Stl Set and struct comparison

I have a struct:

Code:

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struct aCoord {
        int a,b;
};

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and a set of them:

Code:

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std::set<aCoord> myCoords;

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In one of my functions, I would like to check whether a newly generated aCoord is equivalent to one of the elements already in myCoords (meaning check whether firstCoord.a==secondCoord.a && firstCoord.b == secondCoord.b).

I have an iterator and am doing the following:

Code:

---

iterator = myCoords.find(newCoord);
if(iterator == myCoords.end()) {
    myCoords.insert(newCoord);
}

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This doesn't work because I need to overload the comparison function used by find, but despite the research I've done so far, I can't figure out the proper form of the operator override. Can anyone help me out? Thanks.

I may be wrong, but I think that find() simply uses operator< to determine equivalence (if a is not < b and b is not < a, a == b). Try defining operator< for aCoord.

That's the part I can't figure out how to write. What is the form for that? Thanks

It depends on whether you write it as a member function or not. As a member function:

Code:

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bool operator<(const aCoord&) const;

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As a regular function:

Code:

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bool operator<(const aCoord&, const aCoord&);

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Quote:

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Originally Posted by pollypocket4eva

That's the part I can't figure out how to write. What is the form for that? Thanks

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Note that a set already checks for uniqueness for you - you don't have to do this manually.

Quote:

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Originally Posted by EVOEx

Note that a set already checks for uniqueness for you - you don't have to do this manually.

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Good catch. I keep forgetting that insert() will gracefully handle duplicates. You can check the return value of insert() to find out if a new element was actually added to the set.

I'm not sure I understand what you're doing with the member and regular functions.
Where would I declare those? And how do I check the components (firstCoord.a==secondCoord.a && firstCoord.b == secondCoord.b) or do I not have to do that explicitly?

Also, EVOEx, that's very helpful to know, but in this case I want to explicitly know whether the set already contains a particular item.

Oh, so will insert() return false if the element was not added?

The insert() approach is not working for me, I get the error: No match for 'operator<'

Yes, that's because you must define some sort of comparison operation for the elements of a set. You may want to read the operator overloading tutorial.

I've read that, but can't figure out how to apply it here.
Would it be something like this?

Code:

---

struct aCoord {
        int a,b;
        bool operator==(const aCoord &other) const
                { return (a == other.a)&&(b == other.b); }
               
};

---

Anyone?

Something like that, but not that. You need operator<, not operator==. sets determine equivalence (operator==) from comparison.

Ok, but how can the < operator address my struct? I need to test if first.a==second.a && first.b==second.b
The following does not do that:

Code:

---

bool operator<(const aCoord &other) const
                { return (a < other.a)&&(b < other.b); }

---

I can't think of how to structure it.
Thanks very much for your help.

Quote:

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Originally Posted by pianorain

I think that find() simply uses operator< to determine equivalence (if a is not < b and b is not < a, a == b).

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Quote:

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Originally Posted by pianorain

sets determine equivalence (operator==) from comparison.

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I'll put it another way. Take any two numbers (A and B). They can be the same number, or they can be different. Now, either:

1. A < B
2. B < A
3. !(A < B) && !(B < A)

Note that case 3 is the same as A == B. Do you see how every case, including operator==, uses operator<? You just have to make sure that operator< orders elements correctly.

In cases like this, you usually want to use a lexicographical order. In other words, first sort by A, and then sort by B.

Thanks. I see your point that every case is covered. But I guess I'm having a hard time wrapping my mind around how the operator is applied by find() and so the ordering has me confused. Sorry and thanks for your patience.

If the operator is evaluating <, then I want it to return true only when !(a<b)&&!(b<a) ?
You see where I'm confused?

Yes, I see where you're confused; I'm just failing to find the words to explain it well.

find() does what you want it to do; it returns a dereferencable iterator when it finds the element you specified, or else it returns the end iterator. find() uses operator< to do this by only returning a dereferencable iterator when !(a<b)&&!(b<a). You don't have to worry telling find() to return when !(a<b)&&!(b<a); it's already programmed to do that. The important part is providing an operator< for the set to use.

So just to flesh it out (and don't worry, I'm not asking you to do my homework, I just want to understand the structure better and I'm not otherwise getting it:

I have:

Code:

---

struct aCoord {
        int a,b;
        bool operator<(const aCoord &other) const
        { return // SOME LOGIC HERE; }
};

---

and:

Code:

---

iterator = myCoords.find(newCoord);
if(iterator == myCoords.end()) {
    myCoords.insert(newCoord);
}

---

What is the proper logic for the above (if the whole thing is set up correctly in the first place)?

Thanks again, I appreciate it.

Your first block of code is correct. Read above where I mentioned lexicographical ordering to get an idea of the kind of logic you need in there.

Your second block of code is fine, but remember that you don't really need to find() before you insert(). insert() will return a value indicating whether or not it actually added a new element to the set.