ArrayList selection Sort

[Bryan Peach]

Hey there,
I created an ArrayList called names. The idea is that the user enters a bunch of names in any order and the program sorts them and prints a report sheet with the names sorted. Here is what I have for my sort method so far.

    
public void sortList()
    {
    	for(int k=0;k<names.size();k++)
    	{
    		int max=0;
    		for(int l=0;l<names.size()-1;l++)
    			if (names.get(l)>names.get(max))
    				max=l;	
    	}
    }

I get an error telling me that I cannot use > on the line

    
if (names.get(l)>names.get(max))

I realize this is probably some simple error but i don't understand why i cannot do that or how to get around it... help please....

[Tim Sparg]

Hi Bryan

I'm not too sure what you are trying to use the '>' operator for.
that operator works for numerics, not string values.

what you can do is

   
if (names.get(l).compareTo(names.get(max)) > 1) 
 max=l; 
 

But the best solution is to use the Collections class

   
Collections.sort(names);
 

This will sort the list by its natural ordering which should be alphabetical

[Bryan Peach]

Ok so now I have my sort method and a swap method... the sort method is supposed to figure out the order of where the strings should be... and my swap method is supposed to swap the contents in the string list i have PLUS make the same changes in two other lists i have made to keep the data compatible... here is the code for the two methods.

    
public void sortList()
    {
    	for(int k=0;k<names.size();k++)
    	{
    		int max=0;
    		for(int l=0;l<names.size()-1;l++)
    			if (names.get(l).compareTo(names.get(max))>1)
    				max=l;	
    	}
    	someSalesList.swapList();
    }
    
    public void swapList()
    {
    	String temp;
    	temp=names.get(k);
    	names.set(k,names.get(l));
    	names.set(l,temp);
    	
    	Integer I=sales.get(l);
    	sales.set(I,sales.get(l));
    	sales.set(l,temp);
    	
    	Float F=expenses.get(l);
    	expenses.set(F,expenses.get(l));
    	expenses.set(l,temp);	
    }

I get a few errors from these when trying to compile. First of all i get a error from the method call within the sort method saying it cannot find the symbol variable someSalesList. but thats the call i used all throughout the program for my methods and it worked... do you have to use a different call when calling a method from within a method?
The other errors i get are cannot find symbol method set in my swap method... any ideas?
Also, this sorting and swapping thing is very new to me... and while i understand the concept and how it is supposed to work the code seems very confusing to me... does the code look alright? or are there some serious problems going on... Thanks

[Campbell Ritchie]

Not quite alphabetical, but what is jocularly called asciibetical. More details here, as well as the Google search earlier.

Note many of these questions are class exercises where using Collections#sort is prohibited.

[Campbell Ritchie]

You ought to use a static swapList method which takes the List, and the two indices as its parameters.

 . . .
public static void swapTwoElementsInList(List l, int i, int j)
{
    if (i != j)
    {
        // Work it out for yourself
    }// end if
}//end swap two elements in list method

Use int rather than Integer.

Use i as your index in the for loop, not l because l can be difficult to distinguish from 1.

In a selection sort, you will probably want to start the loop with max = k;

It looks like a very awkward way to arrange sorting, trying to sort three Lists in parallel. Very difficult to maintain. You want a List of SalesAndExpenses and a SalesAndExpenses object which incorporates the three data you are using.

public class SalesAndExpenses implements Comparable<SalesAndExpenses>
{
    private Sales sales;
    private Expenses Expenses;
    private String name;
 . . .
    @Override public int compareTo(SalesAndExpenses other)
    {
        return this.name.compareTo(other.name);
    }//end compare to

Maybe a different name for the SalesAndExpenses class would be appropriate.

Haev you looked through the List interface for set methods? Note how many parameters set requires.

[John de Michele]

I get a few errors from these when trying to compile. First of all i get a error from the method call within the sort method saying it cannot find the symbol variable someSalesList. but thats the call i used all throughout the program for my methods and it worked... do you have to use a different call when calling a method from within a method?

Did you define someSalesList in your sortList() method? Is it an instance variable of your class?

John.

[Bryan Peach]

John, I defined someSalesList in the main method and thats what i used to call all my other methods and it worked. So i'm not sure if there is a different way to call it from within a method. Here is the entire code. That might help to see what is going on here.

    
import java.util.ArrayList;
 import java.util.Scanner; 
public class SalesList {
	static Scanner in;		//Scanner variable called in
	static float totalsales=0.0f;		//total sales in list
	static float totalexpenses=0.0f;		//total expenses in list
	static int k;
	static int l;
	
	static ArrayList<String> names;		//declaring a list called names
	static ArrayList<Integer> sales;		//declaring a list called sales
	static ArrayList<Float> expenses;	//declaring a list called expenses
    
    public void getInput()	//a method to prompt for and add input to the lists
    {
    	String name;	//name variable for input
		int sale;		//sale variable for input
		Float exp;		//expense variable for input
    	
    	System.out.println("Please enter employee's name. Last name first. i.e. Smith Adam.");	//Prompt for input
    	name=in.next(); //name is equal to input
    	names.add(name);	//add name to the names list
    	
