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Chapters:
</br>
<a href="Introduction.html">1: Introduction</a><br>
<a href="SimpleExample.html">2: Simple example</a><br>
<a href="InvokingGri.html">3: Invocation</a><br>
<a href="GettingMoreControl.html">4: Finer Control</a><br>
<a href="X-Y.html">5: X-Y Plots</a><br>
<a href="ContourPlots.html">6: Contour Plots</a><br>
<a href="Images.html">7: Image Plots</a><br>
<a href="Examples.html">8: Examples</a><br>
<a href="Commands.html">9: Gri Commands</a><br>
<a href="Programming.html">10: Programming</a><br>
<a href="Environment.html">11: Environment</a><br>
<a href="Emacs.html">12: Emacs Mode</a><br>
<a href="History.html">13: History</a><br>
<a href="Installation.html">14: Installation</a><br>
<a href="Bugs.html">15: Gri Bugs</a><br>
<a href="TestSuite.html">16: Test Suite</a><br>
<a href="Acknowledgments.html">17: Acknowledgments</a><br>
<a href="License.html">18: License</a><br>
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Indices:</br>
<a href="ConceptIndex.html"><i>Concepts</i></a><br>
<a href="CommandIndex.html"><i>Commands</i></a><br>
<a href="BuiltinIndex.html"><i>Variables</i></a><br>
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<h3>10.11.5: Altering command arguments -- the `<font color="#82140F"><code>&</code></font>' syntax</h3>
The Gri language permits a newcommand to change variables and synonyms
passed as arguments, using a syntax that is quite similar to that
employed by the C++ language.
<p>
<UL>
<LI><a href="ChangeableCommandArguments.html#TheAmpersandSyntax">The Ampersand Syntax</a>: Denoting changeable arguments
<LI><a href="ChangeableCommandArguments.html#DoublingAVariable">Doubling A Variable</a>: Variables (e.g. `<font color="82140F"><code>&.variable.</code></font>')
<LI><a href="ChangeableCommandArguments.html#ManipulatingASynonym">Manipulating A Synonym</a>: Synonyms (e.g. `<font color="82140F"><code>&\synonym</code></font>')
<LI><a href="ChangeableCommandArguments.html#Nesting">Nesting</a>: Newcommands called by newcommands
<LI><a href="ChangeableCommandArguments.html#UsingNewAndDelete">Using New And Delete</a>: Isolating local variables and synonyms
<LI><a href="ChangeableCommandArguments.html#DeterminingCallingInformation">Determining Calling Information</a>: The `<font color="82140F"><code>\&.word?.</code></font>' and `<font color="#82140F"><code>\&&.word?.</code></font>' syntax
<LI><a href="ChangeableCommandArguments.html#ImplementationofAmpersandSyntax">Implementation of Ampersand Syntax</a>: Algorithm Gri uses
</UL>
<!-- @node The Ampersand Syntax, Doubling A Variable, Changeable Command Arguments, Changeable Command Arguments -->
<a name="TheAmpersandSyntax" ></a>
<h4>10.11.5.1: Overview of the `<font color="#82140F"><code>&</code></font>' syntax</h4>
Normally the arguments to a newcommand are parsed into either numerical
values or strings, before execution is passed into the newcommand. This
is a akin to the scheme called "call by value" in some programming
languages. Gri also provides a syntax, borrowed from C++, that permits
a newcommand to alter the contents of variable or synonym arguments.
<p>
The technique is simple. To permit a newcommand to modify an argument
that is a variable or a synonym, just put a `<font color="#82140F"><code>&</code></font>' to the left of the
item on the calling line. Then, within the newcommand, the
corresponding local synonym (i.e. `<font color="#82140F"><code>\.word1.</code></font>', etc.) will behave as
though it were the instance of the original variable or synonym.
<p>
The `<font color="#82140F"><code>&</code></font>' is placed to the left of the variable-name or synonym-name
without intervening space. For example `<font color="#82140F"><code>foo &.var. &\syn</code></font>' tells
the parser that the newcommand named `<font color="#82140F"><code>foo</code></font>' may possibly alter the
values of the variable `<font color="#82140F"><code>.var.</code></font>' and the synonym `<font color="#82140F"><code>\syn</code></font>', as they
exist in the calling context.
