Tcl list Command

last modified April 3, 2025

The Tcl list command creates and manipulates lists, which are fundamental data structures in Tcl. Lists are ordered collections of elements that can contain any Tcl values.

Basic Definition

The list command creates a properly formatted list from its arguments. It handles quoting and escaping automatically, making it safer than manual list creation.

Syntax: list ?arg1 arg2 ... argN?. With no arguments, it returns an empty list. With arguments, it creates a list containing those elements.

Creating a Simple List

This example demonstrates basic list creation with the list command.

basic_list.tcl

set colors [list red green blue]
puts $colors

This creates a list containing three color names. The puts command prints the entire list. The list elements are properly quoted and separated.

Creating Nested Lists

The list command can create nested lists (lists within lists).

nested_list.tcl

set matrix [list [list 1 2 3] [list 4 5 6] [list 7 8 9]]
puts $matrix

This creates a 3x3 matrix as a nested list structure. Each inner list represents a row of the matrix. The outer list contains these row lists as elements.

List Concatenation

Lists can be concatenated using the list command with multiple lists.

concat_list.tcl

set fruits [list apple orange]
set veggies [list carrot potato]
set combined [list {*}$fruits {*}$veggies]
puts $combined

This combines two separate lists into one. The {*} operator expands each list's elements. The result is a single list with all elements.

Creating Lists with Special Characters

The list command properly handles elements containing spaces or special characters.

special_list.tcl

set files [list "my document.txt" "important file.doc" "data.csv"]
puts $files

This creates a list of filenames containing spaces. The list command automatically handles the quoting needed for elements with spaces.

Empty List Creation

An empty list can be created by calling list with no arguments.

empty_list.tcl

set empty [list]
puts "Empty list has [llength $empty] elements"

This demonstrates creating an empty list, which is often used as an initial value. The llength command confirms the list contains zero elements.

List with Variable References

Lists can contain variable references that are evaluated when the list is used.

var_list.tcl

set name "Alice"
set age 30
set person [list $name $age]
puts $person

This creates a list containing the values of variables name and age. The variables are evaluated when the list is created.

Best Practices

Use list for creation: Always use list to create lists safely.
Proper quoting: Let list handle quoting automatically.
Nested lists: Use nested lists for complex data structures.
List commands: Use Tcl's list commands (lindex, lappend, etc.) for manipulation.
Variable expansion: Use {*} for safe list expansion.

This tutorial covered the Tcl list 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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