How should I correct this code that causes "value computed not used" warning?

Question

I have an array of doubles and need to do a calculation on that array and then find the min and max value that results from that calculation. Here is basically what I have:

double * array;
double result;
double myMin;
double myMax;

// Assume array is initialized properly...
for (int i = 0; i < sizeOfArray; ++i) {
    result = transmogrify(array[i]);
    if (i == 0) {
        myMin = result;
        myMax = result;
    }
    else if (result < myMin) {
        myMin = result;
    }
    else if (result > myMax) {
        myMax = result;
    }
}

I'm getting a warning that the value computed for result is never used, and since we treat all warnings as errors, this doesn't compile. How can I fix this code to avoid the warning? I'm using g++ for my compiler.

Here's the warning text:

cc1plus: warnings being treated as errors
foo.cc:<lineno of transmogrify call>: error: value computed is not used

Edit: I don't understand the down votes, but I've got things working now. Thanks to everyone for taking the time to help me out.

Answer 1

Assuming you don't need result outside of the loop, you could declare result inside the loop thusly:

for( int i=0; i < sizeOfArray; ++i ) {
    double result = transmogrify( array[i] );
    ...
}

Answer 2

I don't think the code as posted should produce the error, unless the compiler is doing some phenomenal flow analysis. It certainly compiles OK with g++, but I'm not sure that g++ even supports the warning you are getting.

The following adaptation of your code, which preserves its structure, produces no error with g++:

int main() {
double * array;
double result;
double myMin;
double myMax;

double t(double);

// Assume array is initialized properly...
for (int i = 0; i < 10; ++i) {
    result = t(array[i]);
    if (i == 0) {
        myMin = result;
        myMax = result;
    }
    else if (result < myMin) {
        myMin = result;
    }
    else if (result > myMax) {
        myMax = result;
    }
}

}

Answer 3

Removing the first else only impacts the running time of the first run through the loop. I'd eliminate it rather.

if (i == 0) {
    myMin = result;
    myMax = result;
}
if (result < myMin) 
    myMin = result;
else if (result > myMax) 
    myMax = result;

Answer 4

result = [...]
if (i == 0) {
    [... do something with result ...]
}
else if (result < myMin) {

In both branches of the if(), result is used. In the 1st case it's assigned to a variable, in the 2nd it's used in a comparison. So the compiler shouldn't warn.

I suspect you might have misdiagnosed the problem. Please can you say exactly what the error message is (copy-paste it). Also, please try to post the smallest piece of code you can that actually compiles and gives the warning? (Simply trying to do that will probably let you find the problem)

EDIT: Is it possible that transmogrify() is a macro that uses result internally?

Answer 5

Initialize myMin and myMax with DBL_MAX and DBL_MIN respectively and get rid of the first time through the loop check.

Answer 6

I'm getting a warning that the value computed for result is never used because (theoretically) it's possible that none of the if/else branches will be selected

That can't be the reason for the warning, because result is also used in the if conditions. Even if none of the branches are taken, result is still used to decide that they should not be taken.

Answer 7

Before the "if" statement:

result = 0.0;

or some other value. It is always good form to set a variable to some value before using it.

Answer 8

I'm getting a warning that the value computed for result is never used because (theoretically) it's possible that none of the if/else branches will be selected, and since we treat all warnings as errors, this doesn't compile. How can I fix this code to avoid the warning? I'm using g++ for my compiler

The value used for result is always used. If not assigned it's used in the comparator. Therefore the compiler is faulty.

Answer 9

A quick solution might be to unroll the first iteration, like this:

double * array;
double result = transmogrify(array[0]);
double myMin = result;
double myMax = result;
int i;

for (i = 1; i < sizeOfArray; ++i) {
    result = transmogrify(array[i]);

    if (result < myMin) {
        myMin = result;
    }

    if (result > myMax) {
        myMax = result;
    }
}

EDIT: I'll expand a bit on this. You haven't given any detailed information on sizeOfArray, but my guess is that it's a signed integer type.

I believe you've misunderstood the cause of the warning, result might be unused because sizeOfArray might be less than or equal to zero, not because of the if..else clauses inside the loop. In the code above, it might be a bit clearer why you need careful handling of the case (sizeOfArray <= 0), but it's equally important in the original code snippet as well.