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Dr. Wayne's dot variables

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    Dr. Wayne's dot variables

    below is Dr. Waynes script which works. But I don't understand why it works.

    we send it the pointer ptr, declare a new pointer (p1), and assign values to the new pointer, but how do ptr's values get assigned the values of the new pointer. It returns a t/f value and does not address the ptr values in the function.

    Its probably simple. I think I am stuck with the conception of a regular variable, whose value would not change unless I directly changed it.

    I sense it is important that I understand this, as version 5 does a lot with compound pointers. Any help appreciated!

    dim ptr as p
    dim ptr.name as c
    dim ptr.num as n
    ptr.name="peter"
    ptr.num=1
    ' pointer-based function
    function change_ref as l(p1 as p)
    p1.name="Captain Marvel"
    p1.num=3
    change_ref=.t.
    end function
    change_ref(ptr)
    ui_msg_box("after change_ref","name is "+ptr.name)
    end
    Cole Custom Programming - Terrell, Texas
    972 524 8714
    [email protected]

    ____________________
    "A young man who is not liberal has no heart, but an old man who is not conservative has no mind." GB Shaw

    #2
    RE: Dr. Wayne's dot variables

    think of a pointer variable as an address. imagine a street
    address, like "100 Main St.", and let's say we wrote

    p=
    in which i use the angle brackets to indicate a physical
    street address.this is not xbasic syntax! in this example, p is a physical address
    pointer.

    i could write
    p.owner="peter"
    p.mortgage="citibank"

    now, if i have a function to change the ownership of
    the building, i could write it as
    function change_ownership(b as )
    b.owner="martin"
    b.mortgage="fleet bank"
    end function

    and then i could write a script like

    dim myhouse as
    myhouse=
    myhouse.owner="peter"
    myhouse.mortgage="citibank"

    change_ownership(myhouse)
    ? myhouse.owner
    = "martin"
    ? myhouse.mortgage
    = "fleet bank"

    i hope this helps.

    Comment


      #3
      RE: Dr. Wayne's dot variables

      oops, that didn't format as well as i had hoped. let me
      try again:

      if i have a function to change the ownership of
      the building, i could write it as

      function change_ownership(b as {address})
      b.owner="martin"
      b.mortgage="fleet bank"
      end function

      and then i could write a script like

      dim myhouse as {adddress}

      myhouse={100 Main St}
      myhouse.owner="peter"
      myhouse.mortgage="citibank"

      change_ownership(myhouse)
      ? myhouse.owner
      = "martin"
      ? myhouse.mortgage
      = "fleet bank"

      i hope this helps.

      Comment


        #4
        RE: Dr. Wayne's dot variables

        Here is my current level of understanding:

        the values are ostensibly assigned "backwards" because you are not really referencing a "variable value" but a "point in memory value" - therefore it doesn't make any difference what name you give it, you are really referencing the value of that point in memory!

        with a regular variable it wouldn't work.
        dim var1 as c
        var1="Martin"
        function test as l(var2 as c)
        var2="Peter"
        end function
        test(var1)
        ui_msg_box("","" + var1) '= "Martin"
        end

        but if I said
        dim var3 as p
        var3.name="Martin"
        function test2 as l(var4 as p)
        var4.name="Peter"
        end function
        test2(var3)
        ui_msg_box("","" + var3.name) '="Peter"
        end

        so I am saying with the line test2(var3):
        pass and assign the value at the same point in memory as var3 to var4. Therefore the values I assign to that point in memory will change the current value of that point in memory.

        var4.name becomes the same address in memory as var3.name

        I am still trying to grasp why one would use dot variables but I am beginning to get the picture. I am currently studying your scripts from learn alpha regarding same.

        I would be very interested to see a script where you use in your every day app.

        Thanks!
        Cole Custom Programming - Terrell, Texas
        972 524 8714
        [email protected]

        ____________________
        "A young man who is not liberal has no heart, but an old man who is not conservative has no mind." GB Shaw

        Comment


          #5
          RE: Dr. Wayne's dot variables

          Martin,
          A while ago on the board someone wanted a way to put one of six sequential dates to a field. Although there were better answers, I suggested this one. It might be an example of what you are lookking for:

          1)Define a form session variable getdate as character.
          2)Create a Multistate button for variable getdate object named getdatenow. Multi-state object is in essence a dot variable.
          3) On init form event use the following script:

          dim dateval as d
          getdatenow.choice.list.redim(6)
          for x = 1 to 6
          dateval = date() + x -1
          getdatenow.choice.list[x].text=dtoc(dateval)
          getdatenow.choice.list[x].value=dtoc(dateval)
          next x
          end
          4) Onchange event of multi-state button to populate another field:
          "datefield".value = ctod(var->getdate)
          John

          Comment


            #6
            RE: Dr. Wayne's dot variables


            Thanks John.
            Yes - that helps. Once I grasp something I can usually use it on the fly, creatively - and my sense is that thats where this could have power.
            Cole Custom Programming - Terrell, Texas
            972 524 8714
            [email protected]

            ____________________
            "A young man who is not liberal has no heart, but an old man who is not conservative has no mind." GB Shaw

            Comment

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