Is it in general wisest to not use member functions in control statements/functions, but rather store their value in a variable and then use that?

I am creating a function that takes two sorted linked lists and merges them into a combined linked list. Here are my two implementations. Only the second one works in all cases I have tested. Left and right are the lists, front() returns the data in the node the head pointer points to, pushBack() adds a new node to the end of the list, and popFront() deletes the node at the front of the list. Merged is the new list they are combined into.

Implementation 1:

else {      if ((left.front() = right.front())) {        merged.pushBack(left.front());        left.popFront();        merged.pushBack(right.front());        right.popFront();      }      else if (left.front() < right.front()) {        merged.pushBack(left.front());        left.popFront();      }      else {        merged.pushBack(right.front());        right.popFront();      }    }  

Implementation 2:

else {        int leftFront = left.front();        int rightFront = right.front();                if (leftFront == rightFront) {          merged.pushBack(leftFront);          left.popFront();          merged.pushBack(rightFront);          right.popFront();        }        else if (leftFront < rightFront) {          merged.pushBack(leftFront);          left.popFront();        }        else {          merged.pushBack(rightFront);          right.popFront();        }      }  

It makes sense to me that I should not be using the member functions in control statements or as function arguments, since they are not static and might change during runtime, but I cannot see for the life of me why, in this particular situation I get different results.

In one test, with left = [1, 5, 10, 20] and right = [2, 4, 11, 19], the merge produces the list merged = [2, 2, 4, 4, 11, 11, 19, 19]. Obviously incorrect, but I don't see why it can't even get the first comparison correct. As far as I can tell, the else if is going to evaluate if 1 < 2, it is, and then push 1 to the back of merged, and then delete 1.

In the future I will follow implementation 2, as it seems safer to me, but I am confused why method 1 does not work in this situation

https://stackoverflow.com/questions/67411284/is-it-in-general-wisest-to-not-use-member-functions-in-control-statements-functi May 06, 2021 at 11:06AM