UDF triggering twice

[Dave Booth]

Hello

I am trying to write a UDF to write to a log file when a button is pressed however the UDF is trigging twice and therefore I get two entries. If I test the UDF in the interactive window I get only a single entry. I have been able to reproduce the problem and attach an example. From the form reportip pressing either button calls the UDF. Looking at the logname table shows the double entry. Logref is a time stamp. I am trying to keep a log when a form is printed (using a report copy of the form)

Is a bug with the operation of the button or my code?
(I also get double entries when using the Onprintinit event on the report on the main database - Not included in the example)

Thanking you in advance for any help

Dave

[Tom Cone Jr]

Re: UDF triggering twice

Dave, I think this behavior results from the way Alpha implemented your first "action" statement for you. Alpha produced the following code when you told it to assign the function's return value to your variable:

Reportno = convert_type(a5_eval_expression("=Logrptip(\"Report1\")",local_variables()),"c")

Notice that the function call is inside a nested expression. I think the UDF is being called by both the a5_eval_expression() function, and by the convert_type() function. Hence, two records are written to the table.

I've attached a revised form of the database, to show you a simpler approach using standard Xbasic code. Check out the script beneath the Print Report3 button.

-- tom

[Dave Booth]

Re: UDF triggering twice

Thank You I will modify the master database

[csda1]

Re: UDF triggering twice

Hi Tom,

Dave, I think this behavior results from the way Alpha implemented your first "action" statement for you. Alpha produced the following code when you told it to assign the function's return value to your variable:

Reportno = convert_type(a5_eval_expression("=Logrptip(\"Report1\")",local_variables()),"c")

Notice that the function call is inside a nested expression. I think the UDF is being called by both the a5_eval_expression() function, and by the convert_type() function. Hence, two records are written to the table.

I think the reason for the two firing is that 1st, the function is not a "Pure" function, and because of that, every time Alpha calls it, it's results are not the same (in other words, it creates a new record). The reason for this is that eval_valid(), along with eval() both cause the code to be executed, and it is very typical to check validity before actual running the code for final results. But this has unintended consequences for this case.

The proper way would be to do error trapping and just run the code once, which can be done in many cases. This is just another example of Action Scripts having unintended consequences or not working as expected per my tips here.

[Dave Booth]

Re: UDF triggering twice

Thank you

I have it working now using Tom's code on the Onprintinit event on the report.

[Tom Cone Jr]

Re: UDF triggering twice

Dave, I'm glad you have it working.

I write again because, after considering Ira's post and giving the matter a bit more thought, I think I reached the right conclusion but misunderstood why. Your UDF is being called twice by the code generated by your action scripting "action". But the convert_type() function has nothing to do with it.

Here's the code that action scripting supplied:

Reportno = convert_type(a5_eval_expression("=Logrptip(\"Report1\")",local_variables()),"c")

The expression that the function "a5_eval_expression()" tests includes another function, namely your UDF. So, Alpha runs your UDF first (because it's the most deeply nested component in the expression), and then the a5_eval_expression() runs it again. Oridinarly, this is a matter of no consquence since a classic or "pure" function Ira references would return the same result no matter how many times it is run. But... in this case your function writes records to a table, so you get multiple copies.

So, in my previous post I reached the right conclusion, but gave you a faulty explanation. Anyway, good luck with your project!

-- tom

[MikeC]

Re: UDF triggering twice

Thanks Tom! As I use UDFs quite often (and for some unusual things at times I might add!), this will be something I had better be aware of as well.

[csda1]

Re: UDF triggering twice

Hi Tom,

....The expression that the function "a5_eval_expression()" tests includes another function, namely your UDF. So, Alpha runs your UDF first (because it's the most deeply nested component in the expression), and then the a5_eval_expression() runs it again.

That's not correct either. It has no relevance to the nesting of the expressions. The only executions occur within the a5_eval_expression() function.

(As a side note, Alpha executes expressions from right to the left, which is really confusing in a few cases where items to the left depend on values created by items to the right - this is very rare (happens for a use of a non-pure function of mine), and never happens with pure functions).

Below, I am showing you a script of what Alpha basically does, as opposed to a better way Alpha could do it. But it comes down to this. Eval_valid() and similar testing functions are generally followed up by an eval() or similar, and this is what a5_eval_expression() does internally.

For non-pure functions, it always will execute twice for good code with the side effects of the non-pure function (writing a record each time for this case). Non-pure functions are OK, but you must be aware of side effects.

dim testcode as c
dim codeok as L
dim result as n

testcode="ui_msg_box(\"Test 1\",\"Test\")"

' This executes the code in test code as a consequence
' of testing it being runnable without errors
codeok=eval_valid(testcode)

' This executes the code in test code
result=eval(testcode)
'============================================================
' This is kind of what what Alpha does in A5_eval_expression()
' If the code is good, it will run twice.
' If bad code, it will run the code up to the point of the 1st error
testcode="ui_msg_box(\"Test 2\",\"Test\")"
IF eval_valid(testcode)
    result=eval(testcode)
END IF
'============================================================
' This is kind of what what Alpha does in A5_eval_expression()
' If the code is good, it will run twice.
' If bad code, it will run the code up to the point of the 1st error
testcode="ui_msg_box(\"Test 3\",\"Test\")"
result=a5_eval_expression("="+testcode)
'====================================
' This is what Alpha should have done
' (and is essentially what happens in eval_valid(), except the result
' is thrown away, and just the logical flag that indicates it ran is
' returned)
' If the code is good, it will run once
' If bad code, it will run the code up to the point of the 1st error

testcode="ui_msg_box(\"Test 4\",\"Test\")"

' Set code OK initially to false
codeok=.f.

' Turn on error trapping
ON error goto finish_error

result=eval(testcode)
codeok=.t.

finish_error:
' Turn off error trapping either way
on error goto 0

' eval() would return result 
' whereas eval_valid() would return codeok

Does this explain it more clearly?