Superscripts and subscripts

Marks after a token that apply an operation to it or select from it

A superscript, such as ², is a mark written straight after a token that applies an operation to it. A subscript, such as ₁, selects from it instead. Both bind more tightly than any function. v₁² is the square of item 1 of v:

v←10 20 30
v₁²

Typing glyphs gives their keys.

Superscripts

On an array

On an array, a superscript digit is a power:

x←3
≥x²

A power of ¯1, written ⁻¹, is the reciprocal:

2⁻¹

0.5

ᵀ swaps rows and columns:

m←[1 2
   3 4]
mᵀ

1 3
2 4

In 2 3², the superscript touches only the 3:

2 3²

2 9

In [2 3]², it touches the whole list:

[2 3]²

4 9

After a space, a superscript applies to the whole run on its left:

2 3 ²

4 9

The run can be any expression, such as ≥x:

≥x ²

A strand of names is several runs, one for each name:

y←5
x y ²

3 25

A spaced superscript also needs a space after it:

2 3 ² + 1

5 10

On a function

On a function, a superscript digit is a repeat count, as in Power. f² is f⍣2:

inc←≥
inc² 5

f⁻¹ is the inverse of f, the same as f⍣¯1. f⁻¹ Y gives the X for which f X is Y. Inverses follow BQN’s specification of inferred properties. The inverse of the exponential * is the natural logarithm:

*⁻¹1

Monadic Encode, ⊤, gives a number’s binary digits. Its inverse reads the digits back into a number:

⊤⁻¹1 0 1

In a dyadic call, X f⁻¹ Y is the Z for which X f Z is Y. The inverses of 2× and 2* are division by 2 and the base-2 logarithm:

2×⁻¹10
2*⁻¹8

Inverting Replicate puts fill where the mask has zeros:

1 0 1#⁻¹10 20

10 0 20

A superscript applies to the whole function on its left, as a monadic operator does. +\⁻¹ turns running sums back into the numbers they came from:

+\⁻¹1 3 6

1 2 3

The same goes for a derived function such as -¨:

-¨⁻¹1 2 3

¯1 ¯2 ¯3

A bound function such as 2↣× inverts as a whole too:

2↣×⁻¹10

A superscript just after a number applies to the number, as a power. Without parentheses, +↢3⁻¹ adds a third. To invert a function that ends in a number, put the function in parentheses:

(+↢3)⁻¹ 10

A space before the superscript also works:

+↢3 ⁻¹ 10

With the axis operator, the inverse can also go before ⍠. #⁻¹⍠¯1 is the same function as (#⍠¯1)⁻¹.

Inverses follow the rules of Power. They pass through bound arguments, Each, Rank and the other compositions that Power lists. f⁻¹←g attaches an inverse to any function that a name holds.

ᵇ, Bitwise, is a monadic operator that applies a Boolean function to integer bits. For example, 6∧ᵇ3 is 2ₓ.

A function as a scalar

fᵘ is a scalar holding the function f, as ⊂ encloses a subject. ⊂ can’t enclose a function, because written next to one it forms a train. Like any scalar, +ᵘ has an empty shape:

⍴+ᵘ

⍬ₓ

Its depth counts the one layer around the function:

≡+ᵘ

1ₓ

Errors

DOMAIN: a function with no known inverse, ᵀ on a function, or ᵘ on an array
SYNTAX: a superscript after a space that doesn’t stand alone, as in 2 3 ²+1

Subscripts

A subscript selects a major cell of the item just before it. v₁ is 1⌷v:

v₁

A negative subscript counts from the end:

v₋₁

On a matrix, a subscript selects a row:

m₁

3 4

After a dotted path, a subscript selects from the whole path:

T←["a":[10 20]]
T.a₁

With a space before it, a subscript selects from the whole run on its left:

2 3 ₁

As with a superscript, the run can be any expression:

≥v ₁

A spaced subscript also needs a space after it.

A subscripted item can be an assignment target:

v₁←5
v

10 5 30

Modified assignment works too:

v₁+←1
v

10 6 30

Use a subscript for one literal position on the leading axis. Use Index, ⌷, for a computed position, several positions, a key or another axis.

The subscript letters ₓ, ⱼ, ₑ and ᵣ are parts of numbers, as in 3ₓ and 1ᵣ3. Numbers covers them. Between two other items, such as names, ⱼ, ₑ and ᵣ join the items into one number: xⱼy is x+y×0ⱼ1. After a name, or any item but a number or a list of literals, ₓ is ⌊:

x←3.7 ¯2.5
xₓ

[3 ¯3]ₓ

Errors

SYNTAX: a subscript after a space that doesn’t stand alone, as in 2 3 ₁+1, or that is an assignment target, as in v ₁←5
SYNTAX: ₋ with no digit after it
SYNTAX: ⱼ, ₑ or ᵣ beside a function or a space, as in f←- ⋄ fⱼ1
INDEX: the position is outside the leading axis
LENGTH: the item is a unit, which has no axis to select from
DOMAIN: ₓ after a function, as in +ₓ

Differences from APL

BPL also accepts Dyalog’s x*2, f⍣2 and f⍣¯1. BQN writes the inverse as 𝔽⁼. Neither language has subscripts.