Tcl for Command

last modified April 3, 2025

The Tcl for command is used to create loops that execute a block of code repeatedly. It's one of the primary looping constructs in Tcl. The command provides initialization, condition, and increment sections.

Basic Definition

The for command executes a script repeatedly while a condition is true. It combines initialization, condition checking, and iteration in one construct.

Syntax: for start test next body. The start script initializes variables. The test expression determines when to stop. The next script updates variables after each iteration.

Basic for Loop

This example shows the simplest usage of for to count from 1 to 5.

basic_for.tcl

for {set i 1} {$i <= 5} {incr i} {
    puts "Iteration $i"
}

This creates a loop that initializes i to 1, continues while i is less than or equal to 5, and increments i after each iteration. The loop body prints the current iteration number.

Counting Down

The for loop can count downward by using a negative increment.

count_down.tcl

for {set i 5} {$i >= 1} {incr i -1} {
    puts "Countdown: $i"
}

This example counts down from 5 to 1. The incr command with -1 as the second argument decrements the variable. The condition checks when i is greater than or equal to 1.

Iterating Over a List

A for loop can iterate over a list by combining it with llength and lindex.

list_iteration.tcl

set colors {red green blue yellow}
for {set i 0} {$i < [llength $colors]} {incr i} {
    puts "Color [expr {$i + 1}]: [lindex $colors $i]"
}

This demonstrates how to process each element in a list. The loop uses the list length as the termination condition. lindex retrieves each element by its index position.

Nested for Loops

for loops can be nested to handle multi-dimensional iterations.

nested_for.tcl

for {set i 1} {$i <= 3} {incr i} {
    for {set j 1} {$j <= 3} {incr j} {
        puts "i=$i, j=$j"
    }
}

This shows two nested loops creating a 3x3 grid of combinations. The outer loop runs three times, and for each iteration, the inner loop also runs three times. This produces nine total iterations.

Loop With Multiple Variables

The initialization and increment sections can handle multiple variables.

multi_var.tcl

for {set i 1; set j 10} {$i <= 5} {incr i; incr j -2} {
    puts "i=$i, j=$j, sum=[expr {$i + $j}]"
}

This example manages two variables in the loop. i counts up while j counts down. The initialization and increment sections use semicolons to separate multiple commands. The loop body calculates their sum.

Infinite Loop With Break

A for loop can run indefinitely until a break condition.

infinite_loop.tcl

set count 0
for {} {1} {incr count} {
    puts "Count: $count"
    if {$count >= 5} {
        break
    }
}

This creates an infinite loop by using 1 (always true) as the condition. The loop breaks when count reaches 5. This pattern is useful when the termination condition is complex or determined within the loop body.

Best Practices

Initialization: Keep initialization code in the start section.
Braces: Use braces {} for expressions to prevent substitution.
Complex Logic: Consider while for complex termination logic.
Performance: Minimize work in the test condition.
Readability: Keep loop bodies short for clarity.

This tutorial covered the Tcl for 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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