Variables

In programming languages, variables are symbolic "containers" for storing information. Like algebra from school where x=4, y=6, and z=x+y; x and y are variable names 4 and 6 are values and z=x+y is an expression.

Miva Script variables are used to hold values which can be strings, numbers, lists of data, or even other variables. Variables can be populated by a simple assignment (like x=3 or tree="oak") or by the result of an expression (like name = 'Miva '+ 'Script').

Variable Names

• Names start with one of these prefixes and a period ( g., l., s., or d. as explained below ). Periods can only be used after the prefix.
• Names are case in-sensitive (g.name and g.NAME are the same variable).
• After the prefix, names must begin with a letter or the underscore character.
• Variable names consist of the letters a through z (upper and lower case), the digits 0 through 9, or the underscore ( _ )
• Brackets characters are use to designate array indexes [ ]
• The colon : is used to indicate a member of a structure.

Other characters are permitted if they are 'escaped' with a backslash, but this is not common practice nor recommended.

<MvEVAL EXPR="{age\-old}">

This evaluates to the variable age-old; without the backslash, this expression would be interpreted as the value of age minus the value of old.

Variable Scope

Prefixes in the variable name define its scope in the current script or function. If scope is not defined, the virtual machine checks to see if a previously scoped variable of same name exists and has scope in current location, in the order below. The prefixes are:

• System: (Prefix s. or system.): Used to access built-in system variables. These are read only.
• Local: (Prefix l. or local.) Only used inside the user defined function <MvFUNCTION> block. Local variables only exist within the local context.
• Database: ( .d. or .database.) Not a prefix really, but a separator between a database alias and a field name (e.g. products.d.code )
• Global: ( Prefix g. ) Scope is the entire program.
• Dynamic: (Prefix dyn_): Used to access dynamically generated time values. See Time Variables below.

<MvEVAL EXPR="{ _username }">
<MvEVAL EXPR="{ user_name }">
<MvEVAL EXPR="{ g._user_name }">
<MvEVAL EXPR="{ l.user_name }">
<MvEVAL EXPR="{ l.users }">
<MvEVAL EXPR="{ l.users[1]:firstname }">
<MvEVAL EXPR="{ l.users[l.index]:lastname }">

<MvEVAL EXPR="{1st_username}">
<MvEVAL EXPR="{user.name}">
<MvEVAL EXPR="{:username}">

Variable Typing

Miva Script does not require that variable be declared or initialized before use, thus they are type-less. The same variable can contain numbers, text, dates, or boolean (true/false) values. In MivaScript a zero or null value is considered false, all other values are considered  true.

<MvASSIGN NAME="g.birthday" VALUE=338961600> Unix style date number.
<MvASSIGN NAME="g.birthday" VALUE="09/28/1980"> String
<MvASSIGN NAME="g.birthday" VALUE="September 28, 1980"> String
<MvASSIGN NAME="g.birthday" VALUE="{ s.dyn_time_t EQ 338961600 }"> This expression is false;

<MvIF EXPR="{ g.birthday }">
   Evaluates as true for the first three g.birthdays and false for the last one. 
</MvIF>

It is up to you to enforce data typing in your programs. The type-less variables can be the source of program bugs for so care must be exercised.  Both examples generate a compiler errors because of mixing string and numeric data in a single expression. However the final example works, producing a string even if all the variables are numbers, however, "9 28, 1980" is probably not the results you want.

<MvASSIGN NAME="g.birthday" VALUE="{ 'September, ' $ 28 $ ', ' $ 1980 }">
Operator type does not match constant type; inefficiency or unexpected rounding may occur.

<MvASSIGN NAME="g.birthday" VALUE="{ 'September, ' + 28 + ', ' + 1980 }">
Operator type does not match constant type; inefficiency or unexpected rounding may occur.

<MvASSIGN NAME="g.birthday" VALUE="{ 'September, ' $ '28, ' $ '1980' }">
This Works because everything is a string.

<MvASSIGN NAME="g.birthday" VALUE="{ g.month $ ' ' $ g.day $ ', ' $ g.year }">
This works even if ALL the variables are numbers but, 
the result "9 28, 1980" is probably not what you wanted.

One possible strategy for avoiding these kinds of problems is to declare string types in the variable name itself. e.g. ( g.str_birthday )

HTML Variable Names

When an HTML forms are submitted, the NAME and VALUE of form fields are received by Miva Script as global variables. In this example the Miva Script program Sales_Reports.mvc receives four global variables; g.screen, g.action, g.username, and g.password. and can take action based on their values.

<form method="get" action="Sales_Reports.mvc">
  <input type="hidden" name="screen" value="report" />
  <input type="hidden" name="action" value="login" />
  <input type="text" name="username" value="" size="25" />
  <input type="password" name="password" value="" size="25" />
  <input type="submit" value="Search">
</form>