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Thank you, Ira.Thank you, Carol.

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My name is Peter and I'm an Alphaholic.

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Variable Scoping and Pointer Variables

Dr. Peter Wayne

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Variable scoping

6 levels of variable scoping:

1) local

2) session

3) global

4) addin

5) layout

6) hidden addin

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Variable scope and lifetime

Local variables exist for the lifetime of the script, and then are destroyed. Exception: script_play_local() preserves the local variables for the next script.

All new variables are local unless explicitly declared otherwise.

Global variables are accessible to all reports, forms, and scripts.

Session variables exist for the lifetime of the "session", which usually means one form or report. Session variables can also be used to pass data from one field rule event to another.

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Scoping_1

x=1

x="alpha"

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Scoping_2 and Scoping_3

dim x as nx=1ui_msg_box("x is",str(x))script_play("scoping_3")

x="alpha" Scoping_3ui_msg_box("x is",x)

Scoping_2

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Script_play_local

dim x as nx=1ui_msg_box("x is",str(x))script_play_local("scoping_3")

x="alpha"ui_msg_box("x is",x)

Scoping_3

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Shared variablesdim shared x as nx=1ui_msg_box("x is",str(x))script_play("scoping_5")

dim shared x as cx="alpha"ui_msg_box("x is",x)

Scoping_5

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Delete statementdim shared x as nx=1ui_msg_box("x is",str(x))script_play("scoping_5")

'Scoping_5:delete xdim shared x as c ' this x is a completely new variablex="alpha"ui_msg_box("x is",x)

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Variable Namespace

x=5name="peter"l=local_variables() ' l is a pointer to the local variable namespace

? variables_enum(l)= xnamel

? typeof(l)= "P"? typeof(x)= "N"

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Pointer sub-elements, or properties, are equivalent to variables within a namespace

defined by the pointer.

dim ptr as pptr.name="cian"ptr.age=40ptr.date={3/21/02}

? properties_enum(ptr)= nameagedate? variables_enum(ptr)= nameagedate

There is a congruence between properties_enum() and variables_enum() for pointer variables.

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With..end with

with ptrage=42lastname="chambliss"

end with

? ptr.lastname= "chambliss"

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delete user ' make sure no 'user' variable exists' now, create a local variable, "user"dim user as cuser="peter"

' create pointers to the global and local namespacesdim g as pg=global_variables()dim l as pl=local_variables()

with gdim user as puser.sex="M"user.home="Bronx"user.name=l.user

end with

Mixing namespaces

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user="ira" ' local variable 'user'ui_msg_box("the local variable, user, is now",user)

ui_msg_box("the global pointer variable, user, \has the following properties",properties_enum(g.user))

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with g.user ui_msg_box("and the value of user.name in the global namespace",\ name)end with

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Accessing a form's layout-level variables

'account' and 'cust_name' are layout-level variables on the form:

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dim account as caccount="0900"name="bo peep"dim l as pl=local_variables()

Referring to a form's layout variables

f=form.load("customer_entry")with customer_entry.variables() ' layout-level variables

account=l.account cust_name=l.name

end with

f.resynch()f.show()f.activate()

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Running the script

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dim account as caccount="0900"name="rabins"dim l as pl=local_variables()dim c as pf=form.load("customer_entry")c=customer_entry.variables()

Try not to use the same variable names in different namespaces. You can confuse Alpha Five (to say nothing of yourself)!

with caccount=account ' this will fail - ambiguous reference to "account"' next assignment will succeed since "name" exists only in 'local_variables() namespace and is therefore unambiguouscust_name=nameend with

f.resynch()f.show()f.activate()

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You can pass pointers to a function to achieve "call by reference"

function increment_value as n (input as n)increment_value=input+1

end function

function increment_by_reference as v(ptr as p)ptr.age=ptr.age+1

end function george.age=20new_age=increment_value(george.age)ui_msg_box("after call by value","george.age is "+str(george.age)+\

crlf()+" and new_age is "+str(new_age))

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increment_by_reference(george)ui_msg_box("after call by reference",\"george.age is "+str(george.age))

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dim person as pperson.name="leslie"person.sex="M"person.age=18

With pointers, one function can change several variables

function change_identity(ptr as p) as Vwith ptr

age=age+1sex=if(sex="M","F","M")

end withend function

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ui_msg_box("leslie is initially",str(person.age)+" y.o. "+person.sex)

change_identity(person)ui_msg_box("leslie is now", str(person.age)+" y.o. "+person.sex)

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Property arrays: arrays of pointer variables

lecture_string=<<%str%selwyn+new features of alpha five v. 5peter+pointer variables and namespacesira+user defined functions %str%

dim lectures[3] as p

lectures.initialize_properties("name+topic",lecture_string)speakers=lectures.dump_properties("name")ui_msg_box("speakers are",speakers)

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found=lectures.find("user defined functions","topic")ui_msg_box("user defined functions is by",lectures[found].name)

found=lectures.find("peter","name")ui_msg_box("peter is speaking on",lectures[found].topic)

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lectures.sort("D","name")ui_msg_box("alphabetically, last speaker is",lectures[1].name)

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prefix="c:\program files\a5v5\xbasiccheckbook\\"checks=table.open(prefix+"mycheckbook.dbf")nrecs=checks.records_get()checks.close()dim deposits[nrecs] as pdeposits.initialize_from_table(prefix+"mycheckbook.dbf",\"type='D'","invert(amount)")check_dump=deposits.dump_properties("date:amount")ui_msg_box("deposits by descending value",check_dump)

Property array from table

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deposits.filter("amount<500",.t.)ui_msg_box("deposits of at least 500",\deposits.dump_properties("date:amount"))

Apply a filter to a property array

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Collections

family_list=<<%list%geoff,sonelizabeth,daughterellen,wifegreg,sonted,sonbingley,dogdarcy,dog%list%

dim family as ufamily.initialize("1,2","1","2",family_list)

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function truthval as c(inval as l)truthval=if(inval=.t.,"Yes","No")

end function

ui_msg_box("is selwyn in my family?",truthval(family.exist("selwyn")))

ui_msg_box("what relation is darcy?",family.get("darcy"))

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User-Defined Types

type membername as cage as n address as csex as c dues_date as d

end type

' now, we can writedim aug_mbr as {member}aug_mbr.name="Barry"aug_mbr.address="Pennsylvania"aug_mbr.sex="M"

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aug_mbr_props=properties_enum(aug_mbr)

dim aug_mbr_desc as Ucode=*for_each(tag,"aug_mbr_desc.set("+"\""+tag+"\",\aug_mbr."+tag+")",aug_mbr_props)evaluate_template(code)ui_msg_box("aug_mbr's dump",\ aug_mbr_desc.dump("key"+chr(9)+"value"))

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Alternative to user-defined type:

1) Define a function to initialize a pointer

2) Then create "instances" of that pointer type

function member as p()member.name=""member.age=0member.dues_date={}member.address=""member.sex=""

end function

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dim mbr as pmbr=member() ' creates a pointer with null valuesmbr.name="Barry"mbr.dues_date=date()mbr.address="Pennsylvania"

' now, examine the pointerdim mbr_desc as Ucode=*for_each(tag,"mbr_desc.set("+"\""+tag+"\",mbr."+tag+")",\properties_enum(mbr))evaluate_template(code)ui_msg_box("mbr's dump",mbr_desc.dump("key"+chr(9)+"value"))