Tcl/Tk Notes (3)

    In the last command, $name gets substituted with var. Then the set command returns the value of var, which is the value of var. Another way to achieve a level of indirection like this is with nested set commands. The last set command above can be written as follows:
        set [set name]
        => the value of var

• The unset command 

    You can delete a variable with the unset command:
        unset varName varName2 ...

• Using info to find out about variables

    The existence of a variable can be tested with the info exists command.
e.g. 
        if ![info exists foobar]{
            set foobar 0
        } else {
            incr foobar }

2 Tcl Lists

    The Tcl list has the same structure as a Tcl command. That is, a
list is simply a string with list elements separated by white
space.Braces or quotes can be used to group words with whitespace into
a single list element.

e.g. Constructing a list with the list command
        set x {1 2}
        => 1 2
        set x
        => 1 2
        list $x /$ foo
        => {1 2} {$} foo
[Note]: The braces used to group the list value into one argument to the set command are not part of the list value.

    The lappend command is used to append elements to the end of a list. You can call lappend with the name of an undefined variable and the variable will be created.
e.g.
            lappend new 1 2
            => 1 2
            lappend new 3 "4 5"
            => 1 2 3 {4 5}
            set new
            => 1 2 3 {4 5}

    The concat
command is useful for splicing together lists. It works by
concatenating its arguments together, separating them with spaces. This
multiple lists into one where the top-level list elements in each input
list are also top-level list elements in the resulting list.
e.g. Using concat to splice together lists.
            concat 1 {2 3} {4 5 6}
            => 1 2 3 4 5 6
    It turns out that double quotes behave much like the concat command.
e.g. Double quotes compared to the list command.
            set x {1 2}
            => 1 2
            set y "$x 3"
            => 1 2 3
            set y [concat $x 3]
            => 1 2 3
            set z [list $x 3]
            =>{1 2} 3
    The basic rule is that list and lappend preserve preserve list structure, while concat(or double-quotes)eliminate one level of list structure.

4 Getting List Elements: llength, lindex, and lrange

    The llength command returns the number of elements in a list.
    The lindex command returns a particular element of a list. It takes an index. List indices count from zero. The keyword end means the last element, and it can be used with lindex, linsert, lrange and lreplace.
e.g.
            lindex {1 2 3} 0
            => 1
            lindex {1 2 3} 2
            => 3
    The lrange command returns a range of list elements. It takes a list and two indices as arguments.
e.g.
            lrange {1 2 3 {4 5}} 2 end
            => 3 {4 5}

5 Modifying Lists: linsert and lreplace

    The linsert
command inserts elements into a list value at a specified index. If the
index is 0 or less, then elements are added to the front. If the index
is equal to or greater than the length of the list, then the elements
are appended to the end. Otherwise, the elements are inserted before
the element that is currentl as position index.
    lreplace
is used to replace a range of list elements with new elements. If you
don't specify any new elements, you effectively delete elements from a
list.
e.g.
            linsert {1 2} 0 new stuff
            => new stuff 1 2
            set x [list a {b c} e d]
            => a {b c} e d
            lreplace $x 1 2 B C
            => a B C d
            lreplace $x 0 0
            => {b c} e d

6 Searching Lists: lsearch

    lsearch returns the index of a value in the list, or -1 if it is not present. lsearch supports pattern matching in its search. Glob-style pattern matching is default, and this canbe disabled with the -exact flag.
    The example below uses lreplace to delete elements by not specifying any replacement list elements.
e.g.
        proc delete {list value} {
            set ix [lsearch -exact $list $value]
            if {$ix>=0} {
                return [lreplace $list $ix $ix]
            } else {
                return $list
            }
        }
 

7 Sorting List: lsort

    The three basic types of sorts are specified with the -ascii, -integer, or -real options. The -increasing or -decreasing option indicate the sirting order. The default option set is -ascii -increasing.