Best way to explain declarative fallacy in C++?

Question

How might one craft a good explanation of why the following code is not correct, in that the author is attempting to write C++ code declaratively rather than procedurally?

const double NEWTONS_PER_POUND = 4.448;

int main()
{
   double pounds, newtons;
   pounds = newtons/NEWTONS_PER_POUND; /* pounds equals 'unassigned variable'/4.448 */
   newtons = 10.0;
   cout << pounds << endl;             /* a big number, not 10.0/4.448 */
   return 0;
}

The author expected cout to display a proper calculation, but instead gets a "crazy number."

I would explain this as "C++ is procedural, and therefore at the time of the declaration

pounds = newtons/NEWTONS_PER_POUND;

newtons has not been assigned a value.

Any better suggestions? Or an explanation why C++ isn't "smart" enough to carry out the behavior the user mistakenly expected?

Answer 1

Tell the author that

pounds = newtons/NEWTONS_PER_POUND;

commands the CPU to

• take the value at the address referred to as "newtons"
• take the value at the address referred to as "NEWTONS_PER_POUND"
• divide them
• store the result at the address referred to as "pounds"

what he is looking for is most probably a function in imperative terms:

double newtons_to_pounds(double newtons) {
  return newtons/NEWTONS_PER_POUND;
}

...

newtons = 10.0;
cout << newtons_to_pounds(newtons) << endl;    /* a big number, not 10.0/4.448 */
return 0;