Tcl unset Command

last modified April 3, 2025

The Tcl unset command is used to remove variables or array elements. It helps manage memory by cleaning up unused variables. The command can remove single variables or multiple variables at once.

Basic Definition

The unset command destroys one or more variables. Once unset, the variable no longer exists in the current scope. Attempting to access it will raise an error.

Syntax: unset ?-nocomplain? ?--? ?name name ...?. The optional -nocomplain suppresses errors for non-existent variables.

Basic Variable Removal

This shows the simplest usage of unset to remove a single variable.

basic_unset.tcl

set name "John Doe"
puts "Before unset: $name"
unset name
puts "After unset: [info exists name]"

This creates a variable name, prints it, then removes it with unset. The info exists command checks if the variable still exists.

Removing Multiple Variables

The unset command can remove several variables in one operation.

unset_multiple.tcl

set x 10
set y 20
set z 30
puts "Before: x=$x, y=$y, z=$z"
unset x y z
puts "After: x exists? [info exists x]"

This creates three variables, then removes them all with one unset command. The example verifies removal by checking variable existence.

Unsetting Array Elements

unset can remove individual elements from an array while keeping the array intact.

unset_array.tcl

array set colors {
    red   #ff0000
    green #00ff00
    blue  #0000ff
}
puts "Before: [array get colors]"
unset colors(green)
puts "After: [array get colors]"

This creates an array of colors, then removes the green element. The remaining array elements are still accessible after the operation.

Using -nocomplain Option

The -nocomplain option prevents errors when unsetting non-existent variables.

unset_nocomplain.tcl

# This would normally cause an error
unset -nocomplain non_existent_var
puts "Command completed without error"

set temp_var "temporary"
unset -nocomplain temp_var non_existent_var
puts "Mixed unset completed"

This shows how -nocomplain allows the script to continue even when some variables don't exist. Useful for cleanup scripts.

Unsetting Variables in Procedures

Variables in procedures can be unset to free memory before procedure completion.

unset_proc.tcl

proc calculate {} {
    set a 5
    set b 10
    set result [expr {$a * $b}]
    unset a b
    return $result
}
puts "Result: [calculate]"

This procedure calculates a result, then unsets its temporary variables before returning. This is good practice for memory management in larger scripts.

Unsetting Entire Arrays

The unset command can remove entire arrays with a single operation.

unset_whole_array.tcl

array set user {
    name John
    age 30
    city New York
}
puts "Before: [array exists user]"
unset user
puts "After: [array exists user]"

This creates an array, then removes it completely. The array exists command verifies the array's existence before and after unset.

Best Practices

Cleanup: Unset large variables when no longer needed.
Scope: Be mindful of variable scope when unsetting.
Arrays: Unset entire arrays to free all elements.
Safety: Use -nocomplain for optional cleanup.
Testing: Check with info exists before accessing.

This tutorial covered the Tcl unset command with practical examples showing its usage in different scenarios.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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