From the reference, it seems that they try to be automatically sorted...(they called it weak ordering)
What happens when the compiler does not know how to sort the data it contains ?
Can I rely on the order I specified on the initializer list ?
Printable View
From the reference, it seems that they try to be automatically sorted...(they called it weak ordering)
What happens when the compiler does not know how to sort the data it contains ?
Can I rely on the order I specified on the initializer list ?
It'll fail to compile. Either the type itself supports the necessary comparison, or you pass in a comparison object that knows what to do.Quote:
What happens when the compiler does not know how to sort the data it contains ?
You can specifier your own compare function, if that is what you are asking. See: map constructor.
I was wondering why my example(below) was compiling when I found out that std::string `s do support some comparison.Quote:
Originally Posted by Prelude
//Located outside any function block in a .cpp file
//..where plus, minus... etc are objects of a class Operator;Code:std::map<std::string,Operator> op_ini_map=
{
{std::string("+"),plus},
{std::string("-"),minus},
{std::string("*"),mult},
{std::string("/"),divide}
};
So, my question would better be...
Is there a way to override the sorting and preserve the ordering I put in the initializer list ?
Yes, it is part of std::string. That's why you can do string1==string2 in c++.Quote:
Originally Posted by manasij7479
Yes, override the default comparison by making your own function as both Prelude and I said. Look at the link I posted.Quote:
Originally Posted by manasij7479
If you want to preserve the ordering of the initialization list (ie. an arbitrary order), why are you using a map?Quote:
Is there a way to override the sorting and preserve the ordering I put in the initializer list ?
Because I need to map some strings(Here operators) the user would be allowed to input to some pre-created objects so that the constructor would not be needed to create new ones.Quote:
Originally Posted by Prelude
I did what I needed manually with some 30 lines of code(setting the precedences), but the ordering would have made it possible to make a loop for that.
Bolding "map" and "to" doesn't change the fact that you're probably using the wrong data structure. std::map is basically just a balanced binary search tree. If it were named std::binary_tree instead of std::map, would you still want to use it?
I didn't know that !Quote:
Originally Posted by Prelude
Can you think of a better data structure that would fit what I need to do ?
Why do you need to preserve the order?
What are the most common ways this container will be used?
It may be suitable to make a map<string,pair<int,Operator>, or vector<pair<string,Operator> >, or something else depending on your use case. Also If you're writing Operator yourself, the string or index could be a data member instead of using a pair.
In general, the program will grab the input, if it is an operator, I figured that having it run through a constructor containing dozens of switch-cases/else-ifs would be a waste when I know there is a limited no. of possibilities. So I defined all the possible operator objects and am using the map to directly assign the address to the Operator* pointers (because the same object for all the cases is okay).Quote:
What are the most common ways this container will be used?
I'd try the first one tomorrow, but I'm not inclined to use vectors because it may be necessary to make searches occasionally.Quote:
It may be suitable to make a map<string,pair<int,Operator>, or vector<pair<string,Operator> >, or something else depending on your use case. Also If you're writing Operator yourself, the string or index could be a data member instead of using a pair.
I don't actually 'need' to do so.Quote:
Why do you need to preserve the order?
But it'd make the task of defining the pre-built objects easier.
Have you considered using a hash table? unordered_map is a hash table type.Quote:
Originally Posted by manasij7479
That would mean no iteration through it.Quote:
Originally Posted by whiteflags
But it could potentially alter my design for the better if I can design the whole thing to not need any.
Yeah, a hash map sound like a good choice for this.Quote:
Originally Posted by manasij7479
How? Can this be done with a separate container?Quote:
I don't actually 'need' to do so.
But it'd make the task of defining the pre-built objects easier.
If I make an initialization function for the operators, and have the Operators arranged in order of precedence, I could set the value of the precedence with a loop instead of by hand.Quote:
Originally Posted by King Mir
It could also make the task of inserting new Operators later easier.
But thinking on it for some time, I do realize that it'd be much more complicated that simply running a loop. So I'd go with the manual approach I've already coded.