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  1. #1
    Join Date
    Apr 2000
    Belgium (Europe)

    std::map, find largest value smaller than key

    I have a std::map<int, foo>

    what's the ideal way to get an iterator to the item that has the largest key (int) smaller than a given value.

    basically, the item before upper_bound(). I can use upper_bound() and then decrement, but it needs special cases for both end() and begin(), and in the case of end() I'm not sure how I get it to the last item in the map, afaik, we're not allowed to decrement end().

    auto it = mymap.upper_bound(x);
    if (it==mymap.begin()) // first item in the map is already too large.  reject
    else if (it==mymap.end())
        if (mymap.empty())
           it= ???;  // last item in the map. what do I do here ?
    // here it points to largest item smaller than x.

    I can iterate over the entire map and do a compare, but then I pretty much loose the benefit of the binary search.
    Last edited by OReubens; April 19th, 2013 at 08:01 AM.

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