Rho
⍴ — Shape or Reshape — Keystroke ALT+r — Character 9076 or x2374
Note: This is ALT+r; ALT+SHIFT+R will generate the symbol root (√).
Rho may be used as a monadic or dyadic operator, depending on the functionality desired. Used monadically with the argument to the right returns the shape of the argument. Used dyadically, the argument on the left is used to reshape the argument on the right.
Shape  Monadic ⍴
Symbol ⍴ when used monadically (no left argument) returns the shape of ⍴'s right argument, i.e. the size or dimensions of a scalar, string, vector, array or matrix. Shape returns null for a scalar or a single character, it returns 0 for a null string, the length of a string of at least two characters, the number of elements of an array or vector, and the number of elements of a matrix.
⍴ 1 2 3 4 5 6 ⍝ Rho in the case of vectors represents a Count(# elements) function 6 ⍴'abcdefg' ⍝ Similarly with text strings(vector of chars), Rho represents a Count function 7 ⍴ 1 ⍝ For a scalar right argument, Rho generates a null vector. ⍴⍴ 1 ⍝ The shape of the shape(⍴⍴) of a scalar (because the shape of a scalar is a null)  generates zero. 0 ⍴ 'a' ⍝ Ditto for {numbers, integers, letters}, etc.  for scalar right arguments: Rho also generates a null vector. ⍴⍴ 'a' 0 ⍴, 4.581 ⍝ Rho for a raveled(, or comma used) scalar(real num 4.581)  again correctly represents a Count(# elements) function 1 2 3⍴⍳6 ⍝ Dyadic call for demonstration purposes(see below for dyadic calls to ⍴): result = 2 rows and 3 cols matrix of integers(1..6) 1 2 3 4 5 6 ⍴2 3⍴⍳6 ⍝ Monadic LEFTMOST shape ⍴ yields: matrix has 2 rows and 3 cols; equivalent to ⍴(2 3⍴⍳6) which has parentheses added for clarity 2 3 ⍴⍴(2 3⍴⍳6) ⍝ Monadic LEFTMOST shapeshape or ⍴⍴ generates the RANK of the 2 row, 3 col matrix which = 2 (matrix) 2 ⍴⍴(⍳6) ⍝ Monadic leftmost ⍴⍴ generates the RANK of the vector = 1 (vector) 1
Reshape  Dyadic ⍴
⍴'s Left Argument(L) outputreshapes ⍴'s Right Argument(R).


L is a scalar or vector of nonnegative integers, and R is an arbitrary array.  
If the right argument is empty, the result consists of ×/⍴L copies of the fill item of R. 
For example, in origin 1(where ⎕IO=1)
2 3⍴⍳6
1 2 3
4 5 6
2 3⍴⍳0
0 0 0
0 0 0
The idea for the latter enhancement was taken from Dyalog APL.
See Also  
System Commands  System Variables and Functions  Operators 
Keyboard  
A+S  ⍪  ≡  ≢  ⍒  ⍋  ⌽  ⍉  ⊖  ⍟  ⍱  ⍲  ⍠  ⌹  
Alt  ⋄  ¨  ¯  <  ≤  ∅  ≥  >  ≠  ∨  ∧  ×  ÷  
Sh  ~  !  @  #  $  %  ^  &  *  (  )  _  +  
Key  `  1  2  3  4  5  6  7  8  9  0    =  
A+S  ⍷  √  ⍨  ⍸  ⍥  ⍣  ⍞  ⍬  ⊣  
Alt  ?  ⍵  ∊  ⍴  ∼  ↑  ↓  ⍳  ○  π  ←  →  ⊢  
Sh  Q  W  E  R  T  Y  U  I  O  P  {  }    
Key  q  w  e  r  t  y  u  i  o  p  [  ]  \  
A+S  §  ⍢  ⍙  ⍤  ⌷  
Alt  ⍺  ⌈  ⌊  ∞  ∇  ∆  ∘  ‼  ⎕  ⍎  ⍕  
Sh  A  S  D  F  G  H  J  K  L  :  "  
Key  a  s  d  f  g  h  j  k  l  ;  '  
A+S  ⊆  ⊇  ⍡  ⍭  ⊙  
Alt  ⊂  ⊃  ∩  ∪  ⊥  ⊤  ⍦  ⍝  ⍀  ⌿  
Sh  Z  X  C  V  B  N  M  <  >  ?  
Key  z  x  c  v  b  n  m  ,  .  / 
NARS 2000 Lang Tool Bar 
←  →  +    ×  ÷  *  ⍟  ⌹  ○  !  ?  √    ⌈  ⌊  ⊥  ⊤  ⊣  ⊢  
=  ≠  ≤  <  >  ≥  ≡  ≢  ∨  ∧  ⍱  ⍲  ↑  ↓  ⊂  ⊃  ⌷  ⍋  ⍒  
⍳  ∊  ⍸  ⍷  ∪  ∩  ⊆  ⊇  ~  §  π  ..  ,  ⍪  ⍴  ⌽  ⊖  ⍉  
/  \  ⌿  ⍀  ⊙  ¨  ⍨  ⍤  ⍦  ⍥  .  ∘  ⍠  ‼  ⌻  ⍞  ⎕  ⍎  ⍕  
⋄  ⍝  ∇  ∆  ⍙  _  ⍺  ⍵  ¯  ⍬  ∞  ∅  b  e  g  p  r  v  x 
See Also
NARS 2000 Lang Tool Bar 
←  →  +    ×  ÷  *  ⍟  ⌹  ○  !  ?  √    ⌈  ⌊  ⊥  ⊤  ⊣  ⊢  
=  ≠  ≤  <  >  ≥  ≡  ≢  ∨  ∧  ⍱  ⍲  ↑  ↓  ⊂  ⊃  ⌷  ⍋  ⍒  
⍳  ∊  ⍸  ⍷  ∪  ∩  ⊆  ⊇  ~  §  π  ..  ,  ⍪  ⍴  ⌽  ⊖  ⍉  
/  \  ⌿  ⍀  ⊙  ¨  ⍨  ⍤  ⍦  ⍥  .  ∘  ⍠  ‼  ⌻  ⍞  ⎕  ⍎  ⍕  
⋄  ⍝  ∇  ∆  ⍙  _  ⍺  ⍵  ¯  ⍬  ∞  ∅  b  e  g  p  r  v  x 