The wild, whacky, wonderful world of workarounds

[Steve Litt <slitt@...>]

:-)

Continue statements? We don't need no steenking continue statements. 

Watch this:

The following is the input file:
===========================================
one
two

three
four



five
        six
         
seven
===========================================

The following is the program
===========================================
#!/usr/bin/lua

sf = string.format

Relevant_lines = {}

function Relevant_lines.new(fname)
	local self = {}
	local handle = assert(io.open(fname, "r"),
		"Relevent_lines() Crashed, could not open input file!")
	local thisline
	local prevline
	local prevrelevantline
	local thislineno = 0
	local prevrelevantlineno = 0
	function self.is_relevant(tab)
		return(thisline and string.match(thisline, "%S"))
	end
	function self.next_rline()
		prevline = thisline
		thisline = handle:read("*line")
		thislineno = thislineno + 1
		while thisline and not self.is_relevant() do
			prevline = thisline
			thisline = handle:read("*line")
			thislineno = thislineno + 1
		end
		if thisline then
			return thislineno, thisline
		else
			io.close(handle)
			return nil, nil
		end
	end
	function self.set_is_relevant(fcn)
		self.is_relevant = fcn
	end
	function self.newincr() return self.next_rline end
	return(self)
end

print("=== PRINTING ONLY NONBLANK LINES ===")
local maker = Relevant_lines.new("test.txt")

for lineno, line in maker.newincr() do
	print(sf("%5d: %s", lineno, line))
end


print("=== PRINTING    ALL        LINES ===")
local maker = Relevant_lines.new("test.txt")
allow_all = function(tab) return true end
maker.set_is_relevant(allow_all)

for lineno, line in maker.newincr() do
	print(sf("%5d: %s", lineno, line))
end
===========================================

Here's the output:
===========================================
=== PRINTING ONLY NONBLANK LINES ===
    1: one
    2: two
    4: three
    5: four
    9: five
   10:         six
   12: seven
=== PRINTING    ALL        LINES ===
    1: one
    2: two
    3: 
    4: three
    5: four
    6: 
    7: 
    8: 
    9: five
   10:         six
   11:          
   12: seven
slitt@mydesk:~$
===========================================

It is actually kinda cool for a workaround. You can pass in any callback you 
want in order to implement any criteria for lines passed through, and it still 
keeps track of original line numbers. The fact that you can pass in a table 
means you really can implement pretty much any criteria you want. The fact 
that tables are passed by reference means your callback could even *write* to 
the table.

One addition might be another callback that self.is_relevant() calls on 
rejection.

Obviously I didn't write it in a succinct manner nor a Lua-appropriate manner, 
but I think you can see what I'm talking about.

SteveT

Steve Litt
Recession Relief Package
http://www.recession-relief.US
Twitter: http://www.twitter.com/stevelitt