Question

I want to go through a map with integer keys that are in the range of 14000-18000. I want to go through them and print the relative difference between them. So if there were three keys 14152 and 14153, 14159, the print output would be 0, 1, 7.

I have put my keys and values into a TreeMap, since it stores things in order.

However, with my implementation:

int dayCounter = 0;

for (Entry<Integer, String> entry : map.entrySet())
{
    builder.append(dayCounter);
    dayCounter = entry.getKey();
}

I am going through the map but know no way of getting the “previous” entry. If I was using an array I could get the (i-1)th value and subtract from i to get the relative value. Is there any way to get thie functionality with java maps?

Answer 1

When you iterate, you know you have the smallest value first. So I would do something like this:

Integer first = null;

for(Integer i : map.keySet()) {
    if(first == null) first = i; // save the first value
    builder.append(i - first);
}

Using your example:

import java.util.TreeMap;
class Eggonlegs {
    public static void main(String[] args) {
        TreeMap<Integer,String> map = new TreeMap<Integer,String>();
        map.put(14152,"First");
        map.put(14153,"Second");
        map.put(14159,"Third");

        Integer first = null;

        for(Integer i : map.keySet()) {
            if(first == null) first = i; // save the first value
            System.out.println(i - first);
        }
    }
}

Results in

c:\files>javac Eggonlegs.java

c:\files>java Eggonlegs
0
1
7

c:\files>

Now, it’s possible that’s not what you intended. Perhaps you want the difference between each node, which not what your example shows. In that case, I would leverage the Collections library like this:

List<Integer> list = new ArrayList<Integer>(map.keySet());

for(int i = 0; i < list.size(); i++) {
    if(i == 0) builder.append(0);
    else builder.append(list.get(i) - list.get(i-1));
}

Here’s an example of that, incase it’s what oyu actually intended:

import java.util.ArrayList;
import java.util.List;
import java.util.TreeMap;
class Eggonlegs {
    public static void main(String[] args) {
        TreeMap<Integer,String> map = new TreeMap<Integer,String>();
        map.put(14152,"First");
        map.put(14153,"Second");
        map.put(14159,"Third");

        List<Integer> list = new ArrayList<Integer>(map.keySet());

        for(int i = 0; i < list.size(); i++) {
            if(i == 0) System.out.println(0);
            else System.out.println(list.get(i) - list.get(i-1));
        }
    }
}

Results in

c:\files>javac Eggonlegs.java

c:\files>java Eggonlegs
0
1
6

c:\files>