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recursive function problem

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Join Date: Oct 2008

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Reputation: efus is an unknown quantity at this point

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efus

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recursive function problem

Nov 30th, 2008

Hi, I am having some troubles with a recursive function.
The function needs to get a string as an input. The string contains both symbols and numbers. What the function does is doing some operation on the two children of a node, the node being the operation(+,-,*,/).
The thing is, I need the function to return a double. This couses a problem since I cant just convert the string to a double becouse of the operators.
This is the algoritm:

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 Java Syntax (Toggle Plain Text) 
function Evaluate {
if node is an operator(+,-,*,/) then
    evaluate the right and left child
    apply the operator of the node to the two children
else if the node is a value then
    the result is the value
function Evaluate {
if node is an operator(+,-,*,/) then
    evaluate the right and left child
    apply the operator of the node to the two children
else if the node is a value then
    the result is the value

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Join Date: Aug 2008

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Reputation: mahlerfive is an unknown quantity at this point

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mahlerfive mahlerfive is offline

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Re: recursive function problem

Nov 30th, 2008

You shouldn't ever be returning the operators though, only the result of operations or the number value in the node.

In the first branch, after you apply the operator to the two children, you should get back a double then return that.

In the second branch you should be returning the value (which is a double).

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Join Date: Oct 2008

Posts: 11

Reputation: efus is an unknown quantity at this point

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efus

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Re: recursive function problem

Nov 30th, 2008

You are right. It will never return anything but a double.
I wrote some code. The problem now is that I get a NullPointerException which I dont get. Every operator has two children so the none of the children should be null.

 Help with Code Tags 
 Java Syntax (Toggle Plain Text) 
    public Double eval()
    {
         Double answer = null;
         if(data != null)
         {
            if(data.equals("-")) answer += (rightChild.eval() - leftChild.eval());
            else if(data.equals("+")) answer += (rightChild.eval() + leftChild.eval());
            else if(data.equals("*")) answer += (rightChild.eval() * leftChild.eval());
            else if(data.equals("/")) answer += (rightChild.eval() / leftChild.eval());
            else answer += ((Double)data).doubleValue();
        }
 
 
        return answer;
    }
    public Double eval()
    {
         Double answer = null;
         if(data != null)
         {
            if(data.equals("-")) answer += (rightChild.eval() - leftChild.eval());
            else if(data.equals("+")) answer += (rightChild.eval() + leftChild.eval());
            else if(data.equals("*")) answer += (rightChild.eval() * leftChild.eval());
            else if(data.equals("/")) answer += (rightChild.eval() / leftChild.eval());
            else answer += ((Double)data).doubleValue();
        }
        
       
        return answer;
    }

[EDIT]

I changed the code to this:

 Help with Code Tags 
 Java Syntax (Toggle Plain Text) 
    public Double eval()
    {
         Double answer = 0.0;
 
            if(data.equals("-")) answer += (rightChild.eval() - leftChild.eval());
            else if(data.equals("+")) answer += (rightChild.eval() + leftChild.eval());
            else if(data.equals("*")) answer += (rightChild.eval() * leftChild.eval());
            else if(data.equals("/")) answer += (rightChild.eval() / leftChild.eval());
            else answer += ((Double)data).doubleValue();
 
        return answer;
    }
    public Double eval()
    {
         Double answer = 0.0;
        
            if(data.equals("-")) answer += (rightChild.eval() - leftChild.eval());
            else if(data.equals("+")) answer += (rightChild.eval() + leftChild.eval());
            else if(data.equals("*")) answer += (rightChild.eval() * leftChild.eval());
            else if(data.equals("/")) answer += (rightChild.eval() / leftChild.eval());
            else answer += ((Double)data).doubleValue();
            
        return answer;
    }

now I get "java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Double"

Last edited by efus; Nov 30th, 2008 at 5:37 pm.

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shaun.husain shaun.husain is offline

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Re: recursive function problem

Nov 30th, 2008

I think this is all you need to do, although to be honest I'm not the best at recursion myself, but if everything else is right this will fix your error I believe.
Double.parseDouble(data);