Question

This weekend I spend a few minutes thrashing together an algorithm that would take in a heading (in degrees) and return a String for the cardinal direction (I’m using it in an android compass application I’m using). What I ended up with was this:

private String headingToString(Float heading)
{
    String strHeading = "?";
    Hashtable<String, Float> cardinal = new Hashtable<String, Float>();
    cardinal.put("North_1", new Float(0));
    cardinal.put("Northeast", new Float(45));
    cardinal.put("East", new Float(90));
    cardinal.put("Southeast", new Float(135));
    cardinal.put("South", new Float(180));
    cardinal.put("Southwest", new Float(225));
    cardinal.put("West", new Float(270));
    cardinal.put("Northwest", new Float(315));
    cardinal.put("North_2", new Float(360));

    for (String key: cardinal.keySet())
    {
        Float value = cardinal.get(key);
        if (Math.abs(heading - value) < 30)
        {
            strHeading = key;
            if (key.contains("North_"))
            {
                strHeading = "North";
            }
            break;
        }
    }
    return strHeading;
}

My question is, is this the best way of doing this? It must have been done many times before although I haven’t done a search for examples on the web yet. Have any other people tried this and found a neater solution?

Edit for The Reverand’s Thilo’s, shinjin’s and Chrstoffer’s responses:

The Solution

public static String headingToString2(double x)
{
    String directions[] = {"N", "NE", "E", "SE", "S", "SW", "W", "NW", "N"};
    return directions[ (int)Math.round((  ((double)x % 360) / 45)) ];
}

Answer 1

That’s fine in most cases, though to make it optimized and (IMO) cleaner, what you could do is find a function to relate the input heading to one used in the map.

For example:
(I’m pretty sure this is right, but you’ll want to check it)

45* (int)Math.round((  ((double)x % 360) / 45))

What this does is first x % 360 makes sure the heading is within a valid range. then

45 * round(.../45)

finds the closest multiple of 45.

Now change your map to be

  HashMap<Integer, String> map = new HashMap<Integer, String>()
  map.put(0, "North")
  map.put(45, "Northeast")
  etc...

So, now your algorithm becomes a fast mathemtical calculation rather than iterating through the map. Furthermore, you don’t need as Hashtable here since it provides constructs for concurrency (if I remember correctly) and in your case it would actually cause a performance decrease.

Once again, the performance hit may be completely negligible for your needs.

Edit for Thilo’s and shinjin’s suggestions:

Instead of multiplying by 45, just keep the rest of the equation, which gives you values for 0-7, and make an array of your strings.

String directions[] = {"N", "NE", "E", "SE", "S", "SW", "W", "NW"}
return directions[ (int)Math.round((  ((double)x % 360) / 45)) % 8 ]

and you’ve got your problem solved in two lines.

One note: Modulus won’t work correctly for negative numbers. If our input heading is negative, you’ll need to make it positive first.