sort ← {(⊂⍋⍵)⌷⍵}
group ← {(1,2≠/sort i)⊂⍋i←⍳⍨⍵}
group 'Mississippi'
┌─┬────────┬───────┬───┐
│0│1 4 7 10│2 3 5 6│8 9│
└─┴────────┴───────┴───┘
iself ← {⍳⍨ ⊂[1↓⍳⍴⍴⍵]⍵}
group1 ← {(1,2≠/sort i)⊂⍋i←iself ⍵}
t←'IBM' 'AAPL' 'GOOG' 'GOOG' 'AAPL'
⎕←x←↑t,'MSFT' 'IBM' 'AAPL' 'AAPL' 'IBM'
IBM
AAPL
GOOG
GOOG
AAPL
MSFT
IBM
AAPL
AAPL
IBM
group1 x
┌─────┬───────┬───┬─┐
│0 6 9│1 4 7 8│2 3│5│
└─────┴───────┴───┴─┘
group2 ← ⊂⍯ ⍝ v14.0
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