Summary  edit

The following basic programs written in TCL will give you a general introduction to the TCL syntax:

Comments on Tcl Syntax  edit

aa - Tcl syntax isn't anything like C syntax. It's unlike the syntax of most other popular languages. It's supremely simple and straightforward. These basic programs serve mostly to show the absence of the kind of incantations that are required when programming in a language like C.

aa I think the Endekalogue provides a more useful introduction to Tcl syntax, though it's likely that only someone who is already familiar with Tcl's quoting and substitution rules -- and who isn't burdened by trying to see it in a C-like context -- will understand it easily. emj Exactly, which is why we need examples as well.

Program 1: Hello world  edit

In this example, a file called hello.tcl contains the following text:

#! /bin/env tclsh
puts "Hello world!"
exit 0

To run this script:

tclsh hello.tcl

In the above example, it may be necessary to replace "tclsh" with "tclsh.exe", "tclkit", "tclkit.exe" or whatever your Tcl interpreter is called.

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Program 2: Using variables  edit

Variables hold values (What sort of values? Any!) You read from a variable by putting a $ in front of its name, and you write to it using the set command. Variable names can be made up of almost any characters, but it is usually easiest to use just letters any numbers. The following is an example of various valid variable names:

set age 42
puts $age

set Age 42
puts $Age

set AGE 42
puts $AGE

set "my age" 42
puts ${my age}

set "My AGE" 42
puts ${My AGE}
puts [set "My AGE"]

Variable names with whitespace characters in them need to have curly brackets around them for Tcl to understand them, or else Tcl would assume the variable name ends at the space or any other non-alphanumeric, non-underscore character.

Alternately, use the [set] command with only one argument to get the value of a variable. This is useful if the variable name is computed or contains special characters, especially close brace, that are difficult or impossible to use with normal $dollar sign notation. Quotes are used to force "My AGE" to be interpreted as a single argument even though it contains a space. Braces can also be used for this purpose, or you can put a backslash in front of each space to disable its word separation ability.

See Also

• what kinds of variable names can be used in Tcl?:
• what kinds of variables can Tcl scripts use?:

Program 3: Reading user input  edit

FW: stdin stands for Standard Input, and is where you can read input from with gets (get string). All programming languages support this sort of thing, including C. It means the channel to receive input from (there's also a stdout, and an stderr, which are places to write output and error messages to; puts writes to stdout by default). This is usually commands typed in the console, but a program can be for example invoked by another program, which sends it data.

puts "Please tell me your name."
gets stdin Name
puts "Hello, $Name!"

Program 4: Conditionals  edit

Ask a person his/her age. If it is under 20, reply: "you are a child or a teenager". If it is 20 or over, we reply: "you are an adult now".'''

puts "Hey dude, how old might you be?"
gets stdin Age
if {$Age < 18} {
    puts "You are a child or a teen-ager"
} else { 
    puts "You are an adult now" 
}

If the user entered something that wasn't a number, there would be an error, but that's a topic for another time.

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In other languages, if and else might be considered "statements", but in Tcl, they are just commands. Tcl doesn't have a concept called "statement". Tcl features like the uplevel command makes it possible to design constructs that in other language, require additional concepts like "statement".

A more concise example of the same program:

puts "Hey dude, how old might you be?"
gets stdin Age
puts "You are [expr {$Age<20? {a child or a teen-ager}: {an adult now}}]"

Program 5: Adding Complexity  edit

In the following example, there are three possible conditions, rather than two:

puts "Hey dude, how old might you be?"
gets stdin Age
if {$Age < 12} {
    puts "You are a child"
} elseif {$Age < 19} {
    puts "You are a teen"
} else { 
    puts "You are an adult now" 
}

The following isn't necessarily an example of good style, but it does work:

puts "Hey dude, how old might you be?"
gets stdin Age
puts "You are a[expr {$Age < 12 ? " child" : ($Age < 19 ? " teen" : "n adult now")}]"

Program 6: Checking for an Age Range  edit

You can use && and || in expressions to connect sub-expressions together. && should be read as "and" and || should be read as "or".

puts "Hey dude, how old might you be?"
gets stdin Age
if {$Age >= 0 && $Age <= 12} {
    puts "You are a child."
} elseif {$Age >= 13 && $Age <= 19} {
    puts "You are a teen."
} elseif {$Age > 19}  { 
    puts "You are an adult now."
}

Another way to write the same program:

puts "Hey dude, how old might you be?"
gets stdin Age
if {$Age <= 12} {
    puts "You are a child."
} elseif { $Age <=19 } {
    puts "You are a teen."
} else {
    puts "You are an adult now."
}

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