Elegant way to do variable substitution in a java string

Question

Pretty simple question and my brain is frozen today so I can't think of an elegant solution where I know one exists.

I have a formula which is passed to me in the form "A+B"

I also have a mapping of the formula variables to their "readable names".

Finally, I have a formula parser which will calculate the value of the formula, but only if its passed with the readable names for the variables.

For example, as an input I get

String formula = "A+B"
String readableA = "foovar1"
String readableB = "foovar2"

and I want my output to be "foovar1+foovar2"

The problem with a simple find and replace is that it can be easily be broken because we have no guarantees on what the 'readable' names are. Lets say I take my example again with different parameters

String formula = "A+B"
String readableA = "foovarBad1"
String readableB = "foovarAngry2"

If I do a simple find and replace in a loop, I'll end up replacing the capital A's and B's in the readable names I have already replaced.

This looks like an approximate solution but I don't have brackets around my variables

How to replace a set of tokens in a Java String?

I'm wondering how complex of a formula can be supplied? Is there just two terms or could it be A+B+C? Are the variables always single letters? Is it always + or could it be * or / or whatever? Are spaces allowed? — Lee Meador, Jan 16 '13 at 18:32
Further what you want to do with this expression. Is it ready when the String is build, or do you need to calculate the formula, such that you get a number as result? — AlexWien, Jan 16 '13 at 18:42
@LeeMeador - they can get pretty complex, involving all 4 basic operators, also nonvariable numbers, and parentheses. — Evan Cross, Jan 16 '13 at 19:52
I realized I didn't completely answer your question. There can be whitespace but its always insignificant (if we drop it, its ok) — Evan Cross, Jan 16 '13 at 20:28

score 3 · Answer 1 · answered Jan 16 '13 at 18:24

That link you provided is an excellent source since matching using patterns is the way to go. The basic idea here is first get the tokens using a matcher. After this you will have Operators and Operands

Then, do the replacement individually on each Operand.

Finally, put them back together using the Operators.

score 0 · Answer 2 · answered Jan 16 '13 at 18:25

A somewhat tedious solution would be to scan for all occurences of A and B and note their indexes in the string, and then use StringBuilder.replace(int start, int end, String str) method. (in naive form this would not be very efficient though, approaching smth like square complexity, or more precisely "number of variables" * "number of possible replacements")
If you know all of your operators, you could do split on them (like on "+") and then replace individual "A" and "B" (you'd have to do trimming whitespace chars first of course) in an array or ArrayList.

score 0 · Answer 3 · answered Jan 16 '13 at 18:26

0

A simple way to do it is

String foumula = "A+B".replaceAll("\\bA\\b", readableA)
                      .replaceAll("\\bB\\b", readableB);

answered Jan 16 '13 at 18:26

Peter Lawrey

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This would fail for A+B with A=B and B=something. – Lee Meador Jan 16 '13 at 18:34
That depends if you want to expand values which contain values. – Peter Lawrey Jan 16 '13 at 18:36

score 0 · Answer 4 · answered Jan 16 '13 at 18:28

0

Your approach does not work fine that way
Formulas (mathematic Expressions) should be parsed into an expression structure (eg. expression tree).

Such that you have later Operand Nodes and Operator nodes.
Later this expression will be evaluated traversing the tree and considering the mathematical priority rules.

I recommend reading more on Expression parsing.

answered Jan 16 '13 at 18:28

AlexWien

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This is only worth the trouble if the equations can be complicated beyond a certain degree. – Lee Meador Jan 16 '13 at 18:35
@LeeMeador that depens on the application which the OP did not explain further. I you want to evaluate the result, then you may need the epxression tree. – AlexWien Jan 16 '13 at 18:41

Lee Meador · Answer 5 · 2013-01-17T16:03:28.347

Matching Only

If you don't have to evaluate the expression after doing the substitution, you might be able to use a regex. Something like (\b\p{Alpha}\p{Alnum}*\b)

or the java string "(\\b\\p{Alpha}\\p{Alnum}*\\b)"

Then use find() over and over to find all the variables and store their locations.

Finally, go through the locations and build up a new string from the old one with the variable bits replaced.

Not that It will not do much checking that the supplied expression is reasonable. For example, it wouldn't mind at all if you gave it )A 2 B( and would just replace the A and B (like )XXX 2 XXX(). I don't know if that matters.

This is similar to the link you supplied in your question except you need a different regular expression than they used. You can go to http://www.regexplanet.com/advanced/java/index.html to play with regular expressions and figure out one that will work. I used it with the one I suggested and it finds what it needs in A+B and A + (C* D ) just fine.

Parsing

You parse the expression using one of the available parser generators (Antlr or Sable or ...) or find an algebraic expression parser available as open source and use it. (You would have to search the web to find those, I haven't used one but suspect they exist.)

Then you use the parser to generate a parsed form of the expression, replace the variables and reconstitute the string form with the new variables.

This one might work better but the amount of effort depends on whether you can find existing code to use.

It also depends on whether you need to validate the expression is valid according to the normal rules. This method will not accept invalid expressions, most likely.

Elegant way to do variable substitution in a java string

5 Answers5