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how an lsort callback works
Lists example from a Well House Consultants training course
More on Lists [link]

This example is described in the following article(s):
   • User defined sorting and other uses of callbacks in Tcl and Tk - [link]

Source code: arx Module: T206
# how a sort callback works

# Sample data:

# 5337 FMR BN1 9PU TQ 346088 50.8621241499 -0.0873836155 Falmer 771095 838150 920259 1009988 1045586 1033392

set fh [open r]

# This proc is my own replacement for lsort. It's written as a demo and
# is very inefficient indeed, and limited in what it can do compare to
# lsort - but it's a great teaching tool.

proc mysort {optionselector howtell listtosort} {

        # bubble sort!
        for {set k 0} {$k < [llength $listtosort] -1} {incr k} {
        for {set j 0} {$j < [llength $listtosort] - $k -1} {incr j} {
        set j1 [expr $j + 1]

        # get two items
        set first [lindex $listtosort $j]
        set second [lindex $listtosort $j1]

        # do they need to be swapped (NOTE CALLBACK)
        set swapsie [$howtell $first $second]

        # Swap them!
        if {$swapsie < 0} {
                set listtosort [lreplace $listtosort $j $j $second]
                set listtosort [lreplace $listtosort $j1 $j1 $first]

        return $listtosort

proc byquantity {a b} {
        global uses
        # next line useful as a debug trace if uncommented
        # puts "$a .... $b"
        return [expr $uses($b) - $uses($a)]

while {[set line [gets $fh]] != ""} {
        set fields [split $line "\t"]

        set placename [lindex $fields 6]
        set passengers [lindex $fields 8]

        if {$passengers != "NULL"} {
                set uses($placename) $passengers

# I have replaced "lsort" by "mysort" to demonstratte how a callback works.
# Rather than the internal "lsort" our own "mysort" is called, passing it the
# name of the proc that places two records in order, for it to call repeatedly.

foreach pl [mysort -command byquantity [array names uses South*]] {
        puts [format "%8d %s" $uses($pl) $pl]
Learn about this subject
This module and example are covered on the following public courses:
 * Learning to program in Tcl
 * Tcl Programming
 * Tcl Programming
Also available on on site courses for larger groups

Books covering this topic
Yes. We have over 700 books in our library. Books covering Tcl, Tk and Expect are listed here and when you've selected a relevant book we'll link you on to Amazon to order.

Other Examples
This example comes from our "Lists" training module. You'll find a description of the topic and some other closely related examples on the "Lists" module index page.

Full description of the source code
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Many other training modules are available for download (for limited use) from our download centre under an Open Training Notes License.

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