Creating chronological variable names automatically

Question

I am writing a program that will take input of polynomials. Needing to create a new ArrayList for each polynomial input, I am needing a way to name each list without knowing the amount of polynomial beforehand. If a file has 2 polynomials I will need to name 2 arrays, but if more polynomials I will need to name more arrays. Is there anyway to automatically name arrays or variables with the iteration of a loop. I cannot figure out how. Variable names such as : P1 , P2, P3 etc. as the number of Polynomials increases is what I am searching for. Each polynomial will be read in line by line. I have attached my code, Though it is nowhere near complete. I imagine I will need to move the PolyCalc creation into the while loop and create a new PolyCalc for each line of input. I am looking to add this feature to the while loop in the main method.

import java.io.File;
import java.io.FileNotFoundException;
import java.util.ArrayList;
import java.util.Scanner;


public class PolyProcessor {
    static int polyNum = 0;
public static void main(String[] args) throws FileNotFoundException{


    File polyfile = new File("polyinput.txt");
    Scanner read = new Scanner(polyfile);
    while (read.hasNextLine()){
        PolyCalc c = new PolyCalc();
        String j = read.nextLine();
        c.add(j.split(";"));
        polyNum++;}


    }
    }


class PolyCalc{
    static int polyCount = 0;

    static ArrayList polynomials = new ArrayList();

     static void add(String[] strings){
         for(int i = 0; i<strings.length;i++){
           polynomials.add(strings[i]);
           polyCount++;}
     }
     static Object get(int i){
         return polynomials.get(i);

     }
}

Answer 1

Why not use a (Hash) Map where the key is the variable name?

Map polys = new HashMap();

int count=0;

For ...
    string key = String.format("p%02d",count++);
    polys.put(key, newPoly(...));

I'd have to look up the String.format but something like that.

The order needs to be preserved so just choose long enough zero padded keys that you can sort. And/or use a linkedHashMap which keeps the insertion order.

Answer 2

First of all, you can't do this using variables. Java does not allow you to declare variables dynamically. It is not that kind of programming language ...

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If it is mandatory that the polynomials are stored in an ArrayList then:

• get the user to refer to the polynomials by number (i.e. position in the list) instead of by name, or

• create a hashmap that maps from names to positions in the list, or

• store the polynomials in BOTH an ArrayList<String> AND a HashMap<String, String>.

Actually, I think that you may have misinterpreted the requirements for your programming exercise. I suspect that you are asked to represent each individual polynomial as an ArrayList (or a custom class that has an ArrayList inside it). Representing a polynomial as a String doesn't allow you to do any operations on it ... without first parsing the String and turning it into another form.

Answer 3

As Stephen's answer and you said that arraylist is mandatory you could still use ArrayList

ArrayList numbers = new ArrayList();


HashMap<String,ArrayList> myPolys = new HashMap<String,ArrayList>();

and to use the HashMap

myPolys.put(what do you want to call them or your so "called" variables , numbers);

Answer 4

As you absolutely need to use ArrayList class to store your polynomials you can use its add(int index, E Element) method as follows:

List polynomials= new ArrayList(); 
for(int k=0;k < counter;k++){
    polynomials.add(k, new Poly(...));
}

You won't have P0, P1, ... but polynomials.get(0), polynomials.get(1), ... Thanks to gmhk in this.