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, whose value is
aValue
The $ notation was added to Tcl for coding convenience as a shorter way to express
set v
The following lines are equivalent:
puts $v puts [set v]
When dynamically compoing a variable name, set can be used where $ can't. In the following example, Tcl substitutes the value of i and then set returns the value of var1, var2, var3.
foreach i {1 2 3} {puts [set var$i]}
The following code fails because Tcl tries to substitute $var and then $i, and finds that var is not defined.
#warning: example of bad code foreach i {1 2 3} {puts $var$i} ;# this fails because $var is undefined
Another approach that is not recommended:
foreach i {1 2 3} {eval puts \$var$i}
Using the [set command in place of the $ variable can help to illuminate the behaviour of Tcl:
Replacing the $ notation by a $ call, the following lines are equivalent two by two:
foreach i {1 2 3} {puts $var$i} ;# this fails because $var is undefined foreach i {1 2 3} {puts [set var][set i]} ;# this fails because $var is undefined foreach i {1 2 3} {puts [set var$i]} foreach i {1 2 3} {puts [set var[set i]]}
When accessing an array variable, the element name is computed and then looked up in the array, so arrays are a good fit when programming in a style that leads to composition of variable names:
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 [set var([set i])]}
Although variable indirection is considered bad style by some, it can be implemented as in the following example:
set a something set pointer a puts [set $pointer] ;#good style puts [set [set pointer]] ;#more verbose style eval puts \$$pointer ;#oh blimey, it's "eval"! Run away!