Find
⍷

Find (⍷
) is a dyadic primitive function which indicates where the left argument appears as a contiguous subarray of the right argument.
Examples
Both arguments can be arrays of any shape. The entire left argument is tested against each position in the right argument. The result is a boolean array having the same shape as the right argument, where a 1 indicates the position of the first element of the matched subarray (which can be seen as the "leftmost" or "top left" position in case of a vector or matrix). If the left argument has lower rank, it is treated as if the shape is prepended with ones. If the left argument has higher rank, Find does not error, but it is never found in the right argument (resulting in an allzero array).
'ANA'⍷'BANANA' ⍝ Matches may overlap
0 1 0 1 0 0
WEEK
SUNDAY
MONDAY
TUESDAY
WEDNESDAY
THURSDAY
FRIDAY
SATURDAY
'DAY'⍷WEEK ⍝ Find the pattern 'DAY' in WEEK; right arg may have higher rank
0 0 0 1 0 0 0 0 0
0 0 0 1 0 0 0 0 0
0 0 0 0 1 0 0 0 0
0 0 0 0 0 0 1 0 0
0 0 0 0 0 1 0 0 0
0 0 0 1 0 0 0 0 0
0 0 0 0 0 1 0 0 0
WEEK⍷'DAY' ⍝ WEEK not found in 'DAY'; left arg may have higher rank but it is never found
0 0 0
For nested arrays, Find tests for exact match between the elements.
'BIRDS' 'NEST'⍷'BIRDS' 'NEST' 'SOUP'
1 0 0
Model
Find can be modelled as follows (for nonempty left arguments), where all possible subarrays of the right argument are checked to see if they match the left argument:^{[1]}
ebar←{⎕IO←0
r←(≢⍴⍺)⌈≢⍴⍵ ⍝ maximum rank
r>≢⍴⍺:(⍺⍴⍨(⍴⍺),⍨(r≢⍴⍺)⍴1)∇ ⍵ ⍝ if ⍺ has lesser rank, make it the same rank
(⍴⍺)∨.>r↑(⍴⍵),¯1:(⍴⍵)⍴0 ⍝ return 0s if ⍺ has greater rank or is longer
ww←⍵
(⍴⍵) ↑ ⍺∘{⍺≡(⍴⍺)↑⍵↓ww}¨ ⍳(×⍴⍺)+(⍴⍵)⍴⍺
}
Empty left argument
Implementations differ in their treatment of empty left arguments:
 APL2, GNU APL, NARS2000, and Dyalog APL indicate positions where the left argument can fit, even if the prototypes don't match.
 APLX never finds any empty arrays.
 APL+ finds empty arrays everywhere, even where they would extend beyond the edges of the right argument.
The prototype is never used, in contrast to Match, which in many APLs compares prototypes of empty arrays. The behavior may come from the use of the inner product ∧.=
in early flat APLs where Match is not a primitive; this function naturally checks elements and not the prototype. Adin Falkoff presented code for Findlike functions using ∧.=
at APL79.^{[2]}
Correspondingly, the above model can be amended to match the behaviour of the primitive (in APL2, etc.) by replacing the ≡
with {(⍺≡⍥⍴⍵)∧(∧/⍺≡¨⍥,⍵)}
which ignores prototypes, only comparing shape and elements:^{[3]}
ebar2←{⎕IO←0
r←(≢⍴⍺)⌈≢⍴⍵ ⍝ maximum rank
r>≢⍴⍺:(⍺⍴⍨(⍴⍺),⍨(r≢⍴⍺)⍴1)∇ ⍵ ⍝ if ⍺ has lesser rank, make it the same rank
(⍴⍺)∨.>r↑(⍴⍵),¯1:(⍴⍵)⍴0 ⍝ return 0s if ⍺ has greater rank or is longer
ww←⍵
(⍴⍵) ↑ ⍺∘{⍺ {(⍺≡⍥⍴⍵)∧(∧/⍺≡¨⍥,⍵)} (⍴⍺)↑⍵↓ww}¨ ⍳(×⍴⍺)+(⍴⍵)⍴⍺
}
See also
External links
Documentation
 Dyalog
 APLX
 J Dictionary, NuVoc
 BQN
References
 ↑ Hui, Roger. ⍷ follies. Dyalog Forums. 16 Feb 2021.
 ↑ Falkoff, Adin. A note on pattern matching: Where do you find the match to an empty array? at APL79.
 ↑ Brudzewsky, Adám. RE: ⍷ follies. Dyalog Forums. 23 Aug 2022.