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Finding distances between places base on OS grid references
Lists example from a Well House Consultants training course
More on Lists [link]

This example is described in the following article(s):
   • Working out distance between places, using OS grid references and a program in Tcl - [link]

Source code: distance_between Module: T206
#!/usr/bin/env tclsh
# Finding distances between places base on OS grid references

# Read in the OS big grid table

set fh [open os_letters.txt r]
while {[gets $fh lyne] > 0} {
        regsub "#.*" $lyne "" result
        if {[string length $result] < 1} continue
        lappend squares $result
        }

# Find out about your journey - e.g. ST899646 SU149853 TQ265814

puts -nonewline "What are the grid reference on your journey (space separated?) "
flush stdout
gets stdin osrefs

# Work out the true north and east value of each point
# (This would be much easier using arrays - later on the course!)

foreach osref $osrefs {
                # Need to trap erroneous formats in the user's grid refs!
        set letters [string range $osref 0 1]
                # Scan to sort out issues with leading zeros on easting / northing
        scan [string range $osref 2 4] %d east
        scan [string range $osref 5 end] %d north
        set mega_north 0
        foreach band $squares {
                if {[set mega_east [lsearch $band $letters]] > -1} break
                incr mega_north
                }
        set easting [expr 1000 * $mega_east + $east]
        set northing [expr 1000 * $mega_north + $north]
        lappend elist $easting
        lappend nlist $northing
        }

# And work out the length of each leg

for {set to 1} {$to < [llength $elist]} {incr to} {
        set from [expr $to - 1]
        set east_west [expr [lindex $elist $from] - [lindex $elist $to]]
        set north_south [expr [lindex $nlist $from] - [lindex $nlist $to]]
        set leg_distance [format %.1f [expr 0.1 * hypot($east_west, $north_south)]]
        puts $leg_distance
        }

# Sample results
#
# munchkin:survey grahamellis$ tclsh distance_between
# What are the grid reference on your journey (space separated?) ST899646 SU149853 TQ265814
# 32.46
# 111.67
# munchkin:survey grahamellis$

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
You can learn more about this example on the training courses listed on this page, on which you'll be given a full set of training notes.

Many other training modules are available for download (for limited use) from our download centre under an Open Training Notes License.

Other resources
• Our Solutions centre provides a number of longer technical articles.
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This is a sample program, class demonstration or answer from a training course. It's main purpose is to provide an after-course service to customers who have attended our public private or on site courses, but the examples are made generally available under conditions described below.

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