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<p class=MsoNormal>Hi all,<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>I was looking for an APL-style “iota” function
for array indices. I noticed<o:p></o:p></p>
<p class=MsoNormal>“range” from Data.Ix which, with a zero for the
lower bound (here (0,0)),<o:p></o:p></p>
<p class=MsoNormal>gives the values I need:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>> let (a,b) = (2,3)<o:p></o:p></p>
<p class=MsoNormal>> index ((0,0),(a-1,b-1))<o:p></o:p></p>
<p class=MsoNormal>> [(0,0),(0,1),(0,2),(1,0),(1,1),(1,2)]<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>However, I need the results as a list of lists rather than a
list of tuples; and<o:p></o:p></p>
<p class=MsoNormal>my input is a list of integral values. I ended up writing the
following function<o:p></o:p></p>
<p class=MsoNormal>instead. The function isn’t long, but longer than I
first expected. Did I miss a<o:p></o:p></p>
<p class=MsoNormal>simpler approach?<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>iota :: (Integral a) => [a] -> [[a]]<o:p></o:p></p>
<p class=MsoNormal>iota is = let count = product is<o:p></o:p></p>
<p class=MsoNormal>
tups = zip (tail $ scanr (*) 1
is) is<o:p></o:p></p>
<p class=MsoNormal>
buildRepList (r,i) =
genericTake count $ cycle $<o:p></o:p></p>
<p class=MsoNormal>
[0..i-1]
>>= genericReplicate r<o:p></o:p></p>
<p class=MsoNormal>
lists = map buildRepList tups<o:p></o:p></p>
<p class=MsoNormal>
in transpose lists<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>> length $ iota [2,3,4]<o:p></o:p></p>
<p class=MsoNormal>> 24<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Thanks,<o:p></o:p></p>
<p class=MsoNormal>Paul<o:p></o:p></p>
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