I'm working on a tool in the context of a java project to evaluate a custom domain specific, rule-like expression like
min-5 avg datalist > Number
with the individual tokens meaning the following:
min-5 : optional minimum (or maximum, in that case max-5) occurences of the following term
avg : an optional aggregation function which operates on the following token datalist (can also be sum or anything similar)
datalist : A list of data (type: integer/ double) which will be available before the evaluation of the entire expression starts, can be reduced to a single value by the preceding aggregation function
operator: conditional operator < or > or =
Number: value for the conditional operator
Note(s):
The optional amount of occurrences and the aggregation can not happen both, that would make no sense.
There can be multiple of the above expressions, chained with and/or
These expressions are external input, not pre-defined
The evaluation of this expression should output a boolean
As I am rather new to expression evaluation / parsing I am searching for an elegant way to solve this, possibly with a java framework/tool.
What I've tried so far:
Parsing by hand which turned out not so nicely
Trying to use Janino Expression Evaluator, but I don't know how to handle this programmatically
I am searching for a solution to solve this in an elegant way, I am thankful for any suggestions
what you try to do is a DSL (domain specific language) and the elegant way to solve your issue is to create a grammar for yuor specific language that help you on parsing function.
Take a look at JavaCC or Antlr.
For a programming project in Calculus we were instructed to code a program that models the Simpson's 1/3 and 3/8 rule.
We are supposed to take in a polynomial(i.e. 5x^2+7x+10) but I am struggling conceptualizing this. I have began by using scanner but is there a better way to correctly read the polynomial?
Any examples or reference materials will be greatly appreciated.
I'd suggest that you start with a Function interface that takes in a number of input values and returns an output value:
public interface Function {
double evaluate(double x);
}
Write a polynomial implementation:
public class Poly {
public static double evaluate(double x, double [] coeffs) {
double value = 0.0;
if (coeffs != null) {
// Use Horner's method to evaluate.
for (int i = coeffs.length-1; i >= 0; --i) {
value = coeffs[i] + (x*value);
}
}
return value;
}
}
Pass that to your integrator and let it do its thing.
A simple way (to get you started) is to use an array.
In your example: 5x^2 + 7x + 10 would be:
{10,7,5}
I.e. at index 0 is the factor 10 for x^0 at index 1 is 7 for x^1 at index 2 is 10 for x^2.
Of course this not the best approach. To figure out way figure out how you would represent x^20
In java it would be easiest to pre-format your input and just ask for constants--as in, "Please enter the X^2 term" (and then the X term, and then the constant).
If that's not acceptable, you are going to be quite vulnerable to input style differences. You can separate the terms by String.split[ting] on + and -, that will leave you something like:
[5x^2], [7x], [10]
You could then search for strings containing "x^2" and "x" to differentiate your terms
Remove spaces and .toLowerCase() first to counter user variances, of course.
When you split your string you will need to identify the - cases so you can negate those constants.
You could do two splits, one on + the other on -. You could also use StringTokenizer with the option to keep the "Tokens" which might be more straight-forward but StringTokenizer makes some people a little uncomfortable, so go with whatever works for you.
Note that this will succeed even if the user types "5x^2 + 10 + 7 x", which can be handy.
I believe parsing is my problem. I am somewhat new to java so this is troubling me.
You should use a parser generator.
A parser generator is a tool that reads a grammar specification and converts it to a Java program that can recognize matches to the grammar. In addition to the parser generator itself, JavaCC provides other standard capabilities related to parser generation such as tree building (via a tool called JJTree included with JavaCC), actions, debugging, etc.
JavaCC's FAQ answers How do I parse arithmetic expressions?
See the examples that come with JavaCC.
See any text on compiling.
See Parsing Epressions by Recursive Descent and a tutorial by Theodore Norvell.
