Tcl dict Command

last modified April 3, 2025

The Tcl dict command provides operations for working with dictionary values. Dictionaries are collections of key-value pairs, similar to arrays.

Basic Definition

A dictionary is a Tcl value containing an even number of elements. Each pair consists of a key followed by its associated value. Keys must be unique.

The dict command has multiple subcommands for dictionary manipulation. These include create, get, set, update, merge, and more.

Creating a Dictionary

This example shows how to create a basic dictionary with key-value pairs.

dict_create.tcl

set user [dict create name "John Doe" age 35 email "john@example.com"]
puts $user

Here we create a dictionary with three key-value pairs. The dict create command builds the dictionary structure. The entire dictionary is stored in the user variable.

Accessing Dictionary Values

Dictionary values can be retrieved using the dict get command.

dict_get.tcl

set user [dict create name "John Doe" age 35 email "john@example.com"]
set name [dict get $user name]
puts "User name: $name"

This retrieves the value associated with the "name" key. The dict get command takes the dictionary variable and the key name as arguments.

Updating a Dictionary

Dictionary values can be modified using dict set or dict update.

dict_update.tcl

set user [dict create name "John Doe" age 35]
dict set user age 36
dict set user email "john@example.com"
puts $user

This updates the age and adds a new email field. The dict set command modifies the dictionary in place. The first argument is the dictionary variable.

Dictionary Iteration

The dict for command allows iteration over all key-value pairs.

dict_iterate.tcl

set user [dict create name "John Doe" age 35 email "john@example.com"]
dict for {key value} $user {
    puts "$key: $value"
}

This loops through each key-value pair in the dictionary. The loop variables key and value receive each pair's values in turn.

Checking Dictionary Contents

The dict exists command checks if a key exists in a dictionary.

dict_exists.tcl

set user [dict create name "John Doe" age 35]
if {[dict exists $user email]} {
    puts "Email exists"
} else {
    puts "Email does not exist"
}

This checks for the existence of the "email" key. The command returns 1 if the key exists, 0 otherwise. It's useful for defensive programming.

Merging Dictionaries

Dictionaries can be combined using the dict merge command.

dict_merge.tcl

set user1 [dict create name "John Doe" age 35]
set user2 [dict create email "john@example.com" city "New York"]
set merged [dict merge $user1 $user2]
puts $merged

This combines two dictionaries into one. If keys overlap, values from the later dictionary take precedence. The original dictionaries remain unchanged.

Best Practices

Key names: Use consistent naming conventions for keys.
Nesting: Dictionaries can be nested for complex data.
Performance: Dictionaries are optimized for lookup.
Immutable: Remember that dict commands return new values.
Validation: Check key existence before access.

This tutorial covered the Tcl dict 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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