Evaluating boolean expressions from a string

Hi All,
In my program I am dynamically generating boolean expressions.
Hence, I store the expression as a string. I want to be able to determine if that expression evaluates to true/false.
For eg, String expr = "1 < 2";
I want to be able to evaluate expr and print false(in this case)
How can I do that? Are there any class that come with JDK1.4 for doing this or can you point me to some API that I can use for this.
Thanks,
Vijay

I can't count how many times this has been askedhere.
Try searching the forums.Give yourself credit - you could count it if you
wanted to.
OK, maybe so. But I can't count it off the top of my
head. :-) But easier than counting the hairs on your head (well.....presuming you do have some....)
Really, it's bizarre how much this gets
asked. I think I'll start a new thread about it...Be realistic....the integer to string or string to integer gets asked a lot more often!

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06:25:26 > CREATE OR REPLACE FUNCTION MJT_EVALUATE_EXPR (in_expr_to_evaluate VARCHAR2)
06:26:00   2     RETURN NUMBER
06:26:00   3 IS
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06:26:00   5     csr_dual        SYS_REFCURSOR;
06:26:00   6     l_this_result   NUMBER;
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06:26:00 16        l_this_result := 0;
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06:26:00 18
06:26:00 19     CLOSE csr_dual;
06:26:00 20
06:26:00 21     RETURN l_this_result;
06:26:00 22 END;
06:26:00 23 /
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Hey everyone,
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(1) In his list of "operators that enforce some order of evaluation," what does "and so on" include?
(2) Is short circuit evaluation ALWAYS used with Boolean expressions in PL/SQL, even when they the expression is outside one of these statements? For example:
boolvariable := p OR q;
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CALL foo(p or q);

This is a very interesting paper. To attempt to answer your questions:-
1) I suppose BETWEEN would be included in the "and so on" list.
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SQL> CREATE TABLE t( c1 VARCHAR2(30), c2 VARCHAR2(30) );
Table created.
SQL>
SQL> CREATE OR REPLACE PACKAGE pkg AS
2     FUNCTION f1( p IN VARCHAR2 ) RETURN NUMBER;
3     FUNCTION f2( p IN VARCHAR2 ) RETURN NUMBER;
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5 /
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SQL>
SQL> CREATE OR REPLACE PACKAGE BODY pkg AS
2
3     PROCEDURE ins( p1 IN VARCHAR2, p2 IN VARCHAR2 ) IS
4        PRAGMA autonomous_transaction;
5     BEGIN
6        INSERT INTO t( c1, c2 ) VALUES( p1, p2 );
7        COMMIT;
8     END ins;
9
10     FUNCTION f1( p IN VARCHAR2 ) RETURN NUMBER IS
11     BEGIN
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13        RETURN 0;
14     END f1;
15
16     FUNCTION f2( p IN VARCHAR2 ) RETURN NUMBER IS
17     BEGIN
18        ins( p, 'F2' );
19        RETURN 1;
20     END f2;
21
22 END pkg;
23 /
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SQL> SELECT SUM(
2           CASE
3              WHEN pkg.f1('CASE') = 0
4              OR   pkg.f2('CASE') = 1
5              THEN 0
6              ELSE 1
7           END
8           ) AS just_a_number_1
9 ,      SUM(
10           NVL( pkg.f1('NVL'), pkg.f2('NVL') )
11           ) AS just_a_number_2
12 ,      SUM(
13           COALESCE(
14             pkg.f1('COALESCE'),
15             pkg.f2('COALESCE'))
16           ) AS just_a_number_3
17 FROM    user_objects;
JUST_A_NUMBER_1 JUST_A_NUMBER_2 JUST_A_NUMBER_3
              0               0               0
SQL>
SQL> SELECT c1, c2, count(*)
2 FROM   t
3 GROUP BY
4         c1, c2;
C1                             C2                               COUNT(*)
NVL                            F1                                     41
NVL                            F2                                     41
CASE                           F1                                     41
COALESCE                       F1                                     41We can see that NVL executes both functions even though the first parameter (F1) is never NULL. To see what happens in PL/SQL, I set up the following procedure. In 100 iterations of a loop, this will test both of your queries ( 1) IF ..OR.. and 2) bool := (... OR ...) ).
SQL> CREATE OR REPLACE PROCEDURE bool_order ( rc OUT SYS_REFCURSOR ) AS
2
3     PROCEDURE take_a_bool( b IN BOOLEAN ) IS
4     BEGIN
5        NULL;
6     END take_a_bool;
7
8 BEGIN
9
10     FOR i IN 1 .. 100 LOOP
11
12        IF pkg.f1('ANON_LOOP') = 0
13        OR pkg.f2('ANON_LOOP') = 1
14        THEN
15           take_a_bool(
16              pkg.f1('TAKE_A_BOOL') = 0 OR pkg.f2('TAKE_A_BOOL') = 1
17              );
18        END IF;
19
20     END LOOP;
21
22     OPEN rc FOR SELECT c1, c2, COUNT(*) AS c3
23                 FROM   t
24                 GROUP BY
25                        c1, c2;
26
27 END bool_order;
28 /
Procedure created.Now to test it...
SQL> TRUNCATE TABLE t;
Table truncated.
SQL>
SQL> var rc refcursor;
SQL> set autoprint on
SQL>
SQL> exec bool_order(:rc);
PL/SQL procedure successfully completed.
C1                             C2                                     C3
ANON_LOOP                      F1                                    100
TAKE_A_BOOL                    F1                                    100
SQL> ALTER SESSION SET PLSQL_OPTIMIZE_LEVEL=0;
Session altered.
SQL> exec bool_order(:rc);
PL/SQL procedure successfully completed.
C1                             C2                                     C3
ANON_LOOP                      F1                                    200
TAKE_A_BOOL                    F1                                    200The above shows that the short-circuiting occurs as documented, under the maximum and minimum optimisation levels ( 10g-specific ). The F2 function is never called. What we have NOT seen, however, is PL/SQL exploiting the freedom to re-order these expressions, presumably because on such a simple example, there is no clear benefit to doing so. And I can verify that switching the order of the calls to F1 and F2 around yields the results in favour of F2 as expected.
Regards
Adrian

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