# Subroutines in Tcl are created with the [proc] command,
# which takes a list of formal parameters as its second
# argument.
# On activation, the parameters are bound to the "words"
# of the call (which may contain data, variable/subroutine
# names, executable expressions, etc). This is a variant
# of call-by-name semantics.
proc hypotenuse {x y} {
return [expr {sqrt($x*$x+$y*$y)}] ;# Better still: use hypot()
}
set diag [hypotenuse 3 4]
# => 5.0
# Subroutines may have a variable number of
# arguments, by using the special argument "args":
proc hypotenuse args {
foreach {x y} $args break
return [expr {hypot($x, $y)}]
}
# A subroutine can be applied to a list using [eval],
# which concatenates and then executes its arguments.
set a [list 3 4]
eval hypotenuse $a
# => 5.0
# It is possible to create local references
# to variables in other stack frames using
# [upvar], so the typical idiom for
# pass-by-reference is to pass the variable's
# name as argument, and [upvar] it:
set nums [list 1.4 3.5 6.7]
proc trunc-em {name} {
upvar $name a
set len [llength $a]
for {set i 0} {$i < $len} {incr i} {
lset a $i [expr {int([lindex $a $i])}]
}
}
trunc-em nums
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