Tcl lrange Command

last modified April 3, 2025

The Tcl lrange command extracts a range of elements from a list. It's essential for working with list data structures in Tcl. The command returns a new list containing the specified elements.

Basic Definition

The lrange command returns one or more adjacent elements from a list. It takes a list and two indices specifying the range to extract.

Syntax: lrange list first last. The indices are zero-based. The command includes both the first and last elements in the result.

Extracting a Simple Range

This example shows the basic usage of lrange to extract elements from a list.

basic_lrange.tcl

set colors {red green blue yellow orange}
set subset [lrange $colors 1 3]
puts $subset

This extracts elements at positions 1 through 3 from the colors list. The result is green blue yellow. Remember that list indices start at 0 in Tcl.

Using Negative Indices

Tcl allows negative indices to count from the end of the list. -1 refers to the last element.

negative_indices.tcl

set fruits {apple banana cherry date elderberry}
set last_three [lrange $fruits end-2 end]
puts $last_three

This extracts the last three elements from the fruits list. The end keyword refers to the last element, and end-2 refers to the third from last element.

Extracting a Single Element

When first and last indices are the same, lrange returns a single element as a list.

single_element.tcl

set numbers {1 2 3 4 5 6 7 8 9}
set middle [lrange $numbers 4 4]
puts "The middle number is $middle"

This extracts just the element at index 4 (the fifth element). The result is 5 as a single-element list. Note this differs from lindex which returns the raw value.

Extracting All Elements

Using 0 as first and end as last indices extracts the entire list.

entire_list.tcl

set letters {a b c d e f g}
set all_letters [lrange $letters 0 end]
puts $all_letters

This demonstrates how to get a copy of the entire list using lrange. While not the most efficient way to copy a list, it shows the command's range capabilities.

Out-of-Bound Indices

lrange handles out-of-bound indices gracefully by clamping them to valid list indices.

out_of_bounds.tcl

set short_list {alpha beta gamma}
set result [lrange $short_list -5 10]
puts "Result: $result"

This shows how lrange deals with indices beyond list boundaries. The result will be the entire list, as negative indices are clamped to 0 and large indices to the last element.

Combining with Other List Commands

lrange can be combined with other list commands for more complex operations.

combined_commands.tcl

set data {10 20 30 40 50 60 70 80 90}
set middle [lrange $data 3 6]
set reversed [lreverse $middle]
puts "Middle reversed: $reversed"

This extracts elements 3 through 6, then reverses them using lreverse. The result is 70 60 50 40, demonstrating how lrange can be part of more complex list processing pipelines.

Best Practices

Indexing: Remember Tcl uses zero-based indexing.
Performance: For single elements, consider lindex.
Bounds: No error is thrown for out-of-bound indices.
Copying: Use lrange $list 0 end to copy lists.
Readability: Use end for clarity with dynamic lists.

This tutorial covered the Tcl lrange 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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