Version 0 of Variable Substitution

ulis, 2003-08-31

Variable substitution is the way Tcl gets a value from a name:

  set v aValue
  puts $v

The $v notation tells Tcl to get the value of the variable v (here the value is "aValue").

The $ notation was added to Tcl to simplify the codification and is not needed. The set command (with one argument) returns the value of a variable. And the two following lines are equivalent:

  puts $v
  puts [set v]

Sometimes the $ notation can't be used without difficulty. This is when the name of the variable is composed from other variables:

  foreach i {1 2 3} { set var$i value$i }
  foreach i {1 2 3} { puts $var$i } <-- this fails (var is undefined)

The last line fails because Tcl tries to substitutes $var and then $i and finds that var is not defined.

The right solution is to use set:

  foreach i {1 2 3} { puts [set var$i] }

Here Tcl substitutes the value of i and then set returns the value of var1, var2, var3.

To best understand what appends when Tcl is substituing variables let define a $ alias that does the $ substitution:

  interp alias {} $ {} set

Replacing the $ notation by a $ call, the following lines are equivalent two by two:

  foreach i {1 2 3} { puts $var$i } <-- this fails (var is undefined)
  foreach i {1 2 3} { puts [$ var][$ i] } <-- this fails (var is undefined)

  foreach i {1 2 3} { puts [set var$i] }
  foreach i {1 2 3} { puts [$ var[$ i]] }

Composing the name of a variable is a clear signal that an array could be used:

  foreach i {1 2 3} { set var($i) value$i }
  foreach i {1 2 3} { puts $var($i) }

The last line is equivalent to:

  foreach i {1 2 3} { puts [$ var([$ i])] }