<p>
It is important to note that Gri pays very little attention to the
`<font color="#82140F"><code>&</code></font>' in a syntax-declaration line. All it does is to note that the
item to the right of the `<font color="#82140F"><code>&</code></font>' is not a fixed word in the newcommand
being defined; this follows the usual rules for parsing newcommand syntax
(see <a href="ParsingSynonyms.html#Parsing">Parsing</a>).
<p>
<!-- @node Doubling A Variable, Manipulating A Synonym, The Ampersand Syntax, Changeable Command Arguments -->
<a name="DoublingAVariable" ></a>
<h4>10.11.5.2: Example: doubling a variable</h4>
Consider the task of adding a fixed amount to a variable. If the
variable we wish to double is `<font color="#82140F"><code>.x.</code></font>', we might write
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
`double_a_particular_variable'
{
.x. = {rpn .x. 2 *}
}
.x. = 10
double_a_particular_variable
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
Code such as that presented above occurs in many applications. (Turn
the multiplication into an addition, and change `<font color="#82140F"><code>.x.</code></font>' to
`<font color="#82140F"><code>..ymargin..</code></font>', and you'll start to see the core of an application
that draws multiple graph panels, one above another.) However, the code
is too specific to be of much general use!
<p>
What if we want to double some other variable instead? The code below
shows how to do that.
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
`double &.value.'
{
\.word1. = {rpn \.word1. 2 *} # line 3
}
.x. = 10 # line 5
double &.x.
.y. = 3.14
double &.y.
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
At line 3 Gri interprets the `<font color="#82140F"><code>\.word1.</code></font>' to the <b>left</b> of the
equals sign as a reference to the variable that is set to the value 10
in line 5. Similarly, the `<font color="#82140F"><code>\.word1.</code></font>' to the <b>right</b> of the
equals sign evaluates to 10, the value in the calling program.
<p>
Gri automatically determines whether an item is a variable or a synonym,
and does the correct thing. Thus, for example, if line 3 above were written as
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
\.word1. = "hello" # ERROR
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
an error would be reported, since `<font color="#82140F"><code>double</code></font>' was called with a
variable as an argument, and variables cannot hold strings.
<p>
<!-- @node Manipulating A Synonym, Nesting, Doubling A Variable, Changeable Command Arguments -->
<a name="ManipulatingASynonym" ></a>
<h4>10.11.5.3: Example: manipulating a synonym</h4>
Synonyms are treated in the same way, as is illustrated in the following
example.
<p>
Q: what does the following print?
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
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<TD>
<PRE>
<font color="#82140F">
`add_a_dat &\filename'
{
\.word1. = {rpn "\.word1." ".dat" strcat}
}
\filename = "test"
add_a_dat &\filename
show "\filename"
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
A: it prints `<font color="#82140F"><code>test.dat</code></font>'.
<p>
<!-- @node Nesting, Using New And Delete, Manipulating A Synonym, Changeable Command Arguments -->
<a name="Nesting" ></a>
<h4>10.11.5.4: Nesting</h4>
One newcommand may call another, letting a deeply-nested newcommand
alter values of synonyms and variables that may otherwise be hidden
behind `<font color="#82140F"><code>new</code></font>' items of the same name. This is done by using the
`<font color="#82140F"><code>&</code></font>' notation at each step. The following provides an example of
passing a variable through two levels of newcommands.
<p>
Q: what does the following print?
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
`food critic &food'
{
\.word2. = "\.word2.s"
yummy &\.word2.
}
`yummy &foods'
{
\.word1. = "\.word1. are tasty"
}
\a = "apple"
food critic &\a
show "\a"
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
A: it prints `<font color="#82140F"><code>apples are tasty</code></font>'.
<p>
<!-- @node Using New And Delete, Determining Calling Information, Nesting, Changeable Command Arguments -->
<a name="UsingNewAndDelete" ></a>
<h4>10.11.5.5: About `<font color="#82140F"><code>new</code></font>' and `<font color="#82140F"><code>delete</code></font>'</h4>
If `<font color="#82140F"><code>new</code></font>' and `<font color="#82140F"><code>delete</code></font>' are executed on local synonyms inside
newcommands (e.g. `<font color="#82140F"><code>new \.word1.</code></font>') they create and destroy
variables and synonyms in the context of the <b>calling program</b>.
<p>
For example, consider the following.
<p>
Q: what does the following print?
