Java parse strings stored in an arraylist into a calculation

Question

So, I have an ArrayList that stores information like: {"2.0", "+", "2.0", "-", "1.0"} and I need to parse that into 2 + 2 - 1, however the method I made to do that doesnt work.

Method Code:

public static void ans()
{
    Double cV = Double.parseDouble(calculate.get(0));

    for(int i = 1; i < calculate.size(); i += 2)
    {

        switch(calculate.get(i))
        {
            case "+":
                cV += Double.parseDouble(calculate.get(i + 1));
                break;
            case "-":
                cV -= Double.parseDouble(calculate.get(i + 1));
                break;
        }
    }

    calc.setText("= " + cV);
}

"calculate" here is my arrayList.

What it is doing wrong is just returning the first number rather than the answer to the calculation. Any help would be appreciated!

EDIT: I added System.out.print(calculate.get(i) + ", " + calculate.get(i + 1) + ", "); into the for loop and nothing is happening... For some reason the loop isn't getting run.

EDIT: Full Code: http://pastebin.com/cP3hGgA3

EDIT: So I just added: System.out.println(calculate.size()); into the method, and it is returning 1... What is going on?

EDIT: I think the problem is here:

public static void addTo(String toAdd)
{
    try{
        if(!isNumeric(toAdd))
        {
            if(!isNumeric(calc.get(calc.size() - 1)))
            {
                calc.set(calc.size() - 1, toAdd);
            }
        }else{
            calc.add(toAdd);
        }
    }catch(Exception e){ }
}

public static boolean isNumeric(String str)  
{  
    try{  
        Double.parseDouble(str);  
    }catch(NumberFormatException nfe){  
        return false;  
    }
    return true;  
}

EDIT: Short Code:

    package net.discfiresoftworks.shortcalc;

import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.ArrayList;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;

public class Short extends JFrame
{
    private static final long serialVersionUID = 1L;

    public static ArrayList<String> calc = new ArrayList<String>();

    public static JLabel ans = new JLabel("");

    public static void main(String[] args)
    {
        new Short();
    }

    public Short()
    {
        this.setSize(300, 300);
        this.setDefaultCloseOperation(3);
        this.setLayout(new FlowLayout());

        JButton b1 = new JButton("Click me");
        JButton b2 = new JButton("then me");
        JButton b3 = new JButton("then me.");

        b1.addActionListener(new ActionListener(){

            @Override
            public void actionPerformed(ActionEvent arg0)
            {
                addTo("1");
            }

        });

        b2.addActionListener(new ActionListener(){

            @Override
            public void actionPerformed(ActionEvent arg0)
            {
                addTo("+");
            }

        });

        b3.addActionListener(new ActionListener(){

            @Override
            public void actionPerformed(ActionEvent arg0)
            {
                addTo("1");
                ans();
            }

        });

        this.add(b1);
        this.add(b2);
        this.add(b3);
        this.add(ans);

        this.setVisible(true);
    }

    public static void addTo(String toAdd)
    {
        try{
            if(!isNumeric(toAdd))
            {
                if(!isNumeric(calc.get(calc.size() - 1)))
                {
                    calc.set(calc.size() - 1, toAdd);
                }
            }else{
                calc.add(toAdd);
            }
        }catch(Exception e){ }
    }

    public static boolean isNumeric(String str)  
    {  
        try{  
            Double.parseDouble(str);  
        }catch(NumberFormatException nfe){  
            return false;  
        }
        return true;  
    }

    public static void ans()
    {
        Double cV = Double.parseDouble(calc.get(0));

        System.out.println(calc.size());

        for(int i = 1; i < calc.size(); i += 2)
        {

            switch(calc.get(i))
            {
                case "+":
                    cV += Double.parseDouble(calc.get(i + 1));
                    break;
            }
        }

        ans.setText("= " + cV);
    }
}

Can you have any `*` and `+` operators? If yes what should be result of `{"2.0", "+", "2.0", "*", "2.0"}`? — Pshemo, Jun 07 '14 at 16:52
What actually goes wrong here? Your code seems to be working fine for me. — Pshemo, Jun 07 '14 at 16:57
@StressedProgrammer you may need to check your `calc.setText` method — Sajan Chandran, Jun 07 '14 at 16:59
http://ideone.com/tPcGjl says something else. Are you sure about content of your list? maybe it is something like `2+2-2`? — Pshemo, Jun 07 '14 at 16:59
You need to provide [short but full example](http://stackoverflow.com/help/mcve) which will let us reproduce your problem. Don't make us guess where you made mistake. — Pshemo, Jun 07 '14 at 17:08
No, full code is not welcome here, since it contains too many lines of irrelevant code. Take your time to create [SSCCE](http://sscce.org/) and post it as part of your question, not just link to external site. — Pshemo, Jun 07 '14 at 17:12
@arielnmz I discovered that it's not the method that is causing the problem. — StressedProgrammer, Jun 07 '14 at 17:28
What should `addTo` do? What does it have to receive and what should be its output? — arielnmz, Jun 07 '14 at 17:34
It should receive either a number or a fucntion, and what it does is adds it to the array if the previous element is not of the same type. — StressedProgrammer, Jun 07 '14 at 17:37
@StressedProgrammer If you refresh the page there should be a link at the top. I have to admit though, I'm not seeing why this is a duplicate of that question. Perhaps one of the dupe markers could elaborate? — JonK, Jun 07 '14 at 17:45

arielnmz · Answer 1 · 2014-06-07T17:27:31.897

This should do the trick (I suupose that calculate is the array in the form {"2.0", "+", "2.0", "-", "1.0"}:

public static void ans() {
    Double total = 0.0;
    boolean isSum = true;

    for (String input : calculate) {
        switch (input) {
            case "+":
                isSum = true;
                break;
            case "-":
                isSum = false;
                break;
            default:
                if (isSum)
                    total += Double.parseDouble(input);
                else 
                    total -= Double.parseDouble(input);
                break;
        }
    }

    /*
    * Now you have the total variable which stores the sum. 
    * You can do whatever you want with it, like printing
    * it along with the result.
    */
    for (String input : calculate) {
        System.out.print(input+" ");
    }
    System.out.print(" = "+total);

}

However, this will not work (as expected) if you have an array like this:

{"2.0", "+", "5.0", "7.0"}

As it will sum the 5 and the 7 because it stores the last sign you used, you may want to implement some sort of vallidation method that requires symbols between numbers. But if you're sure your input will always be number, symbol, number then it'll no problem with this code.

Java parse strings stored in an arraylist into a calculation

1 Answers1