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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.

MivaScript 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#

A variable can have a short name, like g.x, or a more descriptive name, like l.product:code, but all variable names follow these rules:

  • Names start with one of these prefixes and a period (g., l., s., or d.). Periods can only be used after the prefix.
  • Names are case-insensitive (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–z (upper and lower case), the digits 0–9, or the underscore (_).
  • Bracket characters are used 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 the same name exists and has scope in the current location, in this order:

  • System (s. or system.): Built-in system variables (read-only).
  • Local (l. or local.): Only inside a user-defined <MvFUNCTION> block.
  • Database (.d. or .database.): Separator between a database alias and field name (e.g., products.d.code).
  • Global (g.): Scope is the entire program.
  • Dynamic (dyn_): Dynamically generated time values (see Time Variables).

Valid Names#

Correct

<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 }">

Invalid Names#

Incorrect

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

Variable Typing#

MivaScript does not require that variables 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. Type-less variables can be the source of program bugs, so care must be exercised. Both examples generate compiler errors because of mixing string and numeric data in a single expression. However, the final example works, producing a string even if all variables are numbers, although “9 28, 1980” is probably not the result you want.

Information

Operator type does not match constant type; inefficiency or unexpected rounding may occur. 

<MvASSIGN NAME="g.birthday" VALUE="{ 'September, ' $ 28 $ ', ' $ 1980 }">

Information

Operator type does not match constant type; inefficiency or unexpected rounding may occur. 

<MvASSIGN NAME="g.birthday" VALUE="{ 'September, ' + 28 + ', ' + 1980 }">

Information

This works because everything is a string. 

<MvASSIGN NAME="g.birthday" VALUE="{ 'September, ' $ '28, ' $ '1980' }">

Information

This works even if ALL the variables are numbers, but the result “9 28, 1980” is probably not what you wanted. 

<MvASSIGN NAME="g.birthday" VALUE="{ g.month $ ' ' $ g.day $ ', ' $ g.year }">

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 form is submitted, the name and value of form fields are received by MivaScript as global variables. In this example, 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 Field Variables:#

<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>