Tcl switch Command

last modified April 3, 2025

The Tcl switch command provides multi-way branching based on pattern matching. It's more efficient than multiple if-else statements when testing a single value against multiple possibilities.

Basic Definition

The switch command evaluates an expression and matches it against patterns to execute corresponding scripts. It supports exact, glob-style, and regexp pattern matching.

Syntax: switch ?options? string pattern body ?pattern body ...?. The command compares string against each pattern and executes the first matching body.

Basic Exact Matching

This example demonstrates simple exact matching with the switch command.

basic_switch.tcl

set day "Tuesday"

switch $day {
    "Monday"    { puts "Start of work week" }
    "Tuesday"   { puts "Second day" }
    "Wednesday" { puts "Midweek" }
    "Thursday"  { puts "Almost Friday" }
    "Friday"    { puts "Weekend coming" }
    default     { puts "Weekend day" }
}

The script matches the exact value of day against the patterns. When it finds "Tuesday", it executes the corresponding body. The default case handles any unmatched values.

Glob-Style Pattern Matching

This example shows how to use glob-style patterns with the -glob option.

switch_glob.tcl

set filename "report2023.pdf"

switch -glob $filename {
    "*.txt"   { puts "Text file" }
    "*.pdf"   { puts "PDF document" }
    "*.jpg"   { puts "JPEG image" }
    "report*" { puts "Report file" }
    default   { puts "Unknown file type" }
}

The -glob option enables glob-style pattern matching. The script matches both the ".pdf" extension and the "report" prefix patterns. Only the first matching pattern's body is executed.

Regular Expression Matching

This example demonstrates regex pattern matching using the -regexp option.

switch_regex.tcl

set input "user123"

switch -regexp $input {
    "^[a-z]+$"       { puts "All letters" }
    "^[0-9]+$"       { puts "All digits" }
    "^[a-z]+[0-9]+$" { puts "Letters followed by digits" }
    "^[0-9]+[a-z]+$" { puts "Digits followed by letters" }
    default          { puts "No match" }
}

The -regexp option enables regular expression matching. The script matches the pattern "letters followed by digits" and executes the corresponding body. Regex patterns provide powerful matching capabilities.

Fall-Through Behavior

This example shows how to use the - marker for fall-through behavior.

switch_fallthrough.tcl

set number 3

switch $number {
    1       { puts "One" }
    2       -
    3       { puts "Two or Three" }
    4       { puts "Four" }
    default { puts "Other number" }
}

The - marker allows multiple patterns to execute the same body. Here both 2 and 3 execute the "Two or Three" body. This is useful for grouping similar cases together.

Using Braces for Patterns

This example demonstrates using braces for patterns to prevent substitution.

switch_braces.tcl

set value "red"

switch $value {
    red     { set color_code "#FF0000" }
    green   { set color_code "#00FF00" }
    blue    { set color_code "#0000FF" }
    default { set color_code "#000000" }
}

puts "Color code for $value is $color_code"

The patterns are enclosed in braces to prevent any substitution or interpretation. This is the most common way to write switch commands in Tcl. The braces ensure the patterns are treated literally.

Nested Switch Commands

This example shows how to nest switch commands for complex logic.

switch_nested.tcl

set category "fruit"
set item "apple"

switch $category {
    "fruit" {
        switch $item {
            "apple"  { puts "It's an apple" }
            "banana" { puts "It's a banana" }
            default  { puts "Unknown fruit" }
        }
    }
    "vegetable" {
        switch $item {
            "carrot" { puts "It's a carrot" }
            "potato" { puts "It's a potato" }
            default  { puts "Unknown vegetable" }
        }
    }
    default { puts "Unknown category" }
}

Nested switch commands allow for hierarchical decision making. The outer switch handles the category, while inner switches handle specific items within each category. This structure keeps complex logic organized.

Best Practices

Default case: Always include a default case for unmatched values.
Pattern order: Place more specific patterns before general ones.
Braces: Use braces for patterns to prevent substitution.
Comments: Add comments for complex patterns or logic.
Performance: For many cases, switch is faster than if-else.

This tutorial covered the Tcl switch 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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