Map - finding nearest value?

问题

I am trying find nearest RGB value in QMap (I know it probably should be HSV, but that is not the problem). Here is what I got so far:

        it = images_map.find(current_rgb);

        if(it != images_map.begin()){
            mi = images_map.lowerBound(current_rgb).value();
        }
        else{
            mi = images_map.upperBound(current_rgb).value();
        }

My map looks like this has that indexes:

images_map[ 4283914078 ] 
images_map[ 4284046165 ] 
images_map[ 4284902241 ] 
images_map[ 4289239953 ] 
images_map[ 4282200377 ] 
images_map[ 4289440688 ]

When my current_rgb is for example 4285046165 it is OK, but if there is some value greater than greatest index, program crashes. What am I doing wrong?

回答1:

Possibly because .value() tries to de-reference a non-existing item?

This looks like your own custom map implementation (or wrapper), but your logic appears to be incorrect

You call lowerBound every time - except if the item you are looking for is the first in the map
If it is the first in the map, you do a search again???
If it's not you search again (which if already found is repeating the operation again), else if not found, looks for nearest (which is okay), however do you handle the case where there is none (i.e. in lowerBound)?

The logic should be something like:

it = images_map.find(current_rgb);

if(it == images_map.end())
{
  it = images_map.lowerBound(current_rgb);
  if (it == images_map.begin())
  {
    it = images_map.upperBound(current_rgb);
    if (it == images_map.end()) 
      // throw error
  }
  // now you know you have a valid iterator - de-reference
  mi = *it.value();
}

回答2:

Call

images_map.upperBound(current_rgb)

May return

images_map.end()

In that case you should not call value().

回答3:

You can solve the iterator out of range problem by adding sentinel values 0x000000 and 0xFFFFFF (once). That way, you always have a valid lower- and upperbound. Of course, this may affect the outcome of your algorithm. E.g. if your "smallest" real color was pure blue (0x0000FF), then dark blue (0x00007F) will now find black, not pure blue. This is easily fixed by two comparisons, of course.

With the sentinels in place, call QMap::lower_bound. You need to check whether you've actually found a precise match: if *lower_bound is the value you want, return it. Else, lower_bound points to the first element that's bigger than your input. Therefore, --lowerbound points to the last element that's smaller than your input. Check which of the two is closer.

Note that the only way lower_bound can point to begin is when your input is precisely 0x000000 (the sentinel), in which case you won't get to --lower_bound. No range error there. By the same logic, the end sentinel 0xFFFFFF means you'll always find an lower_bound.

来源：https://stackoverflow.com/questions/6730136/map-finding-nearest-value

标签

c++

qmap