Why true statement false?

Question

I have a class , vector which inherited from a "raw vector"

struct vector2raw {
    real_t x, y;
};
struct vector2 : public vector2raw {
    vector2() { null(); }
    vector2(real_t x, real_t y) { this->x = x; this->y = y; }
    vector2(const vector2 &v) { x = v.x; y = v.y; }
and so on

Now I want to compare two numbers, one v.y=4 from v = (5.41, 4), another min.y = 4 from min=(4,4).This is only the strange case when I compare two equal numbers, other cases are executed correctly. I get always false on (4>=4) ( v.y>=min.y) . What can be the problem?

real_t is defined to double UPD: this is written in C++

try debug with setprecision as in: http://stackoverflow.com/a/6722297/2266462 — , Jan 14 '17 at 09:28
You should have given a **reproducible example**. Now it may happen that when you do that, you invalidate my already given answer, which involves some speculation about what you're really doing. That's bad, and remember that: try to always ask clear questions, don't call for speculation and telepathy. Thanks. — Cheers and hth. - Alf, Jan 14 '17 at 09:45

score 1 · Answer 1 · answered Jan 14 '17 at 09:40

Apparently (you're not giving a reproducible example) you're comparing floating point numbers with ==.

That's an ungood idea unless those numbers happen to be integral values, and for beginners it's an ungood idea in general.

Two floating point values can appear to be equal, e.g. they give the same presentation when you don't request presentation of additional decimals, while in reality they differ in some otherwise very insignificant digit.

In the old days beginners who encountered this problem used to be referred to “What every scientist should know about floating point numbers” (or thereabouts, title from fallible memory).

In the last few years I have been criticized for giving that reference, because real technical stuff and so on is, allegedly, too hard for today's students. And people have suggested more easy to digest alternatives, sort of like Wikipedia's simple edition. However, I can't remember any of them.

Why true statement false?

1 Answers1