# 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 railstats.xyz 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]
}