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Can you confirm expression evaluation error?

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  • Can you confirm expression evaluation error?

    Can you confirm these results in expression evaluation in any version of A5? I can't think of any time an expression evaluation error was as simple as this, with straightforward functions failing like this. It happens in a5v8 through a5v10 and I suspect all earlier versions as well.
    text="abcdefg"

    i=3

    ?asc(substr(text,i,1),.t.)+1
    ERROR: Argument is incorrect data type

    ?1+asc(substr(text,i,1),.t.)
    = 100
    Thanks!
    Regards,

    Ira J. Perlow
    Computer Systems Design


    CSDA A5 Products
    New - Free CSDA DiagInfo - v1.39, 30 Apr 2013
    CSDA Barcode Functions

    CSDA Code Utility
    CSDA Screen Capture



  • #2
    Re: Can you confirm expression evaluation error?

    Hi Ira,
    Using v8, I get the same results. Using eval it comes out fine though...odd.
    Code:
    text="abcdefg"
    dim i as n
    i=3
    ?eval("asc(substr(text,i,1),.t.)")+1
    = 100
    Mike
    __________________________________________
    It is only when we forget all our learning that we begin to know.
    It's not what you look at that matters, it's what you see.
    Henry David Thoreau
    __________________________________________



    Comment


    • #3
      Re: Can you confirm expression evaluation error?

      Ira,

      I get the same and have narrowed it down to the positive argument in the asc() function. Take that out and it works. Strange!

      Comment


      • #4
        Re: Can you confirm expression evaluation error?

        Hi Lance,

        [quote=Lance Gurd;509878]I get the same and have narrowed it down to the positive argument in the asc() function. Take that out and it works. Strange!/quote]

        Alpha evaluates an expression from the right. That is why right side portions of expression terms are related to the problem. Simplifying the failure, it is related to use of the 2nd argument in asc().
        ?asc("a",.f.)+1
        ERROR: Argument is incorrect data type
        ?asc("a",.t.)+1
        ERROR: Argument is incorrect data type
        ?asc("a",.t.)
        = 97
        ?asc("a",.f.)
        = 97
        ?asc("a")+1
        = 98
        My guess is that the "C" code that asc() is written in probably messes up the xbasic stack when used with the 2 arguments.
        Regards,

        Ira J. Perlow
        Computer Systems Design


        CSDA A5 Products
        New - Free CSDA DiagInfo - v1.39, 30 Apr 2013
        CSDA Barcode Functions

        CSDA Code Utility
        CSDA Screen Capture


        Comment


        • #5
          Re: Can you confirm expression evaluation error?

          So Ira....Why, in your opinion, does it work when adding the numeric prior instead of after? And why does it work regardless when using Eval()?
          Mike
          __________________________________________
          It is only when we forget all our learning that we begin to know.
          It's not what you look at that matters, it's what you see.
          Henry David Thoreau
          __________________________________________



          Comment


          • #6
            Re: Can you confirm expression evaluation error?

            Hi Mike,

            Originally posted by MikeC View Post
            Why, in your opinion, does it work when adding the numeric prior instead of after? And why does it work regardless when using Eval()?
            Adding a number after it sets up a structure (the XBasic stack I suspect) that gets corrupted by the asc() "c" function when used with 2 parameters as the asc() function is evaluated after the number. (Terms are evaluated from right)

            When it is the last item in the expression (meaning it is evaluated 1st), it does not mess up the structure, so any expression terms to the left that are evaluated are still OK.

            When you use eval(), you are basically evaluating it in it's own space (so it has it's own structure), and returning a number to the current space, which you are adding to another number. But try the whole thing together with eval() so it's added together in the same space and you get
            ?eval("asc('a',.t.)+1")
            ERROR: Argument is incorrect data type
            This is a very rare example of a function corrupting something outside it's internal space, and probably can only happen within non-xbasic functions.

            Why has a bug like this persisted over the years? I'll bet most people have never used the 2nd parameter, nor were they adding values to the value they got (I was using it in font-mapping routines for barcoding).

            I was using this 2nd term, in hopes of speeding the code a bit (relying on an internal function to be faster than my equivalent code). After working through the bug (by rearranging terms), here are simple results.
            ?asc(chr(150),.t.)
            = 150
            ?asc(chr(150),.f.)
            = -106
            ?asc(chr(150))
            = -106
            ?asc(chr(150))+256.and.255
            = 150

            ' This takes .00036 seconds to execute
            ?1+asc(chr(150),.t.)
            = 151
            ' This takes .00029 seconds to execute. Surprisingly, faster than the built in function!
            ?1+(asc(chr(150))+256.and.255)
            = 151
            Regards,

            Ira J. Perlow
            Computer Systems Design


            CSDA A5 Products
            New - Free CSDA DiagInfo - v1.39, 30 Apr 2013
            CSDA Barcode Functions

            CSDA Code Utility
            CSDA Screen Capture


            Comment


            • #7
              Re: Can you confirm expression evaluation error?

              This error has been repaired in A5v10 build 2495-3379
              Regards,

              Ira J. Perlow
              Computer Systems Design


              CSDA A5 Products
              New - Free CSDA DiagInfo - v1.39, 30 Apr 2013
              CSDA Barcode Functions

              CSDA Code Utility
              CSDA Screen Capture


              Comment

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