This chapter takes you through the basics of the Tcl language syntax. Even if you are an expert programmer, it is worth taking the time to read these few pages to make sure you understand the fundamentals of Tcl. The basic mechanisms are all related to strings and string substitutions, so it is fairly easy to visualize what is going on in the interpreter. The model is a little different than some other programming languages you may already be familiar with, so it is worth making sure you understand the basic concepts.
Tcl Commands
The basic syntax for a Tcl command is:
command arg1 arg2 arg3 ...The command is either the name of a built-in command or a Tcl procedure. White space (i.e., space or tab) is used to separate the command name and its arguments, and a newline or semicolon is used to terminate a command. The arguments to a command are just strings.
Tcl has syntax for grouping, which allows multiple words in one argument, and substitution, which is used with programming variables and nested command calls. The Tcl interpreter does grouping first, then substitutions, and finally it calls the command. It is up to the command to interpret its arguments. This model is described in detail in this Chapter.
puts stdout {Hello, World!}
=> Hello, World!In this example, the command is puts, which takes two arguments: an I/O stream identifier and a string. puts writes the string to the I/O stream along with a trailing newline character. There are two points to emphasize:
The braces are not part of the value.The braces are syntax for the interpreter, and they get stripped off before the value is passed to the command. Braces group all characters, including newlines and nested braces, until a matching brace is found. Tcl also uses double quotes for grouping. Grouping arguments will be described in more detail later.
It is not necessary to declare Tcl variables before you use them. The interpreter will create the variable when it is first assigned a value. The value of a variable is obtained later with the dollar-sign syntax illustrated in Example 1-2:
set var 5
=> 5
set b $var=> 5
The second set command assigns to variable b the value of variable var. The use of the dollar sign is our first example of substitution. You can imagine that the second set command gets rewritten by substituting the value of var for $var to obtain a new command.
set b 5The actual implementation is a little different, but not much.
set len [string length foobar]=> 6
In the example, the nested command is:
string length foobarThis command returns the length of the string foobar. The string command is described in detail starting on page 43. The nested command runs first. Then command substitution causes the outer command to be rewritten as if it were:
set len 6If there are several cases of command substitution within a single command, the interpreter processes them from left to right. As each right bracket is encountered, the command it delimits is evaluated. This results in a sensible ordering in which nested commands are evaluated first so their result can be used in arguments to the outer command.
The implementation of expr takes all its arguments, concatenates them into a single string, and then parses the string as a math expression. After expr computes the answer, the answer is formatted into a string and returned:
expr 7.2 / 4=> 1.8
You can include variable references and nested commands in math expressions. The following example uses expr to add 7 to the length of the string foobar. As a result of the innermost command substitution, the expr command sees 6 + 7, and len gets the value 13:
set len [expr [string length foobar] + 7]=> 13
The expression evaluator supports a number of built-in math functions. For a complete listing, see page 19. The following example computes the value of pi:
set pi [expr 2*asin(1.0)]=> 3.1415926535897931
set dollar \$foo
=> $foo
set x $dollar=> $foo
Only a single round of interpretation is done.The second set command in the example illustrates an important property of Tcl. The value of dollar does not affect the substitution done in the assignment to x. In other words, the Tcl parser does not care about the value of a variable when it does the substitution. After the example, the value of x and dollar is the string $foo. In general, you do not have to worry about the value of variables until you use eval, which is described in Chapter 10.
You can also use backslash sequences to specify characters with their hexadecimal or octal value:
set escape \0x1b
set escape \033The value of variable escape is the ASCII ESC character, which has character code 27. The table on page 18 summarizes backslash substitutions.
A common use of backslashes is to continue long commands on multiple lines. This is necessary because a newline terminates a command unless an argument is being grouped as described in the next section. A backslash as the last character in a line is converted into a space. In addition, all the white space at the beginning of the next line is replaced by this substitution. The backslash in the next example is required; otherwise the expr command gets terminated by the newline after the plus sign.
set totalLength [expr [string length $one] + \
[string length $two]]
set s Hello
=> Hello
puts stdout "The length of $s is [string length $s]."
=> The length of Hello is 5.
puts stdout {The length of $s is [string length $s].}
=> The length of $s is [string length $s].In the second command of Example 1-9, the Tcl interpreter does variable and command substitution on the second argument to puts. In the third command, substitutions are prevented so the string is printed as-is.
In practice, grouping with curly braces is used when substitutions on the argument must be delayed until a later time (or never done at all). Examples include loops, conditional statements, and procedure declarations. Double quotes are useful in simple cases like the puts command previously shown.
puts [format "Item: %s\t%5.3f" $name $value]Here format is used to align a name and a value with a tab. The %s and %5.3f indicate how the remaining arguments to format are to be formatted. Note that the trailing \n usually found in a C printf call is not needed because puts provides one for us. For more information about the format command, see page 46.
