Square Brackets Do Not Group; Grouping Before Substitution - Newport XPS-Q8 Manual

XPS-Q8
EDH0307En1041 — 10/17
declarations. Double quotes are useful in simple cases like the puts command shown
previously.
Another common use of quotes is with the format command. This is similar to the C
printf function. The first argument to format is a format specifier, which often
includes special characters like newlines, tabs, and spaces. The easiest way to specify
such characters is with backslash sequences (e.g., \n for newline and \t for tab). The
backslashes must be substituted before the format command is called, so you need to
use quotes to group the format specifier.
puts [format
Here format is used to align a name and a value with a tab. The %s and %5.3f
indicate how the remaining arguments are to be formatted by the format command .
Note that the trailing \n usually found in a C printf call is not needed because
puts provides one for us.
2.2.6.1

Square Brackets Do Not Group

The square bracket syntax used for command substitution does not provide grouping.
Instead, a nested command is considered part of the current group. In the command
below, the double quotes group the last argument, and the nested command is just part
of that group.
puts
stdout "The
If an argument is made up of a nested command, you do not need to group it with
double-quotes because the Tcl parser treats the whole nested command as part of the
group.
puts stdout
The following is a redundant use of double quotes:
puts
stdout
2.2.6.2

Grouping before Substitution

The Tcl parser makes a single pass through a command as it makes grouping decisions
and performs string substitutions. Grouping decisions are made before substitutions are
performed, which is an important property of Tcl. This means the values being
substituted will not affect grouping because the grouping decisions have already been
made.
The following example demonstrates how nested command substitution affects
grouping. A nested command is treated as an unbroken sequence of characters,
regardless of internal structure. It is included with the surrounding group of characters
when collecting arguments for the main command.
Example 1–11: Embedded command and variable substitution.
set x 7; set
puts stdout
⇒ 7+9=16
In Example 1–11, the second argument to puts is:
$x+$y=[expr
The white space inside the nested command is ignored for the purposes of grouping the
argument. By the time Tcl encounters the left bracket, it has already done some variable
substitutions to obtain:
7+9=
When the left bracket is encountered, the interpreter calls itself recursively to evaluate
the nested command. Again, the $x and $y are substituted before calling expr.
Finally, the result of expr is substituted for everything from the left bracket to the
right bracket. The
7+9=16
"Item: %s\t%5.3f" $name $value]
length
of $s is
[string length $s]
"[expr $x + $y]"
y 9
$x+$y=[expr $x + $y]
$x + $y]
puts
command gets the following as the second argument:
6
Tcl Manual
[string length
$s]."