Part 5 Procedures
- A Tcl procedure is a command that is implemented with a Tcl script rather than C code
- Procedures make it easy to package solutions to problems so that they can be reused easily
- Procedures also provide a simple way to prototype new features in an application
- Procedures encapsulate a set of commands and they introduce a local scope for variables
- A Tcl procedure is defined with the proc command
- It takes three arguments:
- The first argument is the procedure name
- The second argument is a list of parameter names
- The last argument is the body of the procedure
Passing parameters by value
- Tcl procedure once defined can be used like any other Tcl command
- When it is invoked each argument is assigned to the corresponding parameter and the body is evaluated
- Eg: proc test { a b} { body }
- Here a and b are the formal arguments which are going to receive the values passed
- test 12 (invoking the procedure)
Example:
proc test { a b }{
puts "$a $b"
}
test 1 2
Output: 1 2
Default values
- Procedures can have default parameters so that the caller can leave out some of the command arguments
- A default parameter is specified with its name and default value proc test { a {b 1} {c 0} } { body }
- The procedure P2 can be called with one two or three arguments
- If it is called with only one argument then the parameters b and c take on the values specified in the proc command
Example:
proc test { a {b 5} {c 4}} {
puts $a
puts Sb
puts $c
}
test 1 2 3
test 1 1
test
Output: 1 2 3
1 1 4
1 5 4
Variable number arguments
- A procedure can take a variable number of arguments by specifying the args keyword as the last parameter
- When the procedure is called, the args parameter is a list that contains all the remaining values
70
Recursive Procedures
Recursive Procedures
An example for recursive procedures is shown below:
Output: 5040
720
Part 6: File Handling
The open command
- The open command opens a file name in the mode specified and returns a file identifier which is used to perform operations on files
- The syntax of the open command is
- open name? Access mode? permissions?
- Eg: set f1 [open filename]
- f1 will be holding the file identifier returned by open command which can be used for further operations on the file
- r opens a file for reading. The file must exist
- r+ opens a file for reading and writing. The file must exist
- w opens for writing. Truncate if it exists Create if it does not exist
- w+ opens for reading and writing .Truncate or create
- a opens for writing. Data is appended to the file
- a+ opens for reading and writing. Data is appended
Reading and Writing
- The standard I/O channels are already open for use
- The standard input channel is stdin
- The standard output channel is stdout
- The standard error channel is stderr
- Other I/O channels are returned by the open command
Reading a File
Writing a File
- Puts command is used to write to an open file.
Part 7: Regular Expressions
- The "regexp" command is used to match a regular expression in Tcl.
- The building block of a regular expression is an atom
- A simple regular expression consist of one or more atoms
- The regular expression pattern matches any string containing the pattern
- The regexp command invokes regular expression matching
- It takes two arguments in its simplest form, the regular expression pattern and an input string
- It returns 0 or 1 to indicate whether or not the pattern matched the input string
- The syntax is regexp pattern string
Example: puts [ regexp vlsi "vlsi4freshers"]
Output: 1
Special characters
- (.) Matches any single character. Eg: regexp v.s vlsi
- ^ Matches the beginning of the string : Eg: regexp ^vl vlsi4freshers
- $ Matches the end of the string : Eg: regexp vls$ companyvlsi
- [chars] Matches any character from the list of chars
- (pattern) matches any string containing pattern
Output: Full Match: vlsi4freshers
Sub Match1: vlsi
Sub Match2: freshers
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