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Databases - Practical PostgreSQL
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Variable Substitution

What may be confusing to experienced programmers at first is that LXP supports the familiar dollar sign notation to substitute a named variable (e.g., $myvariable) with its associated value in a mixed character string.

When using LXP, it is important to understand the contexts in which variables are substituted (and the context in which they are not). Subsequently, it is also important to understand when to use variable substitution and when not to.

The first rule is that variables will never be substituted outside of an LXP tag. Example 13-9 attempts incorrectly to place the value of a variable named variable within an LXP document.

Example 13-9. Invalid variable substitution

<lxp>
  Here is my variable: $variable <!-- Wrong -->
</lxp>

Instead, suppose that the URL https://localhost/test.lxp?setbar=foo is opened in a browser, and that test.lxp contains the snippet of LXP mark-up shown in Example 13-10.

Example 13-10. Valid variable substitution

<lxp>
  <setvar bar="$setbar" /> <!-- sets bar's value to setbar's value -->
  <putvar name="bar" />    <!-- output the value of bar -->
<lxp>

The mark-up in Example 13-10 opens an LXP region and uses the <setvar> tag to assign the value of the variable named setbar to a new variable named bar. Variable substitution is correctly used in this case, because it occurs within an LXP tag.

Since the previously mentioned URL assigned a value of foo to setbar, this means that the new variable bar will be assigned a value of foo.

The use of the <putvar> tag introduces the second rule to watch out for in LXP. Some tags (such as the <putvar> tag) expect to receive a literal variable name in order to perform their job. Remember that dollar signs and at signs are not actually part of variable names; they are only used to substitute values in place of names.

You might be inclined to think that the syntax of the <putvar> tag in Example 13-10 should have read like this:

  <putvar name="$bar" /> <!-- output the value of bar -->

This would actually result, however, in the value of the variable bar being substituted into the value of the name attribute. Since the value of the bar variable is foo, LXP would attempt to insert a variable with the name of foo.

The simplest way to know whether or not to use substitution characters is to remain aware of what the purpose of the tag is. If an attribute should be substituted with a variable's value , use the $ symbol to substitute it. If an attribute is literally specifying a variable by name, as with the <putvar> tag, do not substitute it.

A literal dollar sign ($) may be used within double quotes by placing two of them immediately one after the other, sequentially (e.g., <setvar price="$$99.95" />).

Note: When using substitution, if a variable with the specified name is not found, LXP will check for a cookie with the specified name. If one is found, its value will be substituted.

Databases - Practical PostgreSQL
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