Special variables: Difference between revisions
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{{draft task}}[[Category:Special variables]]
Special variables have a predefined meaning within the programming language. The task is to list the special variables used within the language.
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* SUBSEP - A control variable that specifies the subscript separator for multidimensional arrays
=={{header|Tcl}}==
There are three major categories of special variables in Tcl: global variables special to the Tcl language, global variables set by Tcl-based interpreters, and local variables with special interpretations.
===Language Globals===
These variables are defined by the standard implementation of Tcl, and are present in all Tcl interpreters by default.
;env
:This global array is Tcl's interface to the process's environment variables.
;errorCode
:This global scalar holds a machine-readable description of the last error to occur. (Note that prior to Tcl 8.6, internally-generated exceptions often used <tt>NONE</tt> for this value.)
;errorInfo
:This global scalar holds a stack trace from the last error to occur.
;tcl_library
:This global scalar holds the location of Tcl's own internal library.
;tcl_version, tcl_patchLevel
:This global scalar holds the version of Tcl in use. From Tcl 8.5 onwards, these hold the same (detailed) value.
;tcl_pkgPath
:This global scalar holds a Tcl list of directories where Tcl looks for packages by default. This is used to initialize the '''auto_path''' global variable.
;auto_path
:This global scalar holds a Tcl list of directories where Tcl looks for packages (and auto-loaded scripts, though this facility is deprecated).
;tcl_platform
:This global array holds a description of the platform on which Tcl is executing.
;tcl_precision
:This global scalar holds the number of significant figures to use when converting a floating-point value to a string by default. From Tcl 8.5 onwards it should not be changed. (If you are thinking of using this, consider using the <code>format</code> command instead.)
;tcl_rcFileName
:This global scalar holds the name of a file to <code>source</code> when the interpreter starts in interactive mode.
;tcl_rcRsrcName
:This global scalar is only used on classic Mac OS (now deprecated); consult the documentation for more information.
;tcl_traceCompile
:If enabled at library configuration time, this global scalar allows tracing of the compilation of bytecode in the interpreter.
;tcl_traceExec
:If enabled at library configuration time, this global scalar allows tracing of the execution of bytecode in the interpreter.
;tcl_wordchars, tcl_nonwordchars
:These global scalars hold regular expression fragments that describe the current platform's interpretation of what is and isn't a word.
===Interpreter Globals===
These global variables are only features of the most common Tcl-based shells, [[tclsh]] and [[wish]].
;argc
:This global scalar holds the number of arguments (after the script) passed to the Tcl interpreter.
;argv
:This global scalar holds a Tcl list of the arguments (after the script) passed to the Tcl interpreter.
;argv0
:This global scalar holds the name of the main script to execute that was passed to the Tcl interpreter, or the name of the interpreter itself in interactive mode.
;tcl_interactive
:This global scalar holds whether this interpreter is working in interactive mode (i.e., needs to print command prompts, run a REPL, etc.)
;tcl_prompt1, tcl_prompt2
:These global scalars allow customization of the prompt strings in interactive mode.
;geometry
This global scalar holds the user-supplied preferred dimensions of the initial window. Only used by interpreters that load the [[Tk]] library.
===Local Special Variables===
This is a language feature of procedures.
;args
:This local variable holds the Tcl list of arguments supplied to the current procedure after all the other formal arguments have been satisfied. Note that it needs to be explicitly listed in the formal arguments ''and'' be last in the list of formal arguments to have this behavior.
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