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
`poetry &\s'
{
new \s # line 3
\s = "rose"
\.word1. = "\.word1. is a \s" # line 5
delete \s # line 6
}
\s = "A rose " # line 8
poetry &\s
show "\s"
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
A: it prints `<font color="#82140F"><code>A rose is a rose</code></font>'.
<p>
The key point here is that the instance of the synonym named `<font color="#82140F"><code>\s</code></font>'
in the calling program, set in line 8, is modified by the `<font color="#82140F"><code>poetry</code></font>'
newcommand, in line 6. This modification involves the use of a synonym,
<b>also named</b> `<font color="#82140F"><code>\s</code></font>', that "lives" wholly within the newcommand,
being created in line 3 and destroyed in line 6.
<p>
<!-- @node Determining Calling Information, Implementation of Ampersand Syntax, Using New And Delete, Changeable Command Arguments -->
<a name="DeterminingCallingInformation" ></a>
<h4>10.11.5.6: Determining calling information</h4>
Newcommands may determine the name and the nesting level of changeable
calling arguments. To get the name, put a single ampersand after the
backslash of the local synonym of interest. To get the nesting level (0
for main program, etc.) put two ampersands after the backslash.
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
`NC &.var.'
{
show "\&.word1. (expect '.a.')"
show "\&&.word1. (expect 0)"
}
.a. = 1
NC &.a.
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
<b>Note</b>: neither of these items may be used an lvalue. That is,
they may not be used to the left of an equals sign. But you can always
get around that by clever use of alias synonyms (see <a href="Synonyms.html#AliasSynonyms">Alias Synonyms</a>).
<p>
<!-- @node Implementation of Ampersand Syntax, Hints, Determining Calling Information, Changeable Command Arguments -->
<a name="ImplementationofAmpersandSyntax" ></a>
<h4>10.11.5.7: How Gri implements the `<font color="#82140F"><code>&</code></font>' syntax</h4>
When the parser encounters an unquoted `<font color="#82140F"><code>&</code></font>' followed immediately by
the name of a variable or a synonym, it converts the whole token
(`<font color="#82140F"><code>&</code></font>' plus name) into a specially-encoded string that can be
recognized inside newcommands. (This is <b>only</b> done if the
`<font color="#82140F"><code>&</code></font>' and the variable name are <b>not</b> enclosed in double
quotes.)
<p>
This specially-encoded string contains not just the name of the variable
or synonym, but also the current level of nesting of newcommands. In
this way a newcommand can have its own private versions of variables,
created by `<font color="#82140F"><code>new</code></font>', that won't be misinterpreted for the
identically-named variables in the calling program.
<p>
The format of these specially-encoded strings is
`<font color="#82140F"><code>#\bn\ba\bm\be\b:\bN \b_ \bl\be\bv\be\bl\b:\bL#\b</code></font>',
where `<font color="#82140F"><code>N</code></font>' stands for
the name of the variable/synonym, `<font color="#82140F"><code>L</code></font>' stands for the current level,
and `<font color="#82140F"><code>\b</code></font>' is the backspace character (hexadecimal 08 in the ascii
table). This string is designed to be strange enough that users are
unlikely to use it themselves. The coding scheme is not entirely
arbitrary, however; note that if the backspace characters are ignored
the result has the form `<font color="#82140F"><code>name:N_level:L</code></font>'. It's also worth noting
that if this string were printed on a terminal that erased characters
when typing backspaces the result would be of the form `<font color="#82140F"><code>N_L</code></font>'.
<p>
Examples: `<font color="#82140F"><code>@.a.</code></font>' in the main program
(i.e. at level 0) encodes to
`<font color="#82140F"><code>#\bn\ba\bm\be\b:\b.a. \b_ \bl\be\bv\be\bl\b:\b0#\b</code></font>'
and `<font color="#82140F"><code>&\my_syn</code></font>' inside a newcommand called by the main program
(i.e. at level 1) encodes to
`<font color="#82140F"><code>#\bn\ba\bm\be\b:\b\my_syn \b_ \bl\be\bv\be\bl\b:\b1#\b</code></font>'.
<p>
Inside a newcommand, Gri checks to see if builtin synonyms
(e.g. `<font color="#82140F"><code>\.word1.</code></font>') hold such specially-encoded strings. If so, then
the appropriate versions of the variables are used in preference to any
variables that may have been newly created by `<font color="#82140F"><code>new</code></font>' inside the
newcommand.
<p>
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