How to create loops using Tcl

Introduction

A tcl for loops takes in four arguments in the following order: initialisation, loop ending condition, next loop action, and main body code.

for loops

The following lines of code print integers from 0 until 4

for {set i 0} {$i < 5} {incr i} {
    puts "Counter: $i"
}

Output:

Counter: 0
Counter: 1
Counter: 2
Counter: 3
Counter: 4

foreach loops

foreach loops are commonly used to deal with tcl lists. There is no need to initialize a counter variable or setting the condition to terminate a loop.

set Fruits [ list banana apple orange watermelon honeydew]

foreach fruit $Fruits {
    puts "Fruit: $fruit"
}

Output:

Fruit: banana
Fruit: apple
Fruit: orange
Fruit: watermelon
Fruit: honeydew

foreach loops can also be used to handle multiple list objects:

set Fruits [ list banana apple orange watermelon honeydew]
set Colours [ list yellow red orange red green]

foreach fruit $Fruits colour $Colours {
    puts "Fruit: $fruit; colour: $colour"
}

Output:

Fruit: banana; colour: yellow
Fruit: apple; colour: red
Fruit: orange; colour: orange
Fruit: watermelon; colour: red
Fruit: honeydew; colour: green

while loops

Tcl while loops takes in only two arguments, loop ending condition and the main body code. 

while {condition} {body}

set max_print 3
set counter 0
while { $counter < $max_print } {
    puts "Counter: $counter"
    set counter [expr $counter + 1]
}

 

Output:

Counter: 0
Counter: 1
Counter: 2

 

Exiting or skipping loops

The continue statement can be used to skip a loop if a certain condition met.

Taking below code as example, The continue statement is used to skip printing “orange” .

set Fruits [ list banana apple orange watermelon honeydew]
foreach fruit $Fruits {
    if {$fruit == "orange"} {
        continue
    } else {
        puts "Fruit: $fruit"
    }
}

Output:

Fruit: banana
Fruit: apple
Fruit: watermelon
Fruit: honeydew

Use break to exit a loop completely

set counter 0
while { $counter < 5 } {
    puts "Counter: $counter"
    set counter [expr $counter + 1]
    if { $counter == 3} { break }
}

Output:

Counter: 0
Counter: 1
Counter: 2