Using a string to write to an array

Question

Attempting to tidy up code, originally I was using this method of writing to arrays, which is ridiculously long when I have to repeat it 20 times

        if (ant.getAntNumber() == 3)
        {
            numbers3.add(ant.getCol());
            numbers3y.add(ant.getRow());
        }

        if (ant.getAntNumber() == 4)
        {
            numbers4.add(ant.getCol());
            numbers4y.add(ant.getRow());
        }

I attempted to use a for loop to do it but I cant figure out how to add to the array using the string value, because it thinks its a string rather than trying to use the array

for (int j = 0; j<maxAnts; j++)
        {
            String str = "numbers" + j;
            String str2 = "numbers" + j + "y";
            //this part doesnt work
            str.add(ant.getCol());

        }

Any suggestions would be helpful

Your number3, number4, ..., numberN should belong to an array, that's it. — moonwave99
– moonwave99, Commented Feb 25, 2014 at 17:43
Array of string is declared as String[] str = new String()[SIZE]; — ItachiUchiha
– ItachiUchiha, Commented Feb 25, 2014 at 17:43
String had no method add(...) you should use a cotainer to add several strings in it. (Array or Collection) — pL4Gu33
– pL4Gu33, Commented Feb 25, 2014 at 17:45
I'm confused about what you are trying to do. What exactly are you trying to achieve? — Josh M
– Josh M, Commented Feb 25, 2014 at 17:46

rgettman · Accepted Answer · 2014-02-25 17:47:31Z

1

In Java, you cannot use the value of a String object to reference an actual variable name. Java will think you're attempting to to call add on the String object, which doesn't exist and gives you the compiler error you're seeing.

To avoid the repetition, you need to add your Lists to two master lists that you can index.

In your question, you mention arrays, but you call add, so I'm assuming that you're really referring to Lists of some sort.

List<List<Integer>> numbers = new ArrayList<List<Integer>>(20);
List<List<Integer>> numbersy = new ArrayList<List<Integer>>(20);
// Add 20 ArrayList<Integer>s to each of the above lists in a loop here.

Then you can bounds-check ant.getAntNumber() and use it as an index into your master lists.

int antNumber = ant.getAntNumber();
// Make sure it's within range here.

numbers.get(antNumber).add(ant.getCol());
numbersy.get(antNumber).add(ant.getRow());

answered Feb 25, 2014 at 17:47

rgettman

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8 Comments

Mivox Over a year ago

How would I then get this data back? Currently using this from my old version for (int i = 0; i<numbers.size(); i++) { int x = numbers.get(i); int y = numbersy.get(i); Ant.world[x][y].increasePheromone(2); } This is to get the route back which the ant has taken, and then increase the pheromone value on all of the cells which the ant has traversed, thanks

rgettman Over a year ago

Like populating the lists, call get on your master lists to get the proper lists for each ant, then iterate over both of those lists to get your x and y values back out.

Mivox Over a year ago

the solution you provided is giving me an index out of bounds exception, as it says the size of it is always 0, is this because I'm trying to get it before theres anything stored in it?

rgettman Over a year ago

Yes. Add empty lists where I commented, "Add 20 ArrayList<Integer>s".

Mivox Over a year ago

sorry to be so simple but you have confused me :(

|

aliteralmind · Accepted Answer · 2014-02-25 17:53:58Z

How about this?

Ant[] aAnt = new Ant[20];

//Fill the ant-array

int[] aColumns = new int[aAnt.length];
int[] aRows = new int[aAnt.length];

for(int i = 0; i < aAnt.length; i++)  {
   aColumns[i] = aAnt[i].getCol();
   aRows[i] = aAnt[i].getRow();
}

or with lists:

List<Integer> columnList = new List<Integer>(aAnt.length);
List<Integer> rowList = new List<Integer>(aAnt.length);

for(Ant ant : aAnt)  {
   columnList.add(ant.getCol());
   rowList.add(ant.getRow());
}

or with a col/row object:

class Coordinate  {
   public final int yCol;
   public final int xRow;
   public Coordinate(int y_col, int x_row)  {
      yCol = y_col;
      xRow = x_row;
   }
}

//use it with

List<Coordinate> coordinateList = new List<Coordinate>(aAnt.length);
for(Ant ant : aAnt)  {
   coordinateList.add(ant.getCol(), ant.getRow());
}

Daniël Knippers · Accepted Answer · 2014-02-25 17:54:51Z

1

A straight-forward port of your code would be to use two Map<Integer, Integer> which store X and Y coordinates. From your code it seems like ant numbers are unique, i.e., we only have to store a single X and Y value per ant number. If you need to store multiple values per ant number, use a List<Integer> as value type of the Map instead.

Map<Integer, Integer> numbersX = new HashMap<Integer, Integer>();
Map<Integer, Integer> numbersY = new HashMap<Integer, Integer>();

for(Ant ant : ants) {
    int number = ant.getAntNumber();        
    numbersX.put(number, ant.getCol());
    numbersY.put(number, ant.getRow());
}

answered Feb 25, 2014 at 17:54

Daniël Knippers

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