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<HTML><TITLE>Basic Syntax and I/O: Curly Brackets</TITLE><BODY BGCOLOR="#FFF0E0" VLINK="#0FBD0F" TEXT="#101000" LINK="#0F0FDD">
<A NAME="top"><H1>Curly Brackets</H1></A>
<P> Here is a <TT>while</TT> loop for copying a file line by line. The condition for
continuing the loop is shown partially in pseudocode.
<PRE>
while {-1 != <CITE>the value obtained by executing</CITE> gets $InFile Line} {
puts $OutFile $Line
}
</PRE>
<P> Two questions come immediately to mind:
<OL>
<P> <P><LI> How can this work when it appears on three lines instead of one?
<P> <P><LI> How can this work if the variables are substituted before the
<TT>while</TT> procedure is executed?
</OL>
The answers to both questions lie in the way curly brackets are processed.
<P> <CITE>When curly brackets surround a command argument,</CITE> they cause the Tcl
interpreter to consider everything between the curly brackets as a sealed
string that does not need processing. In particular, it needs no substitutions
and no consideration of end-of-line symbols. More precisely,
<UL>
<P> <P><LI> the interpreter forms everything inside these brackets into a string
<P> <P><LI> while forming the string, it does not do any substitutions or pay
attention to blanks or new lines. (An exception appears below in the
<A HREF="2.10.html" tppabs="http://www.mapfree.com/sbf/tcl/book/select/Html/2.10.html">More about Substitution</A>.)
<P> <P><LI> when done, it throws away the curly brackets and passes the string
as an argument to the procedure
</UL>
<P> This means that the <TT>while</TT> statement shown above looks like
<PRE>
while {<CITE>CONDITION</CITE>} {<CITE>BODY</CITE>}
</PRE>
and there is no variable substitution within the curly brackets. What you
see for <CITE>CONDITION</CITE> and <CITE>BODY</CITE> is what the <TT>while</TT> procedure sees. At
the beginning of each iteration, when <TT>while</TT> is evaluating the
<CITE>CONDITION</CITE> argument, it performs substitutions as a part of that
evaluation. During each iteration, <TT>while</TT> performs substitutions to the
lines in <CITE>BODY</CITE>. These substitutions are normal Tcl substitutions. They
have been suppressed by the curly brackets and are performed by <TT>while</TT>
as it goes about its business.
<P> Returning to the file copying loop, the <TT>while</TT> procedure will execute this
line over and over:
<P> <PRE>
puts $OutFile $Line
</PRE>
<CITE>Each execution</CITE> of the line begins with Tcl's normal substitutions – the
same substitutions that were suppressed by the use of the curly brackets
around the second argument. The curly brackets ensure that it is the strings
<TT>$Outfile</TT> and <TT>$Line</TT> that are available when the line is executed – not the values these strings represent. By delaying the substitution of these
variables, Tcl allows the values to change during execution of the <TT>while</TT>
loop.
<P> Here are some examples that make additional points about curly brackets. As
you read these examples, be aware that in them <TT>puts</TT> has just <CITE>one</CITE>
argument. This fact may not be apparent as the examples appear to you.
<UL>
<P> <P><LI> Curly brackets may be nested. For example,
<PRE>
% puts {a{x}b}
a{x}b
</PRE>
<P> <P><LI> An argument is considered to be surrounded by curly brackets if, and
only if, it begins with a curly bracket. In the next example, the curly
brackets do not surround the argument. Therefore, they do not affect
the substitution of the variable named <TT>X.</TT>
<PRE>
% puts 1{$X}1
1{$X}1
</PRE>
The next example shows an error.
<PRE>
% puts {X}1
extra characters after close-brace
</PRE>
The error message is <TT>puts</TT>' return string. The error is that the
argument begins with a curly bracket but is not surrounded by curly brackets.
</UL>
<P> <P><A NAME="2.5a">
<STRONG>Exercise 2.5a</STRONG> </A><DL><DD>
Write a Tcl command that assigns the string
"Hello World" (without the quotes) to the variable named <TT>X.</TT>
<P>
<A HREF="2.11.html#Sol2.5a" tppabs="http://www.mapfree.com/sbf/tcl/book/select/Html/2.11.html#Sol2.5a">Solution</A></DL>
<P> <P><A NAME="2.5b">
<STRONG>Exercise 2.5b</STRONG> </A><DL><DD>
What are the values of <TT>X,</TT> <TT>Y,</TT> and <TT>Z</TT>
after these statements have executed?
<PRE>
set X 2
set Y {$X}
set Z VAR-{$X}
</PRE>
In the last <TT>set</TT> statement there are just two arguments even though your
browser may show more.
<P>
<A HREF="2.11.html#Sol2.5b" tppabs="http://www.mapfree.com/sbf/tcl/book/select/Html/2.11.html#Sol2.5b">Solution</A></DL>
<P>
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<I>Jun 17, 1998</I>
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