Some functions in APL can be used to select portions of an array. When
associated with assignment, they can be used to assign values to
portions of such array.
Bracket indexing is the easiest example.
Let's assign the first three elements of a vector by using the Take
(dyadic ↑) function.
VEC←⍳5
(3↑VEC)←'ABC'
8⎕CR VEC
┌→──────┐
│ABC 4 5│
└───────┘
Let's use the Ravel (monadic ,) on a matrix to assign a new vector
value to it:
MAT←3 4⍴'ABCDEFGHIJKL'
(,MAT)←'NEW DATAHERE' ⍝ Ravelled matrix appears as a vector
8⎕CR MAT ⍝ Assignment occurs in matrix itself
┌→───┐
↓NEW │
│DATA│
│HERE│
└────┘
Now let's combine Compression (dyadic /) and Ravel (monadic ,) to
select all A's on the matrix, and replace them by asterisk:
(('A'=,MAT)/,MAT)←'*'
8⎕CR MAT
┌→───┐
↓NEW │
│D*T*│
│HERE│
└────┘
We can also combine Take (dyadic ↑) and Ravel (monadic ,) to replace
elements at the top-left 2×2 submatrix of MAT:
(,2 2↑MAT)←'⎕⎕⎕⎕'
8⎕CR MAT
┌→───┐
↓⎕⎕W │
│⎕⎕T*│
│HERE│
└────┘
We can also use the Compression (/) function for selection.
TABLE←3 4⍴⍳12
8⎕CR TABLE
(1 0 1 0/TABLE)←3 2⍴100
8⎕CR TABLE
┌→─────────┐
↓1 2 3 4│
│5 6 7 8│
│9 10 11 12│
└──────────┘
┌→────────────┐
↓100 2 100 4│
│100 6 100 8│
│100 10 100 12│
└─────────────┘
In the next example, we have a vector X. We want to replace the first
⍴X elements of DATA with the contents of X.
DATA←⍳13
X←10×⍳3
8⎕CR DATA
((⍴X)↑DATA)←X
8⎕CR DATA
┌→────────────────────────────┐
│1 2 3 4 5 6 7 8 9 10 11 12 13│
└─────────────────────────────┘
┌→───────────────────────────────┐
│10 20 30 4 5 6 7 8 9 10 11 12 13│
└────────────────────────────────┘
Replace the first X+2 elements of Y with the reverse of a vector
containing numbers 1 up to X+2:
Y←⍳10
X←3
8⎕CR Y
((2+X)↑Y)←⌽⍳X+2
8⎕CR Y
┌→───────────────────┐
│1 2 3 4 5 6 7 8 9 10│
└────────────────────┘
┌→───────────────────┐
│5 4 3 2 1 6 7 8 9 10│
└────────────────────┘
We can use Enlist (monadic ∊) to remove nesting from an array.
8⎕CR NEST←(2 2⍴⍳4) 'TEXT' (3 1⍴⍳3)
(∊NEST)←0
8⎕CR NEST
⍝ Set specific position to number
(6⌷∊NEST)←999
8⎕CR NEST
⍝ Set specific position to character vector (text).
⍝ For that, introduce extra nesting to the new text.
(7⌷∊NEST)←⊂'TEXT'
8⎕CR NEST
┌→───────────────┐
│┌→──┐ ┌→───┐ ┌→┐│
│↓1 2│ │TEXT│ ↓1││
││3 4│ └────┘ │2││
│└───┘ │3││
│ └─┘│
└∊───────────────┘
┌→──────────────────┐
│┌→──┐ ┌→──────┐ ┌→┐│
│↓0 0│ │0 0 0 0│ ↓0││
││0 0│ └───────┘ │0││
│└───┘ │0││
│ └─┘│
└∊──────────────────┘
┌→────────────────────┐
│┌→──┐ ┌→────────┐ ┌→┐│
│↓0 0│ │0 999 0 0│ ↓0││
││0 0│ └─────────┘ │0││
│└───┘ │0││
│ └─┘│
└∊────────────────────┘
┌→─────────────────────────┐
│┌→──┐ ┌→─────────────┐ ┌→┐│
│↓0 0│ │0 999 ┌→───┐ 0│ ↓0││
││0 0│ │ │TEXT│ │ │0││
│└───┘ │ └────┘ │ │0││
│ └∊─────────────┘ └─┘│
└∊∊────────────────────────┘
The function First (monadic ↑) selects the first element of an
array. Here, we shall replace the first 2×2 matrix of NEST by a
character vector.
So, the sky is the limit. For more info, see this page.
.png)
1 day ago
1