Square Brackets Do Not Group
The square bracket syntax used for command substitution does not provide grouping. Instead, a nested command is considered part of the current group. In the command below the double quotes group the last argument, and the nested command is just part of that group.
puts stdout "The length of $s is [string length $s]."In the next example the last argument is a nested command. There is no need to explicitly group the nested command because the Tcl parser treats the whole nested command as part of the group.
puts stdout [string length $s]In general, you can place a bracketed command or variable reference anywhere. The following computes a command name:
[findCommand $x] arg arg
The following example demonstrates how nested command substitution affects grouping. A nested command is treated as an unbroken sequence of characters, regardless of its internal structure. It is included with the surrounding group of characters when collecting arguments for the main command.
set x 7; set y 9
puts stdout $x+$y=[expr $x + $y]=> 7+9=16
In the example the second argument to puts is:
$x+$y=[expr $x + $y]The white space inside the nested command is ignored for the purposes of grouping the argument. By the time Tcl encounters the left bracket, it has already done some variable substitutions to obtain:
7+9=When the left bracket is encountered, the interpreter calls itself recursively to evaluate the nested command. Again, the $x and $y are substituted before calling expr. Finally, the result of expr is substituted for everything from the left bracket to the right bracket. The puts command gets the following as its second argument:
7+9=16Grouping before substitution.
The point of this example is that the grouping decision about puts's second argument is made before the command substitution is done. Even if the result of the nested command contained spaces or other special characters, they would be ignored for the purposes of grouping the arguments to the outer command. Grouping and variable substitution interact the same as grouping and command substitution. Spaces or special characters in variable values do not affect grouping decisions because these decisions are made before the variable values are substituted.
If you want the output to look nicer in the example, with spaces around the + and =, then you can use double quotes to explicitly group the argument to puts:
puts stdout "$x + $y = [expr $x + $y]"The double quotes are used for grouping in this case to allow the variable and command substitution on the argument to puts. Note that it is never necessary to explicitly group a nested command with double quotes if it makes up the whole argument. The following is a redundant use of double quotes:
puts stdout "[expr $x + $y]"
proc name arglist bodyThe first argument is the name of the procedure being defined. The second argument is a list of parameters to the procedure. The third argument is a command body that is one or more Tcl commands.
The procedure name is case sensitive, and in fact it can contain any characters. Procedure names and variable names do not conflict with each other. As a convention, this book begins procedure names with uppercase letters and it begins variable names with lowercase letters. Good programming style is important as your Tcl scripts get larger. Tcl coding style is discussed in Chapter 12.
proc Diag {a b} {
set c [expr sqrt($a * $a + $b * $b)]
return $c
}
puts "The diagonal of a 3, 4 right triangle is [Diag 3 4]"=> The diagonal of a 3, 4 right triangle is 5.0
The Diag procedure defined in the example computes the length of the diagonal side of a right triangle given the lengths of the other two sides. The sqrt function is one of many math functions supported by the expr command. The variable c is local to the procedure; it is only defined during execution of Diag. Variable scope is discussed further in Chapter 7. It is not really necessary to use the variable c in this example. The procedure could also be written as:
proc Diag {a b} {
return [expr sqrt($a * $a + $b * $b)]
}The return command is used to return the result of the prodecure. The return command is optional in this example because the Tcl interpreter returns the value of the last command in the body as the value of the procedure. So, the procedure could be reduced to:
proc Diag {a b} {
expr sqrt($a * $a + $b * $b)
}Note the stylized use of curly braces in the example. The curly brace at the end of the first line starts the third argument to proc, which is the command body. In this case, the Tcl interpreter sees the opening left brace, causing it to ignore newline characters and scan the text until a matching right brace is found. Double quotes have the same property. They group characters, including newlines, until another double quote is found. The result of the grouping is that the third argument to proc is a sequence of commands. When they are evaluated later, the embedded newlines will terminate each command. The other crucial effect of the curly braces around the procedure body is to delay any substitutions in the body until the time the procedure is called. For example, the variables a, b, and c are not defined until the procedure is called, so we do not want to do variable substitution at the time Diag is defined.
The proc command supports additional features such as having variable numbers of arguments and default values for arguments. These are described in detail in Chapter 7.
A Factorial Example
To reinforce what we have learned so far, here is a longer example that uses a while loop to compute the factorial function:
proc Factorial {x} {
set i 1; set product 1
while {$i <= $x} {
set product [expr $product * $i]
incr i
}
return $product
}
Factorial 10=> 3628800
The semicolon is used on the first line to remind you that it is a command terminator just like the newline character.
