New to Haskell - List Comprehension Question
Hi, I'm currently learning Haskell, and I've been trying to work out a function for the following problem for a couple of days now. I want to use a list comprehension method to change the first letter of a string to upper case, and the rest of the string to lower case. Eg: "heLLo" -> "Hello" As I'm trying to learn this, I would appreciate hints rather than the explicit solution if possible? I'm sure I'm close to a solution, I must be missing something though. Driving me crazy! My attempts are something similar to this: upperCase :: String -> String upperCase xs = [toUpper(x):toLower(xs) | x <- xs] I think 'toLower' expects a single character rather than the list which is one place I'm going wrong? Thanks in advance, Steven -- View this message in context: http://www.nabble.com/New-to-Haskell---List-Comprehension-Question-tp2579414... Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.
I don't think a list comprehension is the easiest way to do it, how about upperCase :: String -> String upperCase [] = [] upperCase (x:xs) = toUpper x : map toLower xs -Ross On Oct 7, 2009, at 4:48 PM, Steven1990 wrote:
Hi, I'm currently learning Haskell, and I've been trying to work out a function for the following problem for a couple of days now.
I want to use a list comprehension method to change the first letter of a string to upper case, and the rest of the string to lower case.
Eg: "heLLo" -> "Hello"
As I'm trying to learn this, I would appreciate hints rather than the explicit solution if possible? I'm sure I'm close to a solution, I must be missing something though. Driving me crazy!
My attempts are something similar to this:
upperCase :: String -> String upperCase xs = [toUpper(x):toLower(xs) | x <- xs]
I think 'toLower' expects a single character rather than the list which is one place I'm going wrong?
Thanks in advance, Steven -- View this message in context: http://www.nabble.com/New-to-Haskell---List-Comprehension-Question-tp2579414... Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com .
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Hint: Move the boundary case outside the comprehension, and then use the comprehension to handle the normal case. Also, FYI, a comprehension feeds each value of the list xs into x, and then evaluates the expression to the left of the pipe with that single value of x. Cheers, Greg On Oct 7, 2009, at 1:48 PM, Steven1990 wrote:
Hi, I'm currently learning Haskell, and I've been trying to work out a function for the following problem for a couple of days now.
I want to use a list comprehension method to change the first letter of a string to upper case, and the rest of the string to lower case.
Eg: "heLLo" -> "Hello"
As I'm trying to learn this, I would appreciate hints rather than the explicit solution if possible? I'm sure I'm close to a solution, I must be missing something though. Driving me crazy!
My attempts are something similar to this:
upperCase :: String -> String upperCase xs = [toUpper(x):toLower(xs) | x <- xs]
I think 'toLower' expects a single character rather than the list which is one place I'm going wrong?
Thanks in advance, Steven -- View this message in context: http://www.nabble.com/New-to-Haskell---List-Comprehension-Question-tp2579414... Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.
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2009/10/7 Steven1990
Hi, I'm currently learning Haskell, and I've been trying to work out a function for the following problem for a couple of days now.
I want to use a list comprehension method to change the first letter of a string to upper case, and the rest of the string to lower case.
Eg: "heLLo" -> "Hello"
As I'm trying to learn this, I would appreciate hints rather than the explicit solution if possible? I'm sure I'm close to a solution, I must be missing something though. Driving me crazy!
My attempts are something similar to this:
upperCase :: String -> String upperCase xs = [toUpper(x):toLower(xs) | x <- xs]
I think 'toLower' expects a single character rather than the list which is one place I'm going wrong?
Hi, try to work little things by little things: $ ghci Prelude> let f xs = [x:xs | x <- xs] Prelude> f "hello" ["hhello","ehello","lhello","lhello","ohello"] Prelude> Prelude> :m + Data.Char Prelude Data.Char> :t toLower toLower :: Char -> Char Prelude Data.Char> :t toUpper toUpper :: Char -> Char Prelude Data.Char> So xs is the whole list (the "hello" part of each element in the resilt of f "hello") and x gets the value of each character. toLower and toUpper have the same type; indeed toLower expects a single character while you feed it a list. The part on the left of the pipe is "executed" for each x drawn from the xs list. This means that if you want to make something specific to the first element of xs, you have to provide more information: the x alone is not enough to know it is the first one or not. The easiest way to do that is with pattern matching on the upperCase argument: upperCase (x:xs) = ... (Don't forget for the "other" case, the empty list: upperCase [] = ...) Cheers, Thu
Try just writing a function that will change ALL the characters to uppercase (or lower case) using list comprehension and then see if you can isolate how to factor out the special case of the first character.
Michael
--- On Wed, 10/7/09, minh thu
Hi, I'm currently learning Haskell, and I've been trying to work out a function for the following problem for a couple of days now.
I want to use a list comprehension method to change the first letter of a string to upper case, and the rest of the string to lower case.
Eg: "heLLo" -> "Hello"
As I'm trying to learn this, I would appreciate hints rather than the explicit solution if possible? I'm sure I'm close to a solution, I must be missing something though. Driving me crazy!
My attempts are something similar to this:
upperCase :: String -> String upperCase xs = [toUpper(x):toLower(xs) | x <- xs]
I think 'toLower' expects a single character rather than the list which is one place I'm going wrong?
Hi, try to work little things by little things: $ ghci Prelude> let f xs = [x:xs | x <- xs] Prelude> f "hello" ["hhello","ehello","lhello","lhello","ohello"] Prelude> Prelude> :m + Data.Char Prelude Data.Char> :t toLower toLower :: Char -> Char Prelude Data.Char> :t toUpper toUpper :: Char -> Char Prelude Data.Char> So xs is the whole list (the "hello" part of each element in the resilt of f "hello") and x gets the value of each character. toLower and toUpper have the same type; indeed toLower expects a single character while you feed it a list. The part on the left of the pipe is "executed" for each x drawn from the xs list. This means that if you want to make something specific to the first element of xs, you have to provide more information: the x alone is not enough to know it is the first one or not. The easiest way to do that is with pattern matching on the upperCase argument: upperCase (x:xs) = ... (Don't forget for the "other" case, the empty list: upperCase [] = ...) Cheers, Thu _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
participants (5)
-
Gregory Crosswhite -
michael rice -
minh thu -
Ross Mellgren -
Steven1990