Tcl lindex Command

last modified April 3, 2025

The Tcl lindex command retrieves an element from a list by its index. It's a fundamental command for working with lists in Tcl. The command supports both simple and nested list indexing.

Basic Definition

The lindex command returns the element at the specified index from a list. Tcl lists are zero-indexed, meaning the first element is at index 0.

Syntax: lindex list ?index...?. With one index, it returns that element. Multiple indices can be used for nested lists.

Basic List Indexing

This example shows the simplest usage of lindex to access list elements.

basic_lindex.tcl

set colors {red green blue yellow}
set first [lindex $colors 0]
puts "The first color is $first"

This creates a list of colors and retrieves the first element (index 0) using lindex. The result is stored in first and printed.

Negative Indexing

lindex supports negative indices which count from the end of the list.

negative_index.tcl

set fruits {apple banana cherry date}
set last [lindex $fruits -1]
puts "The last fruit is $last"

Here we use index -1 to get the last element of the list. Negative indices are convenient for accessing elements from the end without knowing the list length.

Nested List Indexing

For nested lists, lindex can take multiple indices to drill down into sublists.

nested_lindex.tcl

set matrix {{1 2 3} {4 5 6} {7 8 9}}
set element [lindex $matrix 1 2]
puts "The element at [1,2] is $element"

This demonstrates accessing an element in a nested list (matrix). The indices 1 and 2 specify row 1 (second row) and column 2 (third element) respectively.

Out-of-Bounds Indexing

When an index is out of bounds, lindex returns an empty string without raising an error.

out_of_bounds.tcl

set nums {10 20 30 40}
set value [lindex $nums 10]
if {$value eq ""} {
    puts "Index out of bounds"
}

This shows how lindex handles invalid indices. The code checks for an empty string result when the index exceeds the list bounds.

Multiple Indices with Variables

Indices can be specified using variables, allowing dynamic list access.

variable_indices.tcl

set data {{a b c} {d e f} {g h i}}
set row 1
set col 0
set item [lindex $data $row $col]
puts "Item at row $row, col $col is $item"

This example uses variables for indices, making the access pattern dynamic. The variables row and col determine which element to retrieve.

End-relative Indexing

Tcl supports end-relative indexing with the end keyword, which can be combined with offsets.

end_relative.tcl

set letters {a b c d e f g}
set third_last [lindex $letters end-2]
puts "The third last letter is $third_last"

This retrieves the third last element using end-2 syntax. The end keyword represents the last element's index, and subtraction works backward from there.

Best Practices

Bounds Checking: Always validate indices when uncertain.
Negative Indices: Use them for convenient end access.
Nested Lists: Document list structure for clarity.
Error Handling: Check for empty string returns.
Performance: Avoid repeated lindex in loops; use foreach.

This tutorial covered the Tcl lindex 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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