Tcl llength Command

last modified April 3, 2025

The Tcl llength command returns the number of elements in a list. It's a fundamental list operation in Tcl. The command takes a single list argument and returns its length.

Basic Definition

The llength command counts the top-level elements in a Tcl list. It doesn't count nested elements recursively. The command is efficient and works in constant time.

Syntax: llength list. The command returns a decimal string representing the number of elements. Empty lists return 0.

Simple List Length

This example shows basic usage of llength with a simple list.

basic_llength.tcl

set colors {red green blue}
set len [llength $colors]
puts "The list has $len elements"

We create a list of colors and use llength to get its size. The result is stored in len and printed. The output will be 3.

Empty List

llength correctly handles empty lists by returning 0.

empty_list.tcl

set empty_list {}
set len [llength $empty_list]
puts "Empty list length: $len"

This demonstrates llength behavior with empty lists. The command returns 0, which is the expected result for an empty list.

Nested Lists

llength only counts top-level elements, not nested ones.

nested_list.tcl

set nested {{a b c} {d e} {f g h i}}
set len [llength $nested]
puts "Nested list length: $len"

The list contains 3 elements (each being a sublist), so llength returns 3. It doesn't count the individual letters in the sublists.

List With Different Data Types

llength works with lists containing mixed data types.

mixed_list.tcl

set mixed {1 "two" 3.0 {four five} six}
set len [llength $mixed]
puts "Mixed list length: $len"

This list contains numbers, strings, and sublists. llength counts each element regardless of type. The result is 5 in this case.

Dynamic List Creation

llength can be used with dynamically created lists.

dynamic_list.tcl

set items [list apple orange [list grape strawberry] banana]
set len [llength $items]
puts "Dynamic list length: $len"

The list is created using the list command. llength counts 4 elements (the sublist counts as one element). This shows how llength works with programmatically created lists.

Using llength in Conditionals

llength is often used in conditional statements to check list size.

conditional_llength.tcl

set data {10 20 30 40 50}

if {[llength $data] > 3} {
    puts "List has more than 3 elements"
} else {
    puts "List has 3 or fewer elements"
}

This example shows how to use llength in a conditional statement. We check if the list contains more than 3 elements and print an appropriate message.

Best Practices

Validation: Use llength to validate list size before processing.
Performance: llength is O(1) operation - fast for any list size.
Nested Lists: Remember it only counts top-level elements.
Empty Check: Combine with empty for comprehensive checks.
Type Safety: Ensure argument is a valid list to avoid errors.

This tutorial covered the Tcl llength 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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