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Steps i:  0 _ _ Index Of Last

i: y is the list of integers from -y to y ; in general, i: a j. b produces the list of numbers from -a to a in b equal steps (or in 1+2*a steps if b is 0). Thus:
   i: 3
_3 _2 _1 0 1 2 3

   i: _2
2 1 0 _1 _2

   i: _2.5j4
2.5 1.25 0 _1.25 _2.5

   i: 0
0
  i: is like i. but gives the index of the last occurrence. Thus:
   1 2 3 4 1 2 3 i: 1 2 2 3 3 3 4 4 5
4 5 5 6 6 6 3 3 7

   1 2 3 4 1 2 3 i. 1 2 2 3 3 3 4 4 5
0 1 1 2 2 2 3 3 7

For example:
   (3 # i.3 4) i: (i.2 4)
2 5

   (3 # i.3 4) i. (i.2 4)
0 3

   (#~ i.@# = i.~) 'eleemosynary'     NB. first occurrence of each letter
elmosynar

   (#~ i.@# = i:~) 'eleemosynary'     NB. last  occurrence of each letter
lemosnary

   (#~ i.~ = i:~) 'eleemosynary'      NB. letters that are unique
lmosnar


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