CODEWITHSUNDEEP
javadictionarytreemap
eggonlegs
eggonlegs

Reputation: 1864

i-1 access to a TreeMap

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?

Upvotes: 0

Views: 157

Answers (4)

eggonlegs
eggonlegs

Reputation: 1864

I didn't explain my question well. Sorry guys. I wanted a cumulative running total. This is what I did by combining your answers:

int hold_previous_key = 0;
int difference = 0;
int running_total = 0;

for (int key : map.keySet())
{
    if (hold_previous_key != 0)
    {
        difference = key - hold_previous_key;
        running_total += difference;
        builder.append(key + " - " + hold_previous_key + " = " + difference + " RUNNING TOTAL = " + running_total + "; ");
    }
    else
    {
        // print 0 for the first date in the data set
        builder.append("first val = 0" + "; ");
    }

    hold_previous_key = key;
}

Upvotes: 0

corsiKa
corsiKa

Reputation: 82599

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>

Upvotes: 2

st0le
st0le

Reputation: 33575

Why don't you hold it in a separate variable?

//Warning : Notepad coding.
int hold_prev_key = -1;

for(int key : map.keySet())
{
   if(hold_prev_key != -1)
      builder.append(key -  hold_prev_key);
   hold_prev_key =  key;
}

Upvotes: 2

amit
amit

Reputation: 178521

simplest and fastest way I think: you can store the last entry in a temp variable, and use it only on next iteration. Don't forget to modify it at the end of each iteration.

Upvotes: 4

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