[RESOLVED] several STL containers questions
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Thread: [RESOLVED] several STL containers questions

  1. #1
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    [RESOLVED] several STL containers questions

    1. should I use hash_map, hash_set, hash_... instead of the normal map, set, multimap, etc. ? Are the hash versions supercede versions of the normal ones? Or there are times when I should use e.g. a map instead of a hash map?

    2. are there occasions when I must write a hash function? If yes, how do I do that and how do I make a good (i.e. not a stupid) hash function?

    3. I've read about these member functions, but I don't understand when or why I should use them: equal_range, lower_bound, upper_bound. well, about lower & upper bounds, I understand that they usually find something if that particular key does not exist - is there more to them?

    4. what are the uses of emplace? for the associative containers, for instance, why shouldn't I simply insert a pair?

    5.what function should I use to find an element of an associative container? at, find, lower_bound, or what?? is there some difference in speed among them?

  2. #2
    Lindley is offline Elite Member Power Poster
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    Re: several STL containers questions

    Quote Originally Posted by Feoggou View Post
    1. should I use hash_map, hash_set, hash_... instead of the normal map, set, multimap, etc. ? Are the hash versions supercede versions of the normal ones? Or there are times when I should use e.g. a map instead of a hash map?
    If you don't care whether the map/set elements are sorted, you should consider using unordered_set etc. (The hash_set etc names are nonstandard.) Hash tables have faster average insertions and lookups than red/black trees, but slower worst-case times.

    2. are there occasions when I must write a hash function? If yes, how do I do that and how do I make a good (i.e. not a stupid) hash function?
    The Boost.Hash library provides some good facilities for creating hash functions, eg hash_combine() etc. I don't know how much of that ended up in the standard library. Certainly, the method of defining a hash function appears to be somewhat different between the std unordered containers and the Boost versions.

    3. I've read about these member functions, but I don't understand when or why I should use them: equal_range, lower_bound, upper_bound. well, about lower & upper bounds, I understand that they usually find something if that particular key does not exist - is there more to them?
    Those are useful for iterating on ranges of values. For instance, if you have a sorted container and you want to iterate over all values in the half-open interval [5,9), you could do
    Code:
    for (auto iter = container.lower_bound(5); iter != container.upper_bound(9); ++iter)
    {
        // stuff
    }
    Note that lower_bound and upper_bound are different only if the argument passed is in the container. So if "9" isn't in the container, lower_bound(9) and upper_bound(9) will behave the same. The difference is that if it is there, then lower_bound(9) will return an iterator to 9, while upper_bound(9) will return an iterator to the next element *past* 9.

    equal_range is useful if the upper and lower bounds are the same, eg, you just want all elements with a given key. This can also be used with unordered containers while lb and ub cannot.

    4. what are the uses of emplace? for the associative containers, for instance, why shouldn't I simply insert a pair?
    emplace is the equivalent of insert for rvalue references. I'm honestly not quite sure why they didn't just overload insert (and push_back rather than adding emplace_back), but that's what they did. The basic idea is you use them if you want to move an element into a container rather than copying it in. (The object must support move semantics for there to be any difference.)

    5.what function should I use to find an element of an associative container? at, find, lower_bound, or what?? is there some difference in speed among them?
    If you're looking for an exact match, use find(). If you're just trying to get as close as you can, lower_bound may be more appropriate.

  3. #3
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    Re: several STL containers questions

    Hash tables have faster average insertions and lookups than red/black trees, but slower worst-case times.
    sorry, I don't understand the "worst-case times". what is exactly slow about the hash versions?

    If you're looking for an exact match, use find(). If you're just trying to get as close as you can, lower_bound may be more appropriate.
    So I guess at() and find() do exactly the same (have the same speed). right?

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    Re: several STL containers questions

    Quote Originally Posted by Feoggou View Post
    sorry, I don't understand the "worst-case times". what is exactly slow about the hash versions?
    "Worst-case time" means: "In the worst possible case, the amount of time it takes".

    There are methods, structures out there that usually perform fast on most data, but if you give it *just* the right data, you can make it slow down to a crawl. Simplistic binary trees, for example, usually suffer from this.

    A (very famous) example is quick_sort. Quick sort is, well, quick, but if you give it just the right data, you can slow it down to a crawl, making a sort that would otherwise take seconds, take centuries. This has been exploited to launch denial of service attacks on various organizations.

    Bottom line:
    Any operation can take a certain average amount of time, which is somewhere between the best-case time, and the worst-case time.
    Is your question related to IO?
    Read this C++ FAQ LITE article at parashift by Marshall Cline. In particular points 1-6.
    It will explain how to correctly deal with IO, how to validate input, and why you shouldn't count on "while(!in.eof())". And it always makes for excellent reading.

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    Re: several STL containers questions

    ok, now I understood what he meant.

    thanks guys for your help.

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