Tcl array Command

last modified October 20, 2026

The Tcl array command provides operations for working with associative arrays. Arrays in Tcl are collections of key-value pairs. They are similar to dictionaries in other languages.

Basic Definition

Tcl arrays are associative arrays where keys can be arbitrary strings. They are different from traditional indexed arrays in other languages.

Syntax: array option arrayName ?arg ...?. The command takes an operation name, array name, and optional arguments depending on the operation.

Creating and Setting Array Elements

This example shows how to create an array and set its elements.

array_set.tcl

array set colors {
    red    #ff0000
    green  #00ff00
    blue   #0000ff
}

puts $colors(red)
puts $colors(green)
puts $colors(blue)

Here we create an array named colors with three elements. The array set command initializes the array with key-value pairs. We then access individual elements using the $array(key) syntax.

Getting Array Size and Names

The array size and array names commands provide information about an array.

array_info.tcl

array set fruits {
    apple  10
    orange 15
    banana 20
}

puts "Number of fruits: [array size fruits]"
puts "Fruit names: [array names fruits]"

This code creates a fruits array and then displays its size and keys. array size returns the number of elements, while array names returns a list of all keys. The order of the keys is not defined, so the output may differ from run to run; use lsort when a stable order is needed.

Iterating Over Array Elements

The array get command converts an array to a list, which can be used for iteration.

array_iterate.tcl

array set temperatures {
    monday    72
    tuesday   68
    wednesday 75
}

foreach {day temp} [array get temperatures] {
    puts "$day: $temp°F"
}

This example shows how to iterate through all elements of an array. The array get command returns a flat list of key-value pairs, which we process with foreach. The elements come in no particular order.

Checking Array Existence

The array exists command checks if a variable is an array.

array_exists.tcl

array set user {
    name    John
    age     30
    city    {New York}
}

if {[array exists user]} {
    puts "User array exists"
} else {
    puts "User array does not exist"
}

This demonstrates how to verify if a variable is an array before attempting to use array operations on it. The command returns 1 if the variable is an array, 0 otherwise.

Searching Array Elements

The array names command accepts a glob pattern, which lets us select the elements whose keys match it.

array_search.tcl

array set books {
    tcl_programming 25
    python_guide    30
    tcl_cookbook    20
    java_basics     15
}

set tcl_books [array names books *tcl*]
foreach book $tcl_books {
    puts "$book: \$[set books($book)]"
}

This example searches for all books with "tcl" in their names. The array names command with a glob pattern returns matching keys (use the -regexp mode for regular expressions). We then display these books with their prices.

Unsetting Array Elements

The array unset command removes the elements whose keys match a glob pattern, or the entire array when no pattern is given.

array_unset.tcl

array set config {
    debug   1
    logfile /var/log/app.log
    timeout 30
}

array unset config logfile
puts "Remaining config: [array get config]"

array unset config
puts "Array exists after unset: [array exists config]"

Here we first remove just the logfile element from the array, then we remove the entire array. The second argument is a glob pattern, not a plain key: array unset config l* would remove every element whose key begins with "l". To remove exactly one element, use unset config(logfile).

Best Practices

Initialization: Use array set for bulk initialization.
Checking: Use array exists to tell an array from a scalar variable or a missing one.
Patterns: Use glob patterns with array names for searching.
Order: Do not rely on the order of array names or array get; sort the keys when order matters.
Naming: Choose descriptive array names for better code clarity.

This tutorial covered the Tcl array 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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