Also, see JavaCC - Parse math expressions into a class structure
Is there any built in support for array in XQuery? For example, if we want to implement
the simple java program in xquery how we would do it:
(I am not asking to translate the entire program into xquery, but just asking
how to implement the array in line number 2 of the below code to xquery? I am
using marklogic / xdmp functions also).
java.lang.String test = new String("Hello XQuery");
char[] characters = test.toCharArray();
for(int i = 0; i<characters.length; i++) {
if(character[i] == (char)13) {
character[i] = (char) 0x00;
}
}
Legend:
hex 0x00 dec 0 : null
hex 0x0d dec 13: carriage return
hex 0x0a dec 10: line feed
hex 0x20 dec 22: dquote
The problem with converting your sample code to XQuery is not the absence of support for arrays, but the fact that x00 is not a valid character in XML. If it weren't for this problem, you could express your query with the simple function call:
translate($input, '', '')
Now, you could argue that's cheating, it just happens so that there's a function that does exactly what you are trying to do by hand. But if this function didn't exist, you could program it in XQuery: there are sufficient primitives available for strings to allow you to manipulate them any way you want. If you need to (and it's rarely necessary) you can convert a string to a sequence of integers using the function string-to-codepoints(), and then take advantage of all the XQuery facilities for manipulating sequences.
The lesson is, when you use a declarative language like XQuery or XSLT, don't try to use the same low-level programming techniques you were forced to use in more primitive languages. There's usually a much more direct way of expressing the problem.
XQuery has built-in support for sequences. The function tokenize() (as suggested by #harish.ray) returns a sequence. You can also construct one yourself using braces and commas:
let $mysequence = (1, 2, 3, 4)
Sequences are ordered lists, so you can rely on that. That is slightly different from a node-set returned from an XPath, those usually are document-ordered.
On a side mark: actually, everything in XQuery is either a node-set or a sequence. Even if a function is declared to return one string or int, you can treat that returned value as if it is a sequence of one item. No explicit casting is necessary, for which there are no constructs in XQuery anyhow. Functions like fn:exists() and fn:empty() always work.
HTH!
Just for fun, here's how I would do this in XQuery if fn:translate did not exist. I think Michael Kay's suggestion would end up looking similar.
let $test := "Hello XQuery"
return codepoints-to-string(
for $c in string-to-codepoints($test)
return if ($c eq 32) then 44 else $c)
Note that I changed the transformation because of the problem he pointed: 0 is not a legal codepoint. So instead I translated spaces to commas.
With MarkLogic, another option is to use http://docs.marklogic.com/json:array and its associated functions. The json:set-item-at function would allow coding in a vaguely imperative style. Coding both variations might be a good learning exercise.
There are two ways to do this.
Firstly you can create an XmlResults object using
XmlManager.createResults(), and use XmlResults.add() to add your
strings to this. You can then use the XmlResults object to set the
value of a variable in XmlQueryContext, which can be used in your
query.
Example:
XmlResults values = XMLManager.createResults();
values.add(new XmlValue("value1"));
values.add(new XmlValue("value2"));
XmlQueryContext.setVariableValue("files", values);
The alternative is to split the string in XQuery. You
can do this using the tokenize() function, which works using a
regular expression to match the string separator.
http://www.w3.org/TR/xpath-functions/#func-tokenize
Thanks.
A little outlook: XQuery 3.1 will provide native support for arrays. See http://www.w3.org/TR/xquery-31/ for more details.
You can construct an array like this:
$myArray = tokenize('a b c d e f g', '\s')
// $myArray[3] -> c
Please note that the first index of this pseudo-array is 1 not 0!
Since the question "How to use or implement arrays in XQuery?" is being held generic (and thus shows up in search results on this topic), I would like to add a generic answer for future reference (making it a Community Wiki, so others may expand):
As Christian Grün has already hinted at, with XQuery 3.1 XQuery got a native array datatype, which is a subtype of the function datatype.
Since an array is a 'ordered list of values' and an XPath/XQuery sequence is as well, the first question, which may arise, is: "What's the difference?" The answer is simple: a sequence can not contain another sequence. All sequences are automatically flattened. Not so an array, which can be an array of arrays. Just like sequences, arrays in XQuery can also have any mix of any other datatype.
