Thread: Strange arithmetic results

I´m seeing arithmetic results that are baffling me.
In my header file, I declare the following:

Code:

#define MAX_CHILDREN 200
#define APPLES_PER_CHILD 25

In my code, I have an attribute to count the number of children served with apples:

Code:

long children_served;

In my tests, number of children served is coming in with the value of 1. But I get strange results from the following printfs:

Code:

printf("The maximum number of children served is %d\n", MAX_CHILDREN);
printf("Apples served per child: %d\n", APPLES_PER_CHILD);
printf("Number of children unserved: %ld\n", (MAX_CHILDREN - children_served));
printf("Number of apples needed to serve unserved children: %ld\n", (MAX_CHILDREN - children_served) * APPLES_PER_CHILD);

The first three printouts are as expected. But the last one is coming in with the wrong value!! It shows 619, while it should be 4975!

Code:

The maximum number of children served is 200
Apples served per child: 25
Number of children unserved: 199
Number of apples needed to serve unserved children: 619

Any ideas what´s going on here? ANd yes, I´ve tried casting, since that´s the only thing I thought could be wrong with the code.

Thanks

I ran this and it prints out correctly. Do you have some other code in there that might giving you problems?

Code:

#define MAX_CHILDREN 200
#define APPLES_PER_CHILD 25

int main(void)
{
    long children_served = 1;

    printf("The maximum number of children served is %d\n", MAX_CHILDREN);
    printf("Apples served per child: %d\n", APPLES_PER_CHILD);
    printf("Number of children unserved: %ld\n", (MAX_CHILDREN - children_served));
    printf("Number of apples needed to serve unserved children: %ld\n", (MAX_CHILDREN - children_served) * APPLES_PER_CHILD);

    return 0;
}

Worked for me!

Rmatze:

Yes, there´s a lot of code between the declaration of the children_served variable and the logs. However, what´s interesting is that everything gets printed out correctly until the last printf. That is, the compiler calculates correctly that MAX_CHILDREN - children_served equals 199. Where it goes wrong is in the next line, where it multiplies that result by APPLES_PER_CHILD (whose value is 25). How is it that the result of that multiplication is 619? That´s what I don´t get.

Try checking the value of the variables before the line that is giving you troubles. Something like this:

Code:

printf("MAX_CHILDREN: %d, children_served: %d, APPLES_PER_CHILD: %ld\n", MAX_CHILDREN, children_served, APPLES_PER_CHILD);

Casting is very rarely the right answer. It's basically telling the compiler you know better than it does, which is rarely true.

You need to post the smallest compilable code that exhibits the error, so we can run/verify it on our systems and help you fix it.

Ok, I´ll tell you guys what I did to make it work. But first let me add that I began by following Timmmay´s suggestion. I substituted the (MAX_CHILDREN - children_served) calculation with a literal to narrow down where the error was occurring:

Code:

printf("The maximum number of children served is %d\n", MAX_CHILDREN);
printf("Apples served per child: %d\n", APPLES_PER_CHILD);
printf("Number of children unserved: %ld\n", (MAX_CHILDREN - children_served));
printf("Number of apples needed to serve unserved children: %ld\n", 199 * APPLES_PER_CHILD);

So the only thing I changed was the last line. Believe it or not, I was still getting the following results:

Code:

The maximum number of children served is 200
Apples served per child: 25
Number of children unserved: 199
Number of apples needed to serve unserved children: 619

So it appears that the problem is with the pre-processor constant multiplication 199 * APPLES_PER_CHILD.

What was the work-around I did? I created two new variables, and initialized them to the constants, as below:

Code:

long children_served;
long maximum_num_children = MAX_CHILDREN;
long apples_per_head = APPLES_PER_CHILD;

I then used those new variables in the logs:

Code:

printf("The maximum number of children served is %ld\n", maximum_num_children);
printf("Apples served per child: %ld\n", apples_per_head);
printf("Number of children unserved: %ld\n", (maximum_num_children - children_served));
printf("Number of apples needed to serve unserved children: %ld\n", (maximum_num_children - children_served) * apples_per_head);

The results came out correct this time:

Code:

The maximum number of children served is 200
Apples served per child: 25
Number of children unserved: 199
Number of apples needed to serve unserved children: 4975

Creating those two new variables and populating them with the constants would seem to me to be unnecessary, and kind of destroys the purpose of pre-processor statements in your code. But that´s the only solution I found to the problem. Something´s happening with that pre-processor statement that´s causing it to produce strange results. So I found a solution, but I would greatly appreciate being able to understand what went wrong.

Thanks everyone for the posts.

Salem, I´m not sure. I´ll have to ask about that on Monday. Seems like something in the way the compiler is pre-processing those directives, huh?