    	System.out.println("Please enter the employee's sales figures in dollars. i.e. 9999.");	//prompt for input
    	sale=in.nextInt();	//sale is equal to input
    	sales.add(sale);	//add sale to the sales list
    	
    	System.out.println("Please enter the employee's total expenses. i.e. 9999.99");	//prompt for input
    	exp=in.nextFloat();	//exp is equal to input
    	expenses.add(exp);	//add exp to the expenses list
    	
    }
    
    public float getTotalSales()	//a method to add the sales list and get the total
    {
    	for(int i=0;i<sales.size();i++)	//loop to iterate through the list
    		totalsales=totalsales+(sales.get(i));	//formula to add the sales together
    		return totalsales;	//return the total when finished
    }
    
    public float getTotalExpenses()	//a method to add the expenses list and get the total
    {
    	for(int j=0; j<expenses.size();j++)	//loop to iterate through the list
    		totalexpenses=totalexpenses+(expenses.get(j));	//formula to add expenses together
    		return totalexpenses;	//return the total when finished
    		
    }
    
    public void sortList()
    {
    	for(int k=0;k<names.size();k++)
    	{
    		int max=0;
    		for(int l=0;l<names.size()-1;l++)
    			if (names.get(l).compareToIgnoreCase(names.get(max))>1)
    				max=l;	
    	}
    	someSalesList.swapList();
    }
    
    public void swapList()
    {
    	String temp;
    	temp=names.get(k);
    	names.set(k,names.get(l));
    	names.set(l,temp);
    	
    	Integer I=sales.get(l);
    	sales.set(I,sales.get(l));
    	sales.set(l,temp);
    	
    	Float F=expenses.get(l);
    	expenses.set(F,expenses.get(l));
    	expenses.set(l,temp);	
    }
    
    public void printReport()	//method to print a report
    {
    	System.out.println("********************************************************************************");	//output	
    	System.out.println("********************************************************************************");	//output	
    	System.out.println("                            Acme Corp. Sales Report.");								//output
    	System.out.println("");																					//blank line
    	System.out.println("                    Total Sales= $"+totalsales+" Total Expenses= $" +totalexpenses);//output
    	System.out.println("");																					//blank line
    	System.out.println("Employee name:		Percentage of total sales:		Percentage of total expenses:");//output
    	
    	for(int k=0;k<names.size();k++)	//loop to iterate through the names list and print the results
    	{
    		System.out.println(names.get(k)+"						"+(sales.get(k))/totalsales*100+
    			"						"+(expenses.get(k))/totalexpenses*100);//loop output
    	}
    	
    	System.out.println("                             End of Sales Report.");		//output
    	System.out.println("********************************************************************************");	//output	
    	System.out.println("********************************************************************************");	//output
    }
    
    public static void main(String[] args) {
    	
    	int action=1;		//Variable to control the switch
    	
    	names=new ArrayList<String>();		//creates names list
    	sales=new ArrayList<Integer>();		//creates sales list
  		expenses= new ArrayList<Float>();		//creates expenses list
    	
    	in=new Scanner(System.in);		//sets scanner object = in
    	
    	SalesList someSalesList=new SalesList();		//instaniates a new Saleslist object
    	
    	
    	while (action>0&&action<3)			//While loop to loop back to the switch after every method call
    	{
    	//Output statements to prompt the user to make their choice
    	System.out.println("Please choose from the options below"+
    		 " and enter the number of your choice");
    	System.out.println("1. Add to the list.");
    	System.out.println("2. Print sales report.");
    	System.out.println("3. Exit program");
    	
    	action=in.nextInt();	//action variable used in switch statement.
    	
    	switch(action)			// Switch to control flow of the program
    	{
    		case 1:someSalesList.getInput();  					//method call for when 1 is entered
    			break;
    		case 2:someSalesList.getTotalSales(); 				//method calls for when 2 in entered
    			   someSalesList.getTotalExpenses();
    			   someSalesList.sortList();
    			   someSalesList.printReport();	
    			break;
    		default:
    			System.out.println("Program will exit now.");	// default case to create an exit for the program
    	}
    	}
    	
    	
    	
    	
    	
    	
    	
    	
    }
}

[Campbell Ritchie]

Do a search for my posts about Scanner. You will not get "Peach Bryan" by using the next() method; you will get "Peach", followed by an Exception when it tries to turn Bryan in to an int. You probably want "nextLine".
Your totalExpenses will give the wrong result; you don't want a field at all, but a local variable, otherwise you get more expenses every tiem that method is called.

You still have the design problem that you don't have a class which incorporates all the data. I now think, having see what you are doing, that a Salesman (or Saleswoman) class is what you want rather than SalesAndExpenses. It will probably also prevent you from trying to add a String to a List<Integer>.

[John de Michele]

Bryan:

It works in your main() method because someSalesList is in scope in your main() method. It doesn't work in your sortList() method because someSalesList is not in scope there. You need to either a) define someSalesList in the sortList() method, or (better) b) pass someSalesList to sortList(), so that it can work on it.

John.