Java: Using created string as loop parameter?

Question

In short, the user will input a number (say 1 through 3). This will decide which range of numbers the loop should search through.

switch(input){
case 1:
    searchTerm = "i<10 && i>5";
case 2:
    searchTerm = "i>=10 && i<19";
case 3:
    searchTerm = "i>19 && i<24";
}
while(searchTerm){
    //some function
}

Is this possible? I I've not been able to find a way to use a string as search parameters.

EDIT: I don't think I did a very good job of explaining why I needed this. What is one to do if there are different numbers of parameters? For example:

case 1:
    searchTerm = "i<5"
case 2:
    searchTerm = "i>25 && i<29"
case 3:
    searchTerm = "(i<50 && i>25) && (i>55 && i<75)"
case 4:
    searchTerm = "(i<20 && i>15) && (i>300 && i<325) && (i>360 && i<380)

Then how does one do it? Multiple loops that call the same function?

Why would you want to use a string for this? Use numeric variables! — T.J. Crowder
– T.J. Crowder, Commented Aug 5, 2015 at 7:48
No, this is not possible, not necessary and a hell to maintain and debug. You define an expression in your string, which will have to be compiled to be used. — Uwe Allner
– Uwe Allner, Commented Aug 5, 2015 at 7:51

T.J. Crowder · Accepted Answer · 2015-08-05 08:15:32Z

9

The correct way to do this is to not use a string at all:

int min, max;

switch(input){
case 1:      // i<10 && i>5
    min = 6;
    max = 10;
    break; // to avoid follow-through to the next case
case 2:     // i>=10 && i<19
    min = 10;
    max = 20;
    break;
case 3:     // i>19 && i<24
    min = 20;
    max = 25;
    break;
default:
    // You need something here in case the value entered wasn't 1-3
}
for (int i = min; i < max; ++i) {
    // ...
}

Re your edit:

I don't think I did a very good job of explaining why I needed this. What is one to do if there are different numbers of parameters?

In that case, you'll have to use an expression evaluator (or write one, which is a non-trivial task). There's one in Spring, for instance (not recommending, just happened to hear about it). A search for "Java expression evaluator" should turn up some options.

Another alternative, which is somewhat amusing given that some folks mistook your question for a JavaScript question, is to use the JavaScript evaluator built into Java (either Rhino or Nashorn). E.g.: Live Example

import javax.script.*;

class Ideone {
    public static void main(String[] args) throws java.lang.Exception {
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = manager.getEngineByName("js");
        String searchTerm = "i >= 19 && i <= 24";
        int i;
        try {
            i = 19;
            engine.put("i", i);
            while ((boolean)engine.eval(searchTerm)) {
                System.out.println("i = " + i);
                ++i;
                engine.put("i", i);
            }
            System.out.println("Done");
        } catch (ScriptException scriptException) {
            System.out.println("Failed with script error");
        }
    }
}

...but you'll still have the problem of determining what initial value to use for i, which I've hardcoded above.

edited Aug 5, 2015 at 8:15

answered Aug 5, 2015 at 7:49

T.J. Crowder

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4 Comments

T.J. Crowder Over a year ago

@Bathsheba: Thanks, yes, I meant to flag that up and forgot. :-)

Jimpy Over a year ago

I didn't use correct examples, but your answer definitely answered the question that was asked. What about the new question?

T.J. Crowder Over a year ago

@Jimpy: I've added to the answer to address the updated question.

Jimpy Over a year ago

Thank you very much for your help! Your answer was extremely thorough and gives me lots of different angles to approach this problem on. It looks like I have a whole new task to tackle. Thank you again.

Dmitrii Bychenko · Accepted Answer · 2015-08-05 10:31:24Z

5

In Java 8 you can select a lambda instead of String:

Predicate<Integer> searchTerm = (Integer v) -> false;

switch (input) {
  case 1:
    searchTerm = (Integer v) -> v < 10 && v > 5;

    break;
  case 2:
    searchTerm = (Integer v) -> v >= 10 && v < 19;

    break;
  case 3:
    searchTerm = (Integer v) -> v > 19 && v < 24;

    break;
}

while (searchTerm.test(i)) {
  ...
}

edited Aug 5, 2015 at 10:31

answered Aug 5, 2015 at 8:03

Dmitrii Bychenko

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1 Comment

T.J. Crowder Over a year ago

Sadly thanks to the edit this no longer answers the question.

Halko Karr-Sajtarevic · Accepted Answer · 2015-08-05 08:08:23Z

You can create an enumeration as below.

public enum SearchTerms {

    None(""),
    Between6And9("i<10 && i>5"),
    Between10And18("i>=10 && i<19"),
    Between20And23("i>19 && i<24");

    private final String stringValue;

    SearchTerms(String stringValue) {
        this.stringValue = stringValue;
    }

    public String getStringValue() {
        return stringValue;
    }

    public static SearchTerms fromStringValue(String stringValue) {
        for (SearchTerms searchTerm : values()) {
            if (searchTerm.getStringValue().equalsIgnoreCase(stringValue)) {
                return searchTerm;
            }
        }
        return SearchTerms.None;
    }

}

Usage:

SearchTerms searchTerm = SearchTerms.fromStringValue("i<10 && i>5");

switch(searchTerm) {
    case Between6And9:
        //dosomething
        break;
}

Shrinivas Shukla · Accepted Answer · 2015-08-05 07:57:08Z

-5

You can use .eval() of JavaScript.

Also don't forget break; at the end of each case:

Check out this fiddle.

Here is the snippet.

function test(input, i) {
  switch (input) { //input=1
    case 1:
      searchTerm = "i<10 && i>5";       //this will be 'searchTerm'
      break;
    case 2:
      searchTerm = "i>=10 && i<19";
      break;
    case 3:
      searchTerm = "i>19 && i<24";
      break;
  }
  while (eval(searchTerm)) {      //'searchTerm' converted to boolean expression
    alert(i);           // alert for i=7,8,9
    i++;
  }
}

test(1, 7);           //pass input=1 and i=7

edited Aug 5, 2015 at 7:57

answered Aug 5, 2015 at 7:53

Shrinivas Shukla

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8 Comments

Raghavendra Over a year ago

can be used but not a good one to use. recommend the down answer

T.J. Crowder Over a year ago

He could, yes, if A) This were JavaScript instead of Java (they're completely different languages and environments), and B) He wanted to completely unnecessarily use a full JavaScript parser where simple variables were all that was needed.

Raghavendra Over a year ago

meaningful to op question why down voting

T.J. Crowder Over a year ago

@raghavendra: The wrong language is useful how, exactly? Shall we also post Fortran and Pascal answers? Perhaps some assembly code?

Shrinivas Shukla Over a year ago

Sorry guys. My mistake. I didn't see the tag. Just saw the code. I took it to be javascript. I will delete this answer.

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