:TITLE: State of the interpreter - info 
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:LESSON_TEXT_START_LEVEL 0:
There are a number of subcommands that provide information about the current
state of the interpreter.  These commands provide access to information like
the current version and patchlevel, what script is currently being executed,
how many commands have been executed, or how far down in the call tree the
current proc is executing.
<P>
Note that the <B>pid</B> command is not part of the <B>info</B> command,
but a command in its own right.
<P>
<B> Commands that return information about the current state of the interpreter</B>
<DL>
<B><DT>info cmdcount</B>
<DD>Returns the total number of commands that have been executed by this
interpreter.
<B><DT>info level</B> <I>?number?</I>
<DD>Returns the stack level at which the compiler is currently evaluating
code.  0 is the top level, 1 is a proc called from top, 2 is a proc called
from a proc, etc.  
<P>
If <I>number</I> number is a positive value, <B>info level</B> returns a the name 
and arguments of the proc at that level on the stack.  <I>Number</I>
is that same value that <B>file level</B> would return if it were called in
the proc being referenced.
<P>
If  <I>number</I> number is a negative value, it refers to the current 
level plus <I>number</I>.  Thus, 
, <B>info level</B> returns a the name 
and arguments of the proc at that level on the stack.
<B><DT>info patchlevel</B>
<DD>Returns the value of the global variable tcl_patchlevel.  This is the
revision level of this interpreter.
<B><DT>info script</B>
<DD>Returns the name of the file currently being evaluated, if one is being
evaluated.  If there is no file being evaluated, returns an empty string.
<B><DT>info tclversion</B>
<DD>Returns the value of the global variable tcl_version.  This is the
patch level of this interpreter.
<B><DT>pid</B>
<DD>Returns the pid of the current Tcl interpreter.
</DL>
:TEXT_END:

:LESSON_TEXT_START_LEVEL 1:
There are a number of subcommands that provide information about the current
state of the interpreter.  These commands provide access to information like
the current version and patchlevel, what script is currently being executed,
how many commands have been executed, or how far down in the call tree the
current proc is executing.
<P>
The <B>info tclversion</B> and <B>info patchlevel</B> can be used to find out
if the revision level of the interpreter running your code has the support
for features you are using.  If you know that certain features are not 
available in certain revisions of the interpreter, you can define your own
procs to handle this, or just exit the program with an error message.
<P>
The <B>info cmdcount</B> and <B>info level</B> can be used while optimizing
a Tcl script to find out how many levels and commands were necessary to
accomplish a function.
<P>
Note that the <B>pid</B> command is not part of the <B>info</B> command,
but a command in its own right.
<P>
<B> Commands that return information about the current state of the interpreter</B>
<DL>
<B><DT>info cmdcount</B>
<DD>Returns the total number of commands that have been executed by this
interpreter.
<B><DT>info level</B> <I>?number?</I>
<DD>Returns the stack level at which the compiler is currently evaluating
code.  0 is the top level, 1 is a proc called from top, 2 is a proc called
from a proc, etc.  
<P>
If <I>number</I> number is a positive value, <B>info level</B> returns a the name 
and arguments of the proc at that level on the stack.  <I>Number</I>
is that same value that <B>file level</B> would return if it were called in
the proc being referenced.
<P>
If  <I>number</I> number is a negative value, it refers to the current 
level plus <I>number</I>.  Thus, 
, <B>info level</B> returns a the name 
and arguments of the proc at that level on the stack.
<B><DT>info patchlevel</B>
<DD>Returns the value of the global variable tcl_patchlevel.  This is the
revision level of this interpreter.
<B><DT>info script</B>
<DD>Returns the name of the file currently being evaluated, if one is being
evaluated.  If there is no file being evaluated, returns an empty string.
<B><DT>info tclversion</B>
<DD>Returns the value of the global variable tcl_version.  This is the
patch level of this interpreter.
<B><DT>pid</B>
<DD>Returns the pid of the current Tcl interpreter.
</DL>
:TEXT_END:

:LESSON_TEXT_START_LEVEL 2:

