Question

I have a relatively simple algorithm that walks an std::vector looking for two neighbouring tuples. Once the tuples left and right of the X value are found I can interpolate between them. Somehow this works:

  std::vector<LutTuple*>::iterator tuple_it;
  LutTuple* left = NULL;
  LutTuple* right = NULL;
  bool found = 0;

  // Only iterate as long as the points are not found
  for(tuple_it = lut.begin(); (tuple_it != lut.end() && !found); tuple_it++) {
    // If the tuple is less than r2 we found the first element
    if((*tuple_it)->r < r) {
        left = *tuple_it;
    }
    if ((*tuple_it)->r > r) {
        right = *tuple_it;
    }
    if(left && right) {
        found = 1;
    }
  }

while this:

  std::vector<LutTuple*>::iterator tuple_it;
  LutTuple* left = NULL;
  LutTuple* right = NULL;

  // Only iterate as long as the points are not found
  for(tuple_it = lut.begin(); tuple_it != lut.end() && !left && !right; tuple_it++) {
    // If the tuple is less than r2 we found the first element
    if((*tuple_it)->r < r) {
        left = *tuple_it;
    }
    if ((*tuple_it)->r > r) {
        right = *tuple_it;
    }
  }

does not. Why is that? I’d expect two NULL ptrs like this to evaluate to true together when negated.

Answer 1

There is a logical issue.

In the first snippet you have (essentially) !(left && right).

In the second snippet you have: !left && !right.

Those are not equivalent.

If you build the truth table, you will realize that !(left && right) is equivalent to (!left || !right).