Tcl while Command

last modified April 3, 2025

The Tcl while command creates a loop that executes as long as a condition remains true. It's a fundamental control structure in Tcl for repetitive tasks. The loop continues until the condition evaluates to false.

Basic Definition

The while command evaluates an expression and executes a body of code repeatedly while the expression is true. The syntax is: while test body.

The test is a boolean expression. The body contains Tcl commands to execute. The body must be enclosed in braces or quotes.

Simple Counter Loop

This example demonstrates a basic while loop that counts from 1 to 5.

basic_while.tcl

set count 1
while {$count <= 5} {
    puts "Count is $count"
    incr count
}

The loop starts with count=1 and continues while count ≤ 5. The incr command increments the counter. Each iteration prints the current count value.

Summing Numbers

This example calculates the sum of numbers from 1 to 10 using a while loop.

while_sum.tcl

set sum 0
set i 1
while {$i <= 10} {
    set sum [expr {$sum + $i}]
    incr i
}
puts "The sum is $sum"

We initialize sum to 0 and i to 1. The loop adds each value of i to sum while i ≤ 10. After the loop, we print the total sum (which should be 55).

Reading Until Condition

This example reads input from the user until they enter "quit".

while_input.tcl

set input ""
while {$input ne "quit"} {
    puts -nonewline "Enter text (quit to exit): "
    flush stdout
    gets stdin input
    puts "You entered: $input"
}

The loop continues as long as input is not equal to "quit". The gets command reads user input. flush stdout ensures the prompt appears immediately.

Infinite Loop with Break

This shows how to create an infinite loop that can be exited with a break.

while_infinite.tcl

set count 0
while {1} {
    incr count
    puts "Iteration $count"
    if {$count >= 5} {
        break
    }
}

The condition {1} is always true, creating an infinite loop. We use break to exit when count reaches 5. This pattern is useful when the exit condition is complex or occurs mid-loop.

Nested While Loops

This example demonstrates nested while loops to create a multiplication table.

while_nested.tcl

set i 1
while {$i <= 5} {
    set j 1
    while {$j <= 5} {
        set product [expr {$i * $j}]
        puts -nonewline [format "%4d" $product]
        incr j
    }
    puts ""
    incr i
}

The outer loop controls rows (i), while the inner loop controls columns (j). We calculate each product and format the output neatly. The result is a 5x5 multiplication table.

Loop Control with Continue

This example shows how to skip iterations using the continue command.

while_continue.tcl

set num 0
while {$num < 10} {
    incr num
    if {$num % 2 == 0} {
        continue
    }
    puts "$num is odd"
}

The loop prints only odd numbers between 1 and 10. When num is even, the continue command skips the rest of the iteration. This shows how to selectively process items in a loop.

Best Practices

Braces: Always use braces for the condition and body.
Variables: Initialize loop variables before the while.
Termination: Ensure the loop will eventually terminate.
Complex Conditions: Use expr for complex conditions.
Performance: Minimize work inside the loop when possible.

This tutorial covered the Tcl while 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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