Tcl set Command

last modified October 20, 2026

The Tcl set command is used to assign values to variables. It's one of the most fundamental commands in Tcl. The command can both set and retrieve variable values.

Basic Definition

The set command creates a variable if it doesn't exist and assigns a value to it. It can also return the value of an existing variable.

Syntax: set varName ?value?. With one argument, it returns the variable's value. With two arguments, it sets the variable's value.

Basic Variable Assignment

This shows the simplest usage of set to assign a value to a variable.

basic_set.tcl

set name "John Doe"
puts $name

This creates a variable name with the value "John Doe". The puts command then prints the variable's value to standard output.

Retrieving Variable Values

The set command can retrieve a variable's value when given only one argument.

set_retrieve.tcl

set age 25
set retrieved_age [set age]
puts "Age is $retrieved_age"

Here we first set the age variable, then retrieve its value using set with one argument. This demonstrates the dual nature of the set command. Reading a variable that does not exist raises the error "can't read "x": no such variable".

Variable Substitution

Tcl performs variable substitution when a variable name is prefixed with $.

set_substitution.tcl

set x 10
set y 20
set sum [expr {$x + $y}]
puts "The sum is $sum"

This example shows how variable substitution works in expressions. The expr command evaluates the mathematical expression using the substituted values; because the expression is in braces, expr itself does the substitution, which is faster and safer than letting the Tcl parser do it.

Setting Multiple Variables

set assigns one variable per command, so several variables are set with separate set commands (or with lassign for a list of values).

set_multiple.tcl

set width 10
set height 5
set area [expr {$width * $height}]
puts "Rectangle area: $area"

This calculates the area of a rectangle by first setting width and height variables. The area is then computed and stored in another variable.

Nested Set Commands

set commands can be nested to create more complex assignments.

set_nested.tcl

set a [set b [set c 100]]
puts "a = $a, b = $b, c = $c"

This demonstrates nested set commands where multiple variables receive the same value. The innermost set is evaluated first.

Dynamic Variable Creation

The set command can create variables dynamically at runtime.

set_dynamic.tcl

for {set i 1} {$i <= 3} {incr i} {
    set "var$i" "Value $i"
}
puts "$var1 $var2 $var3"

This example creates three variables dynamically in a loop. Each variable name is constructed using the loop counter, showing runtime variable creation. Reading such variables back is easiest when the names are known, as here; for a family of related values an array (set var($i) ...) is usually the better design.

Best Practices

Naming: Use descriptive variable names for clarity.
Scope: Be aware of variable scope (global vs local).
Braces: Put expr expressions in braces, so that expr substitutes the variables itself; this avoids double substitution and allows byte-compilation.
Unset: Use unset to remove variables when done.
Arrays: Consider arrays for related data sets.

This tutorial covered the Tcl set 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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