Mastering Tcl: A Beginner's Guide to Tool Command Language part 3

Control Structures in Tcl

Tcl (Tool Command Language) provides various control structures for handling the flow of execution in your scripts. In this post, we'll cover the if_else statement, procedures, and file operations.

If-Else Statement

Example 1

This example demonstrates a simple if_else statement to check if a number is less than 10.

 set number 5
if {$number < 10} {
    puts "Number is less than 10"
} else {
    puts "Number is 10 or greater"
}
    

Output:

Number is less than 10

Example 2

This example uses multiple conditions to check if a number is even or odd and whether it's greater or less than 50.

 set number 40
if {($number % 2 == 0) && ($number > 50)} {
    puts "The number is even and greater than 50"
} elseif {($number % 2 == 0) && ($number < 50)} {
    puts "The number is even and less than 50"
} else {
    puts "The number is odd"
}

    

Output:

The number is even and less than 50

Procedures

Procedures in Tcl act like functions in other programming languages. They help you organize your code into reusable blocks.

Defining and Calling a Procedure

Procedure Syntax:

 proc name_of_the_function {arguments} {
    # Body of the procedure
    puts "$arguments"
}

# Or using return
proc name_of_the_function {arguments} {
    return $arguments
}

    

Example 1

A simple procedure to greet a person by name.

tcl

proc greet {name} {
    puts "Hello, $name!"
}
greet "Alice"

Output:

Hello, Alice!

Example 2

A procedure to add two numbers. Note that passing the arguments correctly is crucial.

tcl

proc plus {a b} {
    return [expr {$a + $b}]
}
puts [plus 3 2]

Output:

5

File Operations

Tcl provides various modes for file operations, each with specific functionality.

Symbols for File Operations

• r: Read operation (file must exist).
• r+: Read and write operation (file must exist).
• w: Write operation (creates a file if it doesn't exist).
• w+: Read and write operation (creates a file if it doesn't exist).
• a: Write operation (file must exist).
• a+: Read and write operation (creates a file if it doesn't exist).

 File Operation Flow 

You need to create a file with specific contents. Start by opening the required file and determining the operation you are going to perform. Once the operation is complete, close the file. 

File operations flowchart

Reading the Entire File at Once

tcl

set fp [open "file.txt" r]
set content [read $fp]
puts $content
close $fp

Reading File Line by Line

tcl

set file [open "example.txt" r]
while {[gets $file data] >= 0} {
    puts $data
}
close $file

Writing File Contents

tcl

set file [open "example.txt" w]
puts $file "This is a test."
close $file

Test for Understanding

Question 1

What would be the output of the following code?

tcl

set A 99
if {[llength $A] != 0} {
    set B $A
}
set A 100
puts $B

Answer:

99

Explanation: The variable B is set to A when A is 99. Changing A afterward does not affect B.

Question 2

Create emails for a list of five employees using a procedure.

Solution:

 proc Email {employees_name} {
    for {set i 0} {$i < [llength $employees_name]} {incr i} {
        set name [lindex $employees_name $i]
        puts "${name}@chipworld.com"
    }
}
Email [list atef mohamed hany dalia Khadija]

    

Output:

atef@chipworld.com
mohamed@chipworld.com
hany@chipworld.com
dalia@chipworld.com
Khadija@chipworld.com

Solution 2 with foreach Loop:

 proc Email {employees_name} {
    foreach name $employees_name {
        puts "${name}@chipworld.com"
    }
}
set employees [list atef mohamed hany dalia Khadija]
Email $employees

    

Output:

atef@chipworld.com
mohamed@chipworld.com
hany@chipworld.com
dalia@chipworld.com
Khadija@chipworld.com

Solution 3: Create List of Emails Instead of Line by Line:

 proc Email {employees_name} {
    set emails_list {}
    foreach name $employees_name {
        set emails [list ${name}@chipworld.com]
        lappend emails_list $emails
    }
    puts "$emails_list"
}
set employees [list Atef Mohamed hany dalia Khadija]
Email $employees

    

Output:

 Atef@chipworld.com Mohamed@chipworld.com hany@chipworld.com dalia@chipworld.com 
Khadija@chipworld.com