<P>
The <B>info</B> command also includes several subcommands which provide
information about the 
state of the interpreter.  These commands provide access to information like
the current version and patchlevel, what script is currently being executed,
how many commands have been executed, or how far down in the call tree the
current proc is executing.
<P>
The <B>info tclversion</B> and <B>info patchlevel</B> can be used to find out
if the revision level of the interpreter running your code has the support
for features you are using.  If you know that certain features are not 
available in certain revisions of the interpreter, you can define your own
procs to handle this, or just exit the program with an error message.
<P>
The <B>info cmdcount</B> and <B>info level</B> can be used while optimizing
a Tcl script to find out how many levels and commands were necessary to
accomplish a function.
<P>
Run the example, and examine the output while you read this description of 
the example code.
<P>
The first two lines show how many instructions have been executed so far.
The initial output is high because of all the commands that the interpreter
executes setting itself up to run the script.  Note that the number of commands 
increases by two between the two lines.  There are two commands in the first
line, the <B>puts</B>, and the <B>[info cmdcount]</B>.
<P>
The <B>patchlevel</B> and <B>tclversion</B> calls will return the same value if
there are no patches applied to the version of the interpreter you are running.
If patches have been applied, then the <B>patchlevel</B> subcommand will
return a string that includes patches, while <B>tclversion</B> will return
the base version level.
<P>
The <B>info script</B> command will return the name of the script that is
being executed.  When you click the <B>Run Example</B> button, the TclTutor
program writes the code in the <B>Example Code</B> window to a file in
the /tmp directory, and executes from that script.
<P>
When programs are invoked under the Unix operating system, they are assigned
a number.  This number is the Program ID number, or PID for short.  When you 
need to interact with a process at the lowest level of the operating system,
it is referred to by that number.  That number is also guaranteed to be unique.
No two processes on a system will be using the same PID at the same time.
<P>
Therefore, the PID can be used to generate a unique file name.  The file
name reported in script is generated with a line of code like this:
<PRE><CODE>set filename "/tmp/xmpl.[pid].tcl"
</CODE></PRE>
<P>
The <B>pid</B> command is shown in this example code printing out the 
PID of the script.  Each time you click the <B>Run Example</B> button,
it will display a new number.
<P>
The proc <B>factorial</B> demonstrates a way to use the <B>info level</B>
command to write an inefficient factorial program.  The <B>factorial</B>
proc will continue calling itself until the current level matches the
value that was passed as the argument.  When this level has been reached,
the nested calls to <B>factorial</B> will unwind as each invocation of
<B>factorial</B> returns the value from the previous call to <B>factorial</B>
times <CODE>($val-$lvl)</CODE>.
<P>
When a function keeps calling itself it is called recursion.
<P>
When <B>factorial</B> is called the first time, the level is 1.  Since
<CODE>$lvl</CODE> is not equal to <CODE>$val</CODE>, line after the
if statement is executed.  The Tcl interpreter sees that the return
statement requires the return from a call to <B>factorial</B>, and
it then invokes <B>factorial</B> again.
<P>
This time, <B>$lvl</B> is 2.  Again, the if statement is false, and 
<B>factorial</B> is invoked again.
<P>
This time, since a proc has called a proc which called a proc, the level is 3.  
In the example code, this matches <CODE>$val</CODE>, so the function returns
the value of $val.
<P>
That value is read in the last line of the previous invocation of <B>factorial</B>,
when <CODE>$lvl</CODE> was 2.  <CODE>($val-$lvl)</CODE> = 3-2, = 1.  So, the
last line becomes <CODE> 1*3</CODE>, and that invocation of <B>factorial</B>
returns 3 to the previous (first) invocation of <B>factorial</B>.
<P>
In the first invocation of <B>factorial</B>, the level was 1.  <CODE>3-1=2</CODE>,
so the last line returns <CODE>2*3</CODE>, which is 6.
<P>
It's obvious that multiplying a number by 1 is an unnecessary step.  Try
modifying the example code to reduce the number of recursions and see how
many fewer commands you can use to calculate the factorial.
<P>
<B> Commands that return information about the current state of the interpreter</B>
<DL>
<B><DT>info cmdcount</B>
<DD>Returns the total number of commands that have been executed by this
interpreter.
<B><DT>info level</B> <I>?number?</I>
<DD>Returns the stack level at which the compiler is currently evaluating
code.  0 is the top level, 1 is a proc called from top, 2 is a proc called
from a proc, etc.  
<P>
If <I>number</I> number is a positive value, <B>info level</B> returns a the name 
and arguments of the proc at that level on the stack.  <I>Number</I>
is that same value that <B>file level</B> would return if it were called in
the proc being referenced.
<P>
If  <I>number</I> number is a negative value, it refers to the current 
level plus <I>number</I>.  Thus, 
, <B>info level</B> returns a the name 
and arguments of the proc at that level on the stack.
<B><DT>info patchlevel</B>
<DD>Returns the value of the global variable tcl_patchlevel.  This is the
revision and patch level of this interpreter.
<B><DT>info tclversion</B>
<DD>Returns the value of the global variable tcl_version.  This is the
revision and patch level of this interpreter.
<B><DT>info script</B>
<DD>Returns the name of the file currently being evaluated, if one is being
evaluated.  If there is no file being evaluated, returns an empty string.
<B><DT>pid</B>
<DD>Returns the pid of the current Tcl interpreter.
</DL>
:TEXT_END:

:CODE_START:
puts "This is how many commands have been executed: [info cmdcount]"
puts "Now  *THIS* many commands have been executed: [info cmdcount]"

puts "\nThis interpreter is revision level: [info tclversion]"
puts "This interpreter is at patch level: [info patchlevel]"

puts "The Pid for this program is [pid]"

proc factorial {val} {
  puts "Current level: [info level] - val: $val"
  set lvl [info level]
  if {$lvl == $val} {return $val;}
  return [expr ($val-$lvl) * [factorial $val]];
  }

set count1 [info cmdcount]
set fact [factorial 3]
set count2 [info cmdcount]
puts "The factorial of 3 is $fact"
puts "Before calling the factorial proc, $count1 commands had been executed"
puts "After calling the factorial proc, $count2 commands had been executed"
puts "It took [expr $count2-$count1] commands to calculate this factorial"


:TEXT_END:

