, price % 100);

 

[Darrell Grainger]

Can someone tell me why I'm having this problem:
When I select drink 5 and deposit too little money, there's a problem
with the 'difference' variable. Try to deposit $.80 instead of the
full $.85 and see what happens if you then keep entering .01 until you
have added the full .85. The program still asks the user to deposit
$0.00. It is telling me for some reason that 0.850000 - 0.850000 =
5.96046e-008.
Any suggestions?

The problem is floating point numbers. There is a thing called
representation error. Think about this, in pure mathematics there are
infinite fractions between the numbers 0 and 1. Since a computer has
finite amounts of storage it cannot represent all real numbers. The way it
works is that there are gaps. Different systems will have different gaps.
You system does not use 0.01. It will round it up or down to some other
value.

Because you can never be sure what numbers the floating point
implementation your system uses, you are best to use integers. You know
that you want to represent a finite set of values. Use an algorithm that
is very straight forward. For example,

int money;

money = 1; /* $0.01 */
money = 85; /* $0.85 */
money = 125; /* $1.25 */

When you go to print it to the user you would use:

printf("$%d.%02.2d\n", money/100, money%100);

/*
machine.txt:
------------------
Cola
0.75 20
Ruby Red Blast
1.00 10
Lemon Fizz
0.75 8
Grape Soda
0.90 5
Citrus Flip
0.85 0
Habanero Surprise
0.80 11
-------------------
*/

/* ================================================================ */
#include <stdio.h>
#include <string.h>
#include <math.h>
/* ================================================================ */
#define MAXNUMDRINKS 6
#define STRINGSIZE 25
#define noGood 0
/* ================================================================ */

typedef char String [STRINGSIZE];
typedef enum bool { false, true } bool;

typedef struct drinkRec
{
String brand;
float price;
int quantity;
} drinkRec;

typedef drinkRec drinkList [ MAXNUMDRINKS ];

/* ================================================================ */

void ReadAllDrinks ( int* numDrinks, drinkList drinks );
void PrintDrinkMenu (int* menuChoice, int numDrinks, drinkList
drinks);
void GetNewQuantity (int menuChoice, drinkList drinks);
void GetMoney (int menuChoice, drinkList drinks);

/* ================================================================ */

int main ( )
{
drinkList drinks;
int numDrinks, menuChoice;
double profit = 0;
printf ("Welcome to the Soda Machine.\n");
ReadAllDrinks ( &numDrinks, drinks );
do
{
PrintDrinkMenu (&menuChoice, numDrinks, drinks);
if (menuChoice != 0){
GetMoney (menuChoice, drinks);
if (drinks[(menuChoice - 1)].quantity > 0)
profit = profit + .25;
GetNewQuantity (menuChoice, drinks);
}
}
while (menuChoice != 0);
printf("\n%s%.2f\n","Current Run's Profit: $", profit);
printf("\nBYE BYE\n\n");
return ( 0 );

}
/* ================================================================ */

void ReadAllDrinks(int *numDrinks, drinkList drinks)
{
String dummy;
float dummy2 = 0;
char *line;
FILE *inFile;
int count = 0;
if ((inFile = fopen("machine.txt", "r"))) {
while ((line = fgets(drinks[count].brand, STRINGSIZE, inFile))) {
drinks[count].brand[strlen(drinks[count].brand) - 1] = '\0';
fscanf(inFile, "%f%d",
&drinks[count].price,&drinks[count].quantity);
fgets(dummy, STRINGSIZE, inFile);
count++;
}
*numDrinks = count;
fclose(inFile);
}
else printf( "machine.txt not read\n");
}
/* ================================================================ */
void PrintDrinkMenu (int* menuChoice, int numDrinks, drinkList drinks)
{
int count, result;
String dummy;
printf("\n Drink Price Amount Left");
printf("\n ------------- ----- -----------\n");
for (count = 0; count < numDrinks; count++){
printf("%d. %-19s$%4.2f", (count + 1),
drinks[count].brand,drinks[count].price);
if (drinks[count].quantity < 1)
printf(" %8s","SOLD OUT\n");
else
printf(" %8d\n",drinks[count].quantity);
}
printf("\n0. Exit\n");
do
{
printf("\nPlease enter your selection: ");
result = scanf("%d", menuChoice);
if (result == noGood) /* This error checks for any input other
than an integer*/
scanf("%s", &dummy); /* If input is non-integer, force scanf to
stop looping */
}
while ((*menuChoice > numDrinks) || (*menuChoice < 0)|| (result ==
noGood));
}
/* ================================================================ */
void GetNewQuantity (int menuChoice, drinkList drinks)
{
int prevQuantity;
prevQuantity = drinks[(menuChoice - 1)].quantity;
drinks[(menuChoice - 1)].quantity = prevQuantity - 1;
}
/* ================================================================ */
void GetMoney (int menuChoice, drinkList drinks)
{
int result;
String dummy;
float moreMoneyIn, moneyIn, difference;
if (drinks[menuChoice - 1].quantity > 0){
do
{
printf("\n%s%.2f: $","Please deposit $",drinks[menuChoice -
1].price );
result = scanf("%f",&moneyIn);
if (result == noGood) /* This error checks for any input other
than an float*/
scanf("%s", &dummy); /* If input is non-float, force scanf to
stop looping */
}
while (result == noGood);
printf("\n%s$%.2f\n","You put in ",moneyIn);
difference = (drinks[menuChoice - 1].price) - moneyIn;
while (difference != 0){
if (difference > 0){
do
{
printf("%s%.2f: ","Please deposit an additional $",
fabs(difference));
result = scanf("%f", &moreMoneyIn);
if (result == noGood) /* This error checks for any input
other than an float*/
scanf("%s", &dummy); /* If input is non-float, force scanf
to stop looping */
}
while (result == noGood);
printf("\n%s$%.2f.\n","You put in ",moreMoneyIn);
moneyIn = moneyIn + moreMoneyIn;
difference = ((drinks[menuChoice - 1].price) - moneyIn);
}
if (difference < 0){
printf("%s%.2f","Your change is $", fabs(difference));
difference = 0;
}
}
printf("\n\nThank you, money accepted. Enjoy your drink!\n");
}
else printf("\n%s %s\n",drinks[menuChoice - 1].brand, "is sold out.
Please choose another.");
}