How can I let the user to enter a maths formula and unknowns to run in Java?

Question

How can I let the user to enter a maths formula and unknowns to run in Java?

Answer 1

With just 2 units what you could do is have a for loop with a nested for loop. Have each and every set of phrases listed in a separate array. Index the first for loop from zero - array1.Length()-1, index the 2nd for loop from zero - array2.Size()-1. Output a string from array1 corresponding to the index of the first for loop and output a string from array2 comparable to the index of the 2d for loop.

Answer 2

some simple starter code for you

Answer 3

/*

* Use the JavaScript Engine to employ calls to the eval() function.

* This is much simpler than writing a full formula parser and lexer

* with the capacity to tokenize, substitute, and evaluate general

* mathematical expressions.

*

* Session example:

* Enter a formula.

* (y-3.25) / x

* Enter the value of the variable x:

* 10.35

* Enter the value of the variable y:

* 13.6

* formula: (y-3.25) / x

* with variables:

* x = 10.35

* y = 13.6

* translates to: (13.6-3.25) / 10.35 and

* evaluates to: 1.0

*/



import java.util.Hashtable;

import java.util.Scanner;

import java.util.Map.Entry;



import javax.script.ScriptEngine;

import javax.script.ScriptEngineManager;

import javax.script.ScriptException;



public class FormulaEvaluator

{

public static void main( String[] args ) {

// Scanner to collect user input

Scanner scan = new Scanner(System.in);



// prompt user for formula string

System.out.println( "Enter a formula.");

String formula = scan.nextLine();

// variables will be presumed to be single, lower-case

// alphabetical characters - kill everything else

String variables = kernel(

formula.replaceAll( "[^a-z]", "" )

);



// to collect values or each variable

String[] values = new String[ variables.length() ];

// for each lower-case variable name, collect value

for (int i = 0; i < variables.length(); i++) {

System.out.println(

"Enter the value of the variable " +

variables.charAt( i ) + ": "

);

values[ i ] = scan.next();

}



// copy original formula

String evaluable = formula;

// substitute the corresponding value for each variable

// in the original formula

for (int i = 0; i < variables.length( ); i++) {

evaluable = evaluable.replaceAll(

"" + variables.charAt( i ), values[ i ]

);

}



// get script engine manager instance

ScriptEngineManager manager = new ScriptEngineManager();

// javascript engine instance to use for evaluation, since

// math ops are mostly the same

ScriptEngine engine = manager.getEngineByName("js");

Object result = null;

try {

result = engine.eval( evaluable );

} catch (ScriptException e) {

System.out.println( e.toString() );

}



// compose output to report

String rpt = "formula: " + formula + "\nwith variables:";

for (int i = 0; i < variables.length( ); i++) {

rpt += "\n " + variables.charAt( i ) + " = " +

values[ i ];

}

rpt += "\ntranslates to: " + evaluable + " and " +

"\nevaluates to: " + String.valueOf( result );

System.out.println( rpt );

}

// Return unique elements of a on first dimension.

public static String kernel( String s ) {

// Set hashtable style associative array working storage.

Hashtable r = new Hashtable();

// For each element in s...

for (int i = 0; i < s.length(); i++) {

// ...assign key/value pair - repeated values have same key and

// replace one another, leaving only single instance of value.

r.put("" + s.charAt( i ), "" + s.charAt( i ));

}

// Initialize return.

StringBuffer sb = new StringBuffer();

// For each key in r...

for (Entry entry : r.entrySet()) {

sb.append( entry.getKey() );

}

return sb.toString();

};



}



UPDATE:

It occurred to me that you might get formulas with more than one instance of a particular variable,e.g., (x+y)/x. So, I added a step that makes sure you only ask for the value of each variable once.



ADDED:

Java version more prettily formatted at http://pastebin.com/sJYnSh5i

HTML/JavaScript version at http://pastebin.com/Q5hHyhMn