In Tcl, the evaluation of a command is done is 2 phases. The first phase is a single pass of substitutions. The second phase is the evaluation of the resulting command. Note that only one pass of substitutions is made.
During the substitution phase, several types of substitutions occur.
A command within square brackets ([]) is replaced with the return from the execution of that command. (This will be explained more fully in the lesson "Results of a Command - Math 101.")
Double quotes ("") and braces ({}) will both group words into a single argument during the substitution phase, however, their behaviors differ. The behavior of braces is covered in the next lesson; the following discussion pertains to double quotes.
Grouping words within double quotes allows substitutions to occur within the quotations. The substituted group is then evaluated as a single argument.
In general, the backslash (\) disables substitution for the single character immediately following the backslash.
The exceptions to this last rule are the "Backslash Sequence" strings, which are substituted as follows:
The final exception is the backslash at the end of a line of text. This causes the interpreter to ignore the newline, and treat the text as a single line of text. The interpreter will insert a blank space at the location of the ending backslash.
If you haven't hit the Run Example button yet, do so now, and examine the results of the example. Play around a bit with the example code, and then go on to the next lesson.
:TEXT_END: :LESSON_TEXT_START_LEVEL 1: This lesson is the first of three which discuss the way TCL handles substitution during command evaluation.
In Tcl, the evaluation of a command is done is 2 phases. The first phase is a single pass of substitutions. The second phase is the evaluation of the resulting command. Note that only one pass of substitutions is made. Thus in the command
puts $varName
During the substitution phase, several types of substitutions occur.
A command within square brackets ([]) is replaced with the return from the execution of that command. (This will be explained more fully in the lesson "Results of a Command - Math 101.")
Words within double quotes or braces are grouped into a single argument. However, double quotes and braces cause different behavior during the substitution phase. In this lesson, we will concentrate on the behavior of double quotes during the substitution phase.
Grouping words within double quotes allows substitutions to occur within the quotations. The substituted group is then evaluated as a single argument. Thus, in the command
puts "The current stock value is $varName"
In general, the backslash (\) disables substitution for the single character immediately following the backslash. Any character immediately following the backslash will stand without substitution.
However, there are specific "Backslash Sequence" strings which are replaced by specific values during the substitution phase. The following backslash strings will be substituted as shown below.
The final exception is the backslash at the end of a line of text. This causes the interpreter to ignore the newline, and treat the text as a single line of text. The interpreter will insert a blank space at the location of the ending backslash.
If you haven't hit the Run Example button yet, do so now, and examine the results of the example. Play around a bit with the example code, and then go on to the next lesson.
:TEXT_END: :LESSON_TEXT_START_LEVEL 2: This lesson is the first of three which discuss the way TCL handles substitution during command evaluation.
In Tcl, the evaluation of a command is done in two phases; a substitution phase and a command evaluation phase.
The substitution phase is where much of the 'work' in a Tcl program is done. The Tcl interpreter looks at each word from left to right and, when it finds a word that should be replaced by another value, it does that replacement. During the command evaluation phase, the compiler evaluates the command, and performs whatever action that command requests. Because of the substitutions, the command that is evaluated may not resemble the command that you typed.
Note that only one pass of substitutions is made.
The Tcl interpreter performs these substitutions:
In the example code, Z is set to "Albany". When puts "$Z_LABEL $Z"
is evaluated the $Z_LABEL is replaced with the string "The Capitol
of New York is: ", and the $Z is replaced by "Albany" before
puts is called to output the string.
Some statements get very complex and hard to follow if they are all typed on a single line. This facility lets you break up really long lines into several shorter lines.
Words within double quotes or braces are evaluated as a single argument. However, double quotes and braces cause different behavior during the substitution phase. In this lesson, we will concentrate on the behavior of double quotes during the substitution phase.
Grouping words within double quotes allows substitutions to occur within the quotes during the substitution phase. The substituted group is then evaluated as a single argument. Thus, in the command
puts "The current stock value is $varName"
Except for the "Backslash Sequence" strings and the backslash at the end of a line (\newline) described above, the backslash (\) disables substitution for the single character immediately following the backslash. Any character immediately following the backslash will stand without substitution. This is the way to print a dollar sign ($). Without the backslash, the $ is interpreted as a variable place holder.
If you haven't hit the Run Example button yet, do so now, and examine the results of the example. Play around a bit with the example code, and then go on to the next lesson. :TEXT_END:
:CODE_START: ;# Show how a \ affects the $ set Z "Albany" set Z_LABEL "The Capitol of New York is: " puts "$Z_LABEL $Z" puts "$Z_LABEL \$Z" ;# The next line needs a backslash to escape the '$' puts "\nBen Franklin is on the $100.00 bill" set a 100.00 puts "Washington is not on the $a bill" ;# This is not what you want puts "Lincoln is not on the $$a bill" ;# This is OK puts "Hamilton is not on the \$a bill" ;# This is not what you want puts "Ben Franklin is on the \$$a bill" ;# But, this is OK puts "\n................. examples of escape strings" puts "Tab\tTab\tTab" puts "This string prints out \non two lines" puts "This string comes out\ on a single line" :TEXT_END: