Tcl lsort Command

last modified October 20, 2026

The Tcl lsort command sorts the elements of a list. It provides various options to control the sorting behavior. The command returns a new sorted list without modifying the original.

Basic Definition

The lsort command sorts list elements according to specified options. By default, it sorts elements as strings, comparing character codes (-ascii). The original list remains unchanged.

Syntax: lsort ?options? list. Options control sorting order, comparison method, and other behaviors. Multiple options can be combined.

Basic List Sorting

This example demonstrates the simplest usage of lsort to sort a list.

basic_lsort.tcl

set colors {red green blue yellow orange}
set sorted [lsort $colors]
puts $sorted

This sorts the list of colors alphabetically. The original list remains unchanged. The sorted result is stored in the sorted variable.

Numerical Sorting

The -integer option sorts elements as integers rather than strings.

lsort_numeric.tcl

set numbers {5 12 3 25 1 8}
set sorted [lsort -integer $numbers]
puts $sorted

Without -integer, numbers would sort as strings (1, 12, 25, 3...). The option ensures proper numerical ordering. The result is 1, 3, 5, 8, 12, 25.

Reverse Order Sorting

The -decreasing option sorts elements in descending order.

lsort_reverse.tcl

set fruits {apple banana cherry date elderberry}
set sorted [lsort -decreasing $fruits]
puts $sorted

This sorts the fruits list in reverse alphabetical order. The result starts with the highest value (elderberry) to lowest (apple). Combine with other options for more control.

Case-Insensitive Sorting

The -nocase option performs case-insensitive sorting of strings.

lsort_nocase.tcl

set mixed {Apple banana Cherry Date elderberry}
set sorted [lsort -nocase $mixed]
puts $sorted

Without -nocase, uppercase letters sort before lowercase. The option makes sorting ignore case differences. Result is Apple, banana, Cherry, Date, elderberry.

Unique Elements Sorting

The -unique option removes duplicate elements from the sorted list.

lsort_unique.tcl

set duplicates {a b c a d b e c f}
set sorted [lsort -unique $duplicates]
puts $sorted

This sorts the list and removes duplicate values. The result contains each element only once. With a comparison option such as -nocase or -integer, elements that compare as equal count as duplicates, and only the last of them is kept. Useful for creating distinct value sets from input data.

Custom Comparison Sorting

The -command option allows specifying a custom comparison script.

lsort_custom.tcl

proc compare_length {a b} {
    return [expr {[string length $a] - [string length $b]}]
}

set words {programming language Tcl is fun to learn}
set sorted [lsort -command compare_length $words]
puts $sorted

This sorts words by their length using a custom comparison procedure. The proc returns negative, zero, or positive to indicate ordering. Result is shortest to longest words.

Best Practices

Options: Combine options like -integer -decreasing.
Performance: The built-in comparisons (-integer, -real, -ascii) are faster than -command, which calls a script for every comparison.
Stability: Tcl's sort is stable (equal elements keep order).
Indexing: Use -index for sorting lists of lists.
Order: -dictionary gives a human-friendly order (file2 before file10); lsort returns a new list, so assign it back to sort a variable in place.

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

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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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