The while loop is used to multiply all the numbers from one up to the value of x. The first argument to while is a boolean expression, and its second argument is a command body to execute. The while command evaluates the boolean expression, and then executes the body if the expression is true (non-zero). The while command continues to test the expression and evaluate the command body until the expression is false (zero). Other control structures are described in Chapter 6.
The same math expression evaluator used by the expr command is used by while to evaluate the boolean expression. There is no need to explicitly use the expr command in the first argument to while, even if you have a much more complex expression.
while {$i < $x} \
{
set product ...
}
Always group expressions and command bodies with curly braces.Curly braces around the boolean expression are crucial because they delay variable substitution until the while command implementation tests the expression. The following example is an infinite loop:
set i 1; while $i<=10 {incr i}
The loop will run indefinitely. The reason is that the Tcl interpreter will substitute for $i before while is called, so while gets a constant expression 1<=10 that will always be true. You can avoid these kinds of errors by adopting a consistent coding style that groups expressions with curly braces:
set i 1; while {$i<=10} {incr i}
The incr command is used to increment the value of the loop variable i. This is a handy command that saves us from the longer command:
set i [expr $i + 1]The incr command can take an additional argument, a positive or negative integer by which to change the value of the variable. Using this form it is possible to eliminate the loop variable i and just modify the parameter x. The loop body can be written like this:
while {$x > 1} {
set product [expr $product * $x]
incr x -1
}
set var {the value of var}
=> the value of var
set name var
=> var
set name
=> var
set $name=> the value of var
This is a somewhat tricky example. In the last command, $name gets substituted with var. Then the set command returns the value of var, which is the value of var. Nested set commands provide another way to achieve a level of indirection. The last set command above can be written as follows:
set [set name]=> the value of var
Using a variable to store the name of another variable may seem overly complex. However, there are some times when it is very useful. There is even a special command, upvar, that makes this sort of trick easier. The upvar command is described in detail on page 79 in Chapter 7.
Funny Variable Names
The Tcl interpreter makes some assumptions about variable names that make it easy to embed variable references into other strings. By default, it assumes that variable names only contain letters, digits, and the underscore. The construct $foo.o represents a concatenation of the value of foo and the literal ".o".
set foo filename
set object $foo.o
=> filename.o
set a AAA
set b abc${a}def
=> abcAAAdef
set .o yuk!
set x ${.o}y
=> yuk!y
unset varName varName2 ...Any number of variable names can be passed to the unset command. However, unset will raise an error if a variable is not already defined.
if {![info exists foobar]} {
set foobar 0
} else {
incr foobar
}In Chapter 7, page 80, there is an example that implements a new version of incr, which handles this case.
More about Math Expressions
This section describes a few fine points about math in Tcl scripts. In Tcl 7.6 and earlier versions math is not that efficient because of conversions between strings and numbers. The expr command must convert its arguments from strings to numbers. It then does all its computations with double precision floating point values. The result is formatted into a string that has, by default, six significant digits. This can be changed by setting the tcl_precision variable to the number of significant digits desired. Seventeen digits of precision are enough to ensure that no information is lost when converting back and forth between a string and an IEEE double precision number:
expr 1 / 3
=> 0
expr 1 / 3.0
=> 0.333333
set tcl_precision 17
=> 17
expr 1 / 3.0
# The trailing 1 is the IEEE rounding digit=> 0.33333333333333331
In Tcl 8.0 and later versions the overhead of conversions is eliminated in most cases by the built-in compiler. The use of tcl_precision is also eliminated so values are always printed with full precision. Even so, Tcl was not designed to support math intensive applications. You may want to implement math-intensive code in a compiled language and register the function as a Tcl command as described in Chapter 41.
if {$answer == "yes"} { ... }
However, the string compare command described in Chapter 4 is more reliable because expr may do conversions on strings that look like numbers. This area has improved in Tcl 8.0. The issues with string operations and expr are discussed on page 45.set y [expr {$x + $y}]
# Here are some parameters
set rate 7.0 ;# The interest rate
set months 60 ;# The loan termOne subtle effect to watch out for is that a backslash effectively continues a comment line onto the next line of the script. In addition, a semicolon inside a comment is not significant. Only a newline terminates comments:
# Here is the start of a Tcl comment \
and some more of it; still in the commentThe behavior of a backslash in comments is pretty obscure, but it can be exploited as shown in Example 2-3 on page 25.
Substitution and Grouping Summary
The following rules summarize the fundamental mechanisms of grouping and substitution that are performed by the Tcl interpreter before it invokes a command:
if {$x > 1}{puts "x = $x"}
set silly a"b
set x xvalue
set y "foo {$x} bar"
=> foo {xvalue} bar
set x [cmd1][cmd2]
set x "This is line one.
This is line two.
This is line three."
set x $