The native XQuery array datatype can be expressed in either of two ways: As [] or via array {}. The difference being, that, when using the former constructor, a comma is being considered a 'hard' comma, meaning that the following array consists of two members:
[ ("apples", "oranges"), "plums" ]
while the following will consist of three members:
array { ("apples", "oranges"), "plums" }
which means, that the array expression within curly braces is resolved to a flat sequence first, and then memberized into an array.
Since Array is a subtype of function, an array can be thought of as an anonymous function, that takes a single parameter, the numeric index. To get the third member of an array, named $foo, we thus can write:
$foo(3)
If an array contains another array as a member you can chain the function calls together, as in:
$foo(3)(5)
Along with the array datatype, special operators have been added, which make it easy to look up the values of an array. One such operator (also used by the new Map datatype) is the question mark followed by an integer (or an expression that evaluates to zero or more integers).
$foo?(3)
would, again, return the third member within the array, while
$foo?(3, 6)
would return the members 3 and 6.
The parenthesis can be left out, when working with literal integers. However, the parens are needed, to form the lookup index from a dynamic expression, like in:
$foo?(3 to 6)
here, the expression in the parens gets evaluated to a sequence of integers and thus the expression would return a sequence of all members from index position 3 to index position 6.
The asterisk * is used as wildcard operator. The expression
$foo?*
will return a sequence of all items in the array. Again, chaining is possible:
$foo?3?5
matches the previos example of $foo(3)(5).
More in-depth information can be found in the official spec: XML Path Language (XPath) 3.1 / 3.11.2 Arrays
Also, a new set of functions, specific to arrays, has been implemented. These functions resinde in the namespace http://www.w3.org/2005/xpath-functions/array, which, conventionally, is being prefixed with array and can be found referenced in here: XPath and XQuery Functions and Operators 3.1 / 17.3 Functions that Operate on Arrays
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Possible Duplicate:
Evaluating a math expression given in string form
Sorry about the long (and also slightly strange :)) title, I couldn't think of a better title for it, but here goes.
I have been making a calculator in Java using a JFrame which has JButtons like a real calculator would. As you click the buttons, the calculation appears in a TextArea. When the 'equals' button is pressed, the whole calculation is taken from the TextArea and calculated. The problem I'm having is how to actually calculate the answer. This may sound a little weird, but say the calculation I'm getting is 36+45/22. How would I write the numbers into variables then tell the computer which operations to perform on the variables, and in what order. Can this be done with an infinite number of variables? Is there any way to do this? Thanks for your help.
You could use ScriptEngine:
ScriptEngine engine = new ScriptEngineManager().getEngineByName("JavaScript");
System.out.println("result = " + engine.eval("36+45/22"));
Another option is Jep.
Try this, I use a simillar implementation. Works just great. :-D
Evaluate expression in java
If you want to evaluate the expression yourself and not depend on external APIs such as ScriptEngine, you must parse the expression first. This parsing gives you an in-memory representation of the expression which you can then evaluate.
A common way to handle arithmetic expressions is a recursive descent parser. This kind of parser has the nice feature that it evaluates the parsed expression in-place in the parsing methods. An example for a recursive descent parser that does not depend on computer science's formal language theory can be found at http://www.savarese.org/articles/1998-2006/2001-05-Recursive_Descent_Parsing/
I am looking for a matrix expression parser/engine. For example,
3 * A + B * C
where A, B, C are matrices is a typical expression. This should be similar to (single value) math expression parser/engine but should handle matrix value and variable. I've already googled in vain. I am also willing to modify existing math expression parser but I am not sure how I can go about it. So if you can give me any clue or hint, I will appreciate it.
See my answer on how to build simple parsers. This especially suited for expression parsers.
It is pretty easy to modify such a parser to compute the answer as it parses. Just add an action routine whenever the parser recognizes syntax, to do what the syntax says.