Newport XPS-Q8, Руководство пользователя

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XPS-Q8 Tcl Manual
put opening curly braces on the line after a while or an if statement, you must
escape the newline with a backslash:
while {$i < $x} \
{
set
product ...
}
Always group expressions and command bodies with curly
braces.
Curly braces around the boolean expression are crucial because they delay variable
substitution until the while command implementation tests the expression. The
following is an example that will result in is an infinite loop:
set i 1; while $i<= 10 { incr i}
The loop will run indefinitely.
*
The reason is that the Tcl interpreter will substitute for
$i
before
while is called, so while gets a constant expression 1<=10 that
will always be true. You can avoid these kinds of errors by adopting a consistent coding
convention that groups expressions with curly braces:
set i 1; while {$i<= 10 } { incr i}
The
incr command is used to increment the value of the loop variable i . This is a
handy command that saves us from the longer command:
set i [ expr $i + 1]
The incr command can take an additional argument, a positive or negative integer by
which to change the value of the variable. In this form, it is possible to eliminate the
loop variable
i
and just modify the parameter
x
. The loop body can be written like
this:
while {$x > 1} {
set
product [
expr
$product * $x]
incr x -1
}
Example 1–14 shows another factorial, this time using a recursive definition. A
recursive function is one that calls itself to complete its work. Each recursive call
decrements
x
by one, until the value of
x
is equal to one, and then the recursion stops.
Example 1–14: A recursive definition of factorial.
proc Factorial {x} {
if {$x <= 1 } {
return 1
} else {
return [ expr $x * [Factorial [ expr $x - 1 ]]]
}
}
2.2.9 More about Variables
The set command will return the value of a variable only if it is passed in single
argument. It treats that argument as a variable name and returns the current value of the
variable. The dollar-sign syntax used to get the value of a variable is really just an easy
way to use the set command. Example 1–15 shows a clever way to circumvent this
restriction by putting the name of one variable into another variable:
Example 1–15: Using set to return a variable value.
set var {the value of var}
⇒ the value of var
set name var
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