Tcl foreach Command

last modified April 3, 2025

The Tcl foreach command is used to iterate over lists and arrays. It's a powerful looping construct that processes each element in a sequence. The command can handle multiple lists simultaneously.

Basic Definition

The foreach command executes a script for each element in one or more lists. It assigns each element to a variable and runs the script body.

Syntax: foreach varList list ?varList list ...? script. It takes variable names, lists to iterate over, and a script to execute for each element.

Basic List Iteration

This example shows the simplest usage of foreach to iterate over a single list.

basic_foreach.tcl

set colors {red green blue yellow}
foreach color $colors {
    puts "Color: $color"
}

This creates a list of colors and iterates over each element. The current color is assigned to the color variable in each iteration.

Multiple List Variables

foreach can iterate over multiple lists simultaneously by using multiple variable names.

foreach_multiple.tcl

set names {Alice Bob Charlie}
set ages {25 30 35}
foreach name $names age $ages {
    puts "$name is $age years old"
}

This pairs elements from two lists together. The command takes elements from both lists in parallel during each iteration.

Nested Lists

foreach can handle nested lists by using multiple loop variables.

foreach_nested.tcl

set points {{1 2} {3 4} {5 6}}
foreach point $points {
    lassign $point x y
    puts "Point at ($x, $y)"
}

This demonstrates processing nested lists. Each point is a sublist that gets split into x and y coordinates using lassign.

Multiple Variables per List

You can specify multiple variables to consume multiple elements from a list in each iteration.

foreach_multivars.tcl

set numbers {1 2 3 4 5 6 7 8}
foreach {a b} $numbers {
    puts "Pair: $a and $b"
}

This processes the list two elements at a time. The loop takes two elements per iteration until the list is exhausted.

Iterating Over Arrays

To iterate over arrays, we first need to get the array keys using array names.

foreach_array.tcl

array set user {
    name "John Doe"
    age 30
    city "New York"
}
foreach key [array names user] {
    puts "$key: $user($key)"
}

This shows how to iterate over an array's keys and values. We first get all keys then access each value using array syntax.

Break and Continue

foreach supports break and continue commands for loop control.

foreach_control.tcl

set nums {1 2 3 4 5 6 7 8 9 10}
foreach num $nums {
    if {$num == 5} {continue}
    if {$num == 8} {break}
    puts "Number: $num"
}

This demonstrates loop control. It skips number 5 with continue and exits the loop at number 8 with break.

Best Practices

Variable names: Use meaningful names for iteration variables.
Large lists: Consider performance with very large lists.
List modification: Avoid modifying the list during iteration.
Error handling: Check list validity before iteration.
Scope: Be aware of variable scope in the loop body.

This tutorial covered the Tcl foreach 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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