Tcl lappend Command

last modified April 3, 2025

The Tcl lappend command appends elements to a list variable. It modifies the variable in place by adding elements to the end of the list.

Basic Definition

The lappend command takes a variable name and one or more values to append. If the variable doesn’t exist, it creates a new list containing the given elements.

Syntax: lappend varName ?value value …?. The command returns the new value of the list variable after appending.

Basic List Append

This example demonstrates the simplest usage of lappend to add elements to a list.

basic_lappend.tcl

set colors [list red green blue] lappend colors yellow puts $colors

We first create a list of colors, then append “yellow” to it. The puts command displays the modified list containing all four colors.

Appending Multiple Elements

lappend can append multiple elements to a list in a single command.

lappend_multiple.tcl

set numbers {1 2 3} lappend numbers 4 5 6 puts $numbers

This starts with a list of three numbers. The lappend command adds three more numbers at once. The result is a list containing numbers 1 through 6.

Creating New Lists

When used with a non-existent variable, lappend creates a new list.

lappend_new.tcl

fruits variable doesn’t exist yet

lappend fruits apple orange banana puts $fruits

Since fruits doesn’t exist initially, lappend creates it as a new list containing the three specified fruits. This is a convenient way to initialize lists.

Appending Nested Lists

lappend can append complex elements including other lists.

lappend_nested.tcl

set data {} lappend data {1 2 3} lappend data {a b c} puts $data

We start with an empty list and append two sublists. The result is a list of lists. Each appended element becomes a single item in the main list.

Building Lists in Loops

lappend is commonly used to build lists incrementally in loops.

lappend_loop.tcl

set squares {} for {set i 1} {$i <= 5} {incr i} { lappend squares [expr {$i * $i}] } puts $squares

This example builds a list of square numbers from 1 to 5. Each iteration calculates a square and appends it to the list. The result is [1 4 9 16 25].

Combining with Other List Commands

lappend can be combined with other list commands for more complex operations.

lappend_combine.tcl

set items {apple banana} set more_items {cherry date} lappend items {*}$more_items puts $items

Here we merge two lists using the {*} expansion operator. This appends each element of more_items individually to items. The result is a single combined list.

Best Practices

• Initialization: Initialize lists before appending when needed.

• Performance: Use lappend for building lists, it’s efficient.

• Expansion: Use {*} to append all elements of another list.

• Readability: Consider line breaks for multiple appends.

• Variables: Ensure variable exists or handle errors.

This tutorial covered the Tcl lappend 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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