How to parse a mathematical expression given as a string and return a number? [duplicate]

Answer 1

public static int calc(String string){
    int result=0; 
    String numbers="0123456789";
    String operations="+-/*";
    for (int i=0;i<string.length();i++){
        if (numbers.contains(string.charAt(i)+"")){
            result=result*10+(Integer.parseInt(string.charAt(i)+""));
            }
        else {
            if (string.charAt(i)=='+'){ result+=calc(string.substring(i+1));}
            if (string.charAt(i)=='-'){ result-=calc(string.substring(i+1));}
            if (string.charAt(i)=='*'){ result*=calc(string.substring(i+1));}
            if (string.charAt(i)=='/'){ try{result/=calc(string.substring(i+1));}
                catch (ArithmeticException e){
                    System.err.println("You cannot devide by Zero!");}
            }  
            break;
        }        
    }
    return result;
}

Answer 2

There is no builtin way of doing that. But you can use one of the many many open source calculators available.

Answer 3

Recently I was using very mature math expression parser library, open source, giving the same API for JAVA and .NET. The library name is mXparser. mXparser provides basic functionalities (simple formulas parsing and calculation) and more advanced ones (i.e. user defined arguments, functions). Additionally it is worth to notice that mXparser has rich built-in math collection (meaning operators, unary / binary / variadic functions, iterated operators such as summation and product).

https://mathparser.org/

https://mathparser.org/mxparser-tutorial/

Please find below a few examples to have more clear view on the syntax.

Example 1 - simple formula

Expression e = new Expression("2+3");
double v = e.calculate();

Example 2 - built-in function

Expression e = new Expression("2+sin(3)");
double v = e.calculate();

Example 3 - built-in constants

Expression e = new Expression("2+sin(pi)");
double v = e.calculate();

Example 4 - user defined arguments and constants

Argument x = new Argument("x = 5");
Constant a = new Constant("a = 2 + sin(3)");
Expression e = new Expression("a + x^2", x, a);
double v1 = e.calculate();
x.setArgumentValue(10);
double v2 = e.calculate();

Example 5 - user defined functions

Function f = new Function("f(x,y) = x^2 + cos(y)");
Expression e = new Expression("f(10,pi) - 3", f);
double v = e.calculate();

Example 6 - user defined recursion

Function factorial = new Function("fact(n) = if( n > 0; n*fact(n-1); 1)");
Expression e = new Expression("fact(10) - 10!", factorial);
double v = e.calculate();

Found recntly - in case you would like to try the syntax (and see the advanced use case) you can download the Scalar Calculator app that is powered by mXparser.

Best regards

LK

Answer 4

You can pass it to a BeanShell bsh.Interpreter, something like this:

Interpreter interpreter = new Interpreter();
interpreter.eval("result = 5+4*(7-15)");
System.out.println(interpreter.get("result"));

You'll want to ensure the string you evaluate is from a trusted source and the usual precautions but otherwise it'll work straight off.

If you want to go a more complicated (but safer) approach you could use ANTLR (that I suspect has a math grammar as a starting point) and actually compile/interpret the statement yourself.

Answer 5

You can use the ScriptEngine class and evaluate it as a javascript string

ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("js");        
Object result = engine.eval("5+4*(7-15)");

Indeed , yu should know that the result of the following instruction in javascript :

   eval('var aa=5+4*(7-15)')
   aa // -27

There may be a better way, but this one works.

Answer 6

i recently developed a expression parser and released it under the apache license. you can grab it at http://projects.congrace.de/exp4j/index.html

hope that helped