# converting capital letters into small letters

Garner, Robin Robin.Garner@crsrehab.gov.au
Fri, 26 Jul 2002 17:27:49 +1000

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> 1. How would I convert capital letters into small letters?
>
> 2. How would I remove vowels from a string?

I expect the first problem is actually to convert the upper case letters in
a string to lower case, as the single-character version isn't a very
interesting problem.

A little thought shows that the first problem can be expressed as both a
catamorphism or an anamorphism.  The anamorphic solution would be [(p,f)]
where
p as = as == [] and
f (a:as) = (makeLowerCase a, bs).

The solution to the second problem is clearly a catamorphism, and one
solution might be (|[],g|) where
g (a:as) = if isVowel a then as else a:as

The definitions of 'makeLowerCase' and 'isVowel' are quite simple (assuming
English/Ascii, as pointed out in other messages), while finding the prelude
functions for the patterns of recursion might take a little more thought.
Suitable, possible, as homework questions in an introductory Haskell course
...

Robin

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<TITLE>RE: converting capital letters into small letters</TITLE>
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<P><FONT SIZE=3D2>&gt; 1. How would I convert capital letters into =
small letters?</FONT>
<BR><FONT SIZE=3D2>&gt; </FONT>
<BR><FONT SIZE=3D2>&gt; 2. How would I remove vowels from a =
string?</FONT>
</P>

<P><FONT SIZE=3D2>I expect the first problem is actually to convert the =
upper case letters in a string to lower case, as the single-character =
version isn't a very interesting problem.</FONT></P>

<P><FONT SIZE=3D2>A little thought shows that the first problem can be =
expressed as both a catamorphism or an anamorphism.&nbsp; The =
anamorphic solution would be [(p,f)] where </FONT></P>

<P><FONT SIZE=3D2>&nbsp;&nbsp;&nbsp; p as =3D as =3D=3D [] and </FONT>
<BR><FONT SIZE=3D2>&nbsp;&nbsp;&nbsp; f (a:as) =3D (makeLowerCase a, =
bs).</FONT>
</P>

<P><FONT SIZE=3D2>The solution to the second problem is clearly a =
catamorphism, and one solution might be (|[],g|) where</FONT>
<BR><FONT SIZE=3D2>&nbsp;&nbsp;&nbsp; g (a:as) =3D if isVowel a then as =
else a:as</FONT>
</P>

<P><FONT SIZE=3D2>The definitions of 'makeLowerCase' and 'isVowel' are =
quite simple (assuming English/Ascii, as pointed out in other =
messages), while finding the prelude functions for the patterns of =
recursion might take a little more thought.&nbsp;&nbsp; Suitable, =
possible, as homework questions in an introductory Haskell course =
...</FONT></P>

<P><FONT SIZE=3D2>Robin</FONT>
</P>

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