Tcl lsort Command

last modified April 3, 2025

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 in ASCII order. 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. 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: For large lists, consider most efficient options.

• Stability: Tcl’s sort is stable (equal elements keep order).

• Indexing: Use -index for sorting lists of lists.

• Memory: lsort creates new lists; watch memory usage.

This tutorial covered the Tcl lsort 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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