On Mar 20, 2006, at 5:15 PM, Neil Rutland wrote:

Hi there,

thank you all for your help with regards to the integer addition question that i asked yesterday it was very good in clarifying my ideas. However i would now like to ask for your expert help once again.

Basically i have a list of 16 tuples of 12 elements each, each tuple representing a train station - the tuple takes the form

station = [string, int, int, bool, bool, bool, bool, bool, bool, boo, bool, bool] I think you'll be better off defining an enumerated type for your rail lines and then having each station contain a list of the lines to which it connects rather than trying to wrangle that huge tuple of booleans. Something like:

data RailLine = Bakerloo | British | Central .... type Station = (String,Int,Float,[RailLine]) victoriaLine :: [Station] victoriaLine = [("Walthamstow Central", 1, 0, [British]), ... ] Or, better: type Station = (String,(Int,Float,[RailLine])) because now you can use the standard list function 'lookup'. Or, better still, use Data.Map: http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Map.html

Basically i have a list called victoria line which is a list of 16 stations. What i want to do is access an element of the list so i can check and output the value that it holds. I'll include all my code below so you can see what i mean more as well as the attempt that i have made to do just that - its the bit called bakerloo!

import Prelude

type Element0 = String type Element1 = Int type Element2 = Float type Element3 = Bool type Element4 = Bool type Element5 = Bool type Element6 = Bool type Element7 = Bool type Element8 = Bool type Element9 = Bool type Element10 = Bool type Element11 = Bool

This all is pretty unnecessary -- as a rule of thumb, introduce a type alias when it makes it easier to read or maintain the resulting code. I don't think these help.

type Station = [ (Element0, Element1, Element2, Element3, Element4, Element5, Element6, Element7, Element8, Element9, Element10, Element11) ]

victoriaLine :: Station

victoriaLine = [ ("Walthamstow Central", 1, 0, False, True, False, False, False, False, False, False, False), ("BlackHorse Road", 2, 3.0, False, True, False, False, False, False, False, False, False), ("Tottenham Hale", 3, 3.3, False, True, False, False, False, False, False, False, False), ("Seven Sisters", 4, 3, False, True, False, False, False, False, False, False, False), ("Finsbury Park", 5, 4.0, False, True, False, False, False, False, False, False, True), ("Highbury and islington", 6, 4.0, False, True, False, False, False, False, False, False, False), ("Kings Corss And St Pancras", 7, 3.3, False, True, False, True, False, False, True, True, True), ("Euston", 8, 1.3, False, True, False, False, False, False, False, True, False), ("Warren Street", 9, 2.0, False, False, False, False, False, False, False, True, False), ("Oxford Circus", 10, 1.45, True, False, True, False, False, False, False, False, False), ("Green Park", 11, 2.0, False, False, False, False, False, True, False, False, True), ("Victoria", 12, 2.3, False, True, False, False, True, True, False, False, False), ("Pimlico", 13, 1.0, False, False, False, False, False, False, False, False, False), ("Vauxhall", 14, 1.45, False, True, False, False, False, False, False, False, False), ("Stockwell", 15, 1.3, False, False, False, False, False, False, False, True, False), ("Brixton", 16, 2.0, False, True, False, False, False, False, False, False, False) ]

mainMenu :: IO() mainMenu = do putStr"\n" putStr"Welcome To the Victoria Line Timetable System\n" putStr"Please Make A Choice from those Listed Below\n\n" putStr"Detail Station Information = stationMenu\n" putStr"For All Stations that link to the Bakerloo Line = Bakerloo\n" putStr"For All Stations that link to British Rail Services = British\n" putStr"For All Stations that link to the Central Line = Central\n" putStr"For All Stations that link to the Circle Line = Circle\n" putStr"For All Stations that link to the District Line = District\n" putStr"For All Stations that link to the Jubilee Line = Jubilee\n" putStr"For All Stations that link to the Metropolitan Line = Metropolitan\n" putStr"For All Stations that link to the Northern Line = Northern\n" putStr"For All Stations that link to the Piccadilly Line = Piccadilly\n" return()

FYI, putStrLn will automatically insert a newline for you, and the final 'return ()' is unnecessary. My favorite idiom for this kind of thing is: mainMenu = putStr $ unlines [ "line 1" , "line 2" , "line 3" ] I think it's easier to read.

stationMenu::IO() stationMenu = do putStr"\n" putStr"Please Select The station You are located at\n\n" putStr"Walthamstow Central\n" putStr"BlackHorse Road\n" putStr"Tottenham Hale\n" putStr"Seven Sisters\n" putStr"Finsbury Park\n" putStr"Higbury and Islington\n" putStr"Kings Cross and St Pancras\n" putStr"Euston\n" putStr"Warren Street\n" putStr"Oxford Circus\n" putStr"Green Park\n" putStr"Victoria\n" putStr"Pimlico\n" putStr"Vauxhall\n" putStr"Stockwell\n" putStr"Brixton\n"

lineMenu::IO() lineMenu = do putStr"\n" putStr"Please Select The Line you wish to connect to\n" putStr"Bakerloo Line = nrBakerloo\n" putStr"British Rail = nrBritish\n" putStr"Central Line = nrCentral\n" putStr"Circle Line = nrCircle\n" putStr"District Line = nrDistrict\n" putStr"Jubilee Line = nrJubillee\n" putStr"Metropolitan Line = nrMetropolitan\n" putStr"Northern Line = nrNorthern\n" putStr"Piccadilly Line = nrPicadilly\n"

bakerloo :: Station->Element0->[Element3] bakerloo dBase findIt = do [ element3 | (element0, element1, element2, element3, element4, element5, element6, element7, element8, element9, element10, element11) <- dBase, element0 == findIt ]

I think that your problem here is that you are mixing the list comprehension and monadic syntax. Delete the 'do' and I think it will work. But you end up with a list of booleans, which is probably not what you want.

stops:: Int->Int->Int stops x y = x-y

main = do mainMenu putStr"\n\nTo Calculate the number of stops between your location and destination enter stop location destination\n" putStr"e.g. Stops 4 14\n" return()

So there it is - any help you can give would be great.

Thanks for all your responses yesterday.

Neil

Rob Dockins Speak softly and drive a Sherman tank. Laugh hard; it's a long way to the bank. -- TMBG