I've got a problem sorting some data structures in a list. They look something like this: class data { //blah float distance; }; list< data > datalist; I want to sort the datalist by the _distance value_. What I've found so far is stl function objects (can pass one to the sort() function), but I can't get them to work.. Thanks in advance! [edited by - Lord Maz on January 11, 2003 6:34:05 PM]

or something like that.

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Or you could do it like this:

  #include <algorithm>#include <iostream>#include <list>using namespace std;struct data{  string   name;  double   val;  data(double v):val(v),name("blah"){}  friend bool operator<(const data& lhs, const data& rhs)  {    return (lhs.val < rhs.val);  }};int main(){  list DataList;  for (int i=0; i<10; ++i)  {     DataList.push_back(data(10-i));  }  DataList.sort();  for (list::iterator it = DataList.begin(); it != DataList.end(); ++it)  {    cout << it->val << ",  ";  }  return 0;}  

ack! I can't get it to look decent no matter what tags I use! Ah well, you get the idea...

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[edited by - fup on January 11, 2003 7:34:37 PM]

While both methods would technically work, most the time it doesn''t make sense to give a lot of objects comparison operators. For example, if you have two vectors, what the heck is v1 < v2 supposed to mean? On the other handed, if you wanted to sort vectors by their distance from the origin, it makes more sense to make a seperate predicate that does the work, rather than make it a property of the object. In addition, you can only have one < operator. What happens when you want to sort by different conditions?

While both methods would technically work, most the time it doesn''t make sense to give a lot of objects comparison operators

I have to disagree with that statement. I use them every day in my work. It depends what you are writing code to do. You can''t make generalizations.

In addition, you can only have one < operator. What happens when you want to sort by different conditions?

The OP didn''t want to sort by different conditions. He explicitly stated his requirements and Fruny and I gave him perfectly acceptable solutions.

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quote:

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Original post by Zipster
While both methods would technically work, most the time it doesn''t make sense to give a lot of objects comparison operators. For example, if you have two vectors, what the heck is v1 < v2 supposed to mean? On the other handed, if you wanted to sort vectors by their distance from the origin, it makes more sense to make a seperate predicate that does the work, rather than make it a property of the object. In addition, you can only have one < operator. What happens when you want to sort by different conditions?

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That''s what Fruny''s method does. SortDataByDistance is a functor which sorts a Data object list by distance...

If I had my way, I''d have all of you shot!

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quote:

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That's what Fruny's method does. SortDataByDistance is a functor which sorts a Data object list by distance...

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Um, yes, it does...

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I have to disagree with that statement. I use them every day in my work. It depends what you are writing code to do. You can't make generalizations.

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I never said overloading the comparison operator for an object doesn't make sense, I said that it doesn't make sense for certain objects to have comparison operators, for clarity's sake if anything. It's like overloading the * operator on a vector to mean the dot product. It might make sense to you, but to other people it might not be clear. Using my previous example, would you add < and > operators to a vector?

quote:

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The OP didn't want to sort by different conditions. He explicitly stated his requirements and Fruny and I gave him perfectly acceptable solutions.

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The point of my post was that I prefer Fruny's solution over adding an operator to a class if the meaning of that operator isn't obvious. And while the original poster did give specific criteria for his class, I simply wanted to point out the difference between the two methods, as well as some limitations, because it was clear that the poster was a little confused about using STL's sort functionality. Don't take it personally.

[edited by - Zipster on January 11, 2003 12:33:08 AM]

quote:

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Original post by Zipster
Um, yes, it does...

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Oh, I assumed when you said "while both methods would technically work..." that you were referring to Fruny''s as well... never mind, then

If I had my way, I''d have all of you shot!

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