Tcl lreplace Command

last modified April 3, 2025

The Tcl lreplace command replaces elements in a list with new elements. It returns a new list with the specified replacements while leaving the original list unchanged. This is useful for list manipulation tasks.

Basic Definition

The lreplace command replaces zero or more elements of a list with new elements. It takes a list and index arguments specifying what to replace.

Syntax: lreplace list first last ?element element …?. The command returns a new list with elements from first to last replaced by the new elements.

Basic List Replacement

This example demonstrates the simplest usage of lreplace to replace a single element in a list.

basic_lreplace.tcl

set colors {red green blue yellow} set new_colors [lreplace $colors 1 1 orange] puts $new_colors

This replaces the element at index 1 (“green”) with “orange”. The original list remains unchanged. The new list contains: red orange blue yellow.

Replacing Multiple Elements

lreplace can replace a range of elements with multiple new elements.

multi_replace.tcl

set numbers {1 2 3 4 5 6} set new_numbers [lreplace $numbers 2 4 7 8 9] puts $new_numbers

This replaces elements from index 2 to 4 (3,4,5) with three new elements (7,8,9). The resulting list is: 1 2 7 8 9 6. The replacement count doesn’t need to match.

Removing Elements

By not providing replacement elements, lreplace can remove elements from a list.

remove_elements.tcl

set fruits {apple banana cherry date elderberry} set trimmed [lreplace $fruits 1 3] puts $trimmed

This removes elements from index 1 to 3 (banana, cherry, date) without adding new elements. The result is a shorter list: apple elderberry.

Inserting Elements

When first and last are the same index, lreplace inserts elements without replacing any existing ones.

insert_elements.tcl

set letters {a b c d} set extended [lreplace $letters 2 2 x y z] puts $extended

This inserts elements x, y, z after index 2 without removing any elements. The result is: a b x y z c d. The original element at index 2 is shifted right.

Working with Nested Lists

lreplace can handle nested lists, replacing entire sublists when needed.

nested_list.tcl

set matrix {{1 2 3} {4 5 6} {7 8 9}} set new_matrix [lreplace $matrix 1 1 {10 11 12}] puts $new_matrix

This replaces the second sublist {4 5 6} with {10 11 12}. The result is a new matrix: {1 2 3} {10 11 12} {7 8 9}. The command works the same with nested lists.

End of List Operations

lreplace can operate on the end of a list using the special index “end”.

end_operations.tcl

set items {pen pencil eraser ruler} set updated [lreplace $items end end marker] puts $updated

This replaces the last element “ruler” with “marker”. The “end” index refers to the last element. The result is: pen pencil eraser marker.

Best Practices

• Immutability: Remember lreplace returns a new list without modifying the original.

• Indexing: Tcl uses 0-based indexing for list operations.

• Performance: For large lists, consider efficiency when replacing many elements.

• Error Handling: Check indices are valid to avoid errors.

• Combination: Combine with other list commands for complex operations.

This tutorial covered the Tcl lreplace command with practical examples showing its usage in different scenarios for list manipulation.

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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