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.
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.
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.
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.
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.
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".
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.
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