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Take a look at the Mercurial repository [1] and especially the file
reverse/README.rst in there.  It contains a lot of hands-on
information for creating a UDF.

[1] http://dev.monetdb.org/hg/MonetDB-extend
either point your browser here, or, better yet, clone it:
hg clone http://dev.monetdb.org/hg/MonetDB-extend

On 30/03/16 14:59, Mark Raasveldt wrote:
> Hey Shmagi,
>
> You are not using the correct function signature for writing a
> MonetDB UDF. The correct function signature is first the return
> value as pointer (this is where you write the return value to), and
> then a list of all the input parameters as pointers. For your
> function, the correct function signature is:
>
> str UDFbitand(flt *ret, lng *a, lng *b);
>
> Where (a,b) are the input parameters, and (ret) is where you write
> the return value. The actual return value of the function is a
> string that is either MAL_SUCCEED (if the function has completed
> correctly), or an error message (if it has not).
>
> I notice in your function that in the end you write the result to a
> local variable:
>
> float final = count/(float)length;
>
> This variable is not exported from the function, and as such cannot
> be read by the database. Instead, the return value of the function
> should be written to the "ret" pointer, as such:
>
> *ret = count / (float) length;
>
> Also, always initialize your variables if you expect them to have a
> specific value. In your code, you have the following variables.
>
> int length; int count;
>
> Which you then increment. As these variables are uninitialized,
> their value is undefined. Some compilers may set their value to 0,
> but you should not rely on this.
>
> I have rewritten your UDF to operate correctly, here's the code.
>
>
> str UDFbitand(flt *ret, lng *a, lng *b) { long long int temp =
> weirdAnd(*a, *b); long long int temp2 = weirdAnd(*a, *b); int
> length = 0; int count = 0;
>
> while (temp != 0) { temp/=10; length++; }
>
> while (temp2 != 0) { if (temp2 % 10 == 1) count++; temp2 /= 10; }
>
> *ret = count/(float)length; return MAL_SUCCEED; }
>
> This gives me the following result.
>
> sql>SELECT bitand(1111, 1010); +---------------------+ |
> bitand_single_value | +=====================+ | 0.5
> | +---------------------+
>
>
> Note that this is a single scalar UDF. While you can use this UDF
> to operate on entire columns, if you want the UDF to perform well
> you should implement a batch UDF that operates on BATs directly.
>
> Hope that helps,
>
> Mark
>
>
>
>
> ----- Original Message ----- From: "Shmagi Kavtaradze"
> <kavtaradze.s@gmail.com> To: "users-list" <users-list@monetdb.org>
> Sent: Wednesday, March 30, 2016 2:31:18 PM Subject: UDF returning 0
> as a result
>
> I wrote a function bitand. When I do "select bitand(1111,1010);" it
> gives "0" instead of "0.5". My function:
>
> --UDF.c--
>
> str UDFbitand(long long int a, long long b) { long long int
> weirdAnd(unsigned int a, unsigned int b) { long long int result =
> 0; int coef = 1; while (a || b) { result += ((a % 10) && (b % 10))
> * coef; coef *= 10; a /= 10; b /= 10; } return result; }
>
>
> long long int temp = weirdAnd(a, b); long long int temp2 =
> weirdAnd(a, b); int length; int count;
>
> while (temp != 0) { temp/=10; length++; }
>
> while (temp2 != 0) { if (temp2 % 10 == 1) count++; temp2 /= 10; }
>
> float final = count/(float)length; return MAL_SUCCEED; }
>
>
> --UDF.mal--
>
> command bitand(a:lng,b:lng):flt address UDFbitand comment
> "bitand";
>
> --80_UDF.sql--
>
> create function bitand(a BIGINT,b BIGINT) returns REAL external
> name udf.bitand;
>
> When I test my C code in netbeans it works as supposed. Any ideas?
> Thanks in advance.
>
> _______________________________________________ users-list mailing
> list users-list@monetdb.org
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>

- --
Sjoerd Mullender
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