# Fifty Shades of J/Chapter 27

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### Principal Topics

- “ (rank conjunction) . (suffix), |: (transpose), ~ (passive) determinants, minors, cofactors, subtotallng.

A frequent requirement in applied mathematics and statistics is to evaluate sums and products omitting just one variable or dimension. The notion of ‘all but one’ can be interpreted in two ways, depending whether the ‘one’ is to be systematically omitted, or obtained by a merge with an existing dimension, that is by reduction.

### Retaining all but one

As an example of the first case, adding or multiplying ‘all but one’ items in a list progressively can be done by using the hook f^{-1} ~f which means {f/x}f^{-1} x, for example :

i.5 0 1 2 3 4 (-~+/)i.5 NB. sum 5 items omitting one at a time 10 9 8 7 6 (%~*/)1+i.5 NB. multiply 5 items omitting one at a time 120 60 40 30 24

This extends readily to objects of higher rank :

]b=:i.3 4 0 1 2 3 4 5 6 7 8 9 10 11 (-"1~+/)b NB. sums of all rows but one 12 14 16 18 8 10 12 14 4 6 8 10 (-"2~+/"1)b NB. sums of all columns but one 6 5 4 3 18 17 16 15 30 29 28 27

The rule is that the rank operand for the left verb should be one greater than that of the right verb.

### Generalising ‘retaining all but one’

Another tool which deals with ‘retaining all but one’ is the suffix adverb which eliminates n items from a list in all possible ways :

(1+\.i.5);(2+\.i.5);(3+\.i.5) ┌───────┬─────┬───┐ │1 2 3 4│2 3 4│3 4│ │0 2 3 4│0 3 4│0 4│ │0 1 3 4│0 1 4│0 1│ │0 1 2 4│0 1 2│ │ │0 1 2 3│ │ │ └───────┴─────┴───┘

In the J line above `+` is monadic, which for real numbers is a ‘do nothing’ verb, that is the left arguments 1, 2 and 3 are not added to elements of `i.5` but are arguments of the derived verb `+\`. which indicate how many items are to be dropped progressively working from the left. The first case with 1 as left argument is thus the ‘all but one’ case.

An application of this is the calculation of minors of a determinant. Consider the rank 2 object `z` :

]z=:3 3$3 6 7 9 3 2 9 7 7 3 6 7 9 3 2 9 7 7 (1&(+\.))"2 z NB. select ‘all but’ one rows 9 3 2 9 7 7 3 6 7 9 7 7 3 6 7 9 3 2

Now combine the structural verb transpose with the adverb suffix to switch rows and columns for all but one row at a time :

transuff=:1&(|:\.)"2 <"2 transuff z ┌───┬───┬───┐ │9 9│3 9│3 9│ │3 7│6 7│6 3│ │2 7│7 7│7 2│ └───┴───┴───┘

and the result is a list of the transposed planes of `(1&(+.))"2 z`.

Do this twice using the power conjunction to generate three boxes within each of the above boxes; the row/column switch is fortuitously reversed and the minors of z obtained :

<"2 transuff^:2 z ┌───┬───┬───┐ │3 2│9 2│9 3│ │7 7│9 7│9 7│ ├───┼───┼───┤ │6 7│3 7│3 6│ │7 7│9 7│9 7│ ├───┼───┼───┤ │6 7│3 7│3 6│ │3 2│9 2│9 3│ └───┴───┴───┘

Define

minors=:transuff^:2 NB. minors unboxed det=:-/ .* NB. determinant

The determinants of the minors are given by

]dz=:det every <"2 minors z 7 45 36 _7 _42 _33 _9 _57 _45

This is verified by using the verb `det` dyadically :

(det z);dz det |:z ┌─┬──────┐ │3│3 0 0│ │ │0 _3 0│ │ │0 0 3│ └─┴──────┘

It is often convenient to use cofactors, that is the signed determinants of minors. This requires multiplication by a matching matrix whose diagonals are alternately +1 and -1. One way of obtaining this matrix is :

signs=:monad : '-_1+2*2|+/~i.#y

'

so that

cof=:signs * det every @<"2@minors cof z 7 _45 36 7 _42 33 _9 57 _45

To summarise this section :

transuff=:1&(|:\.)"2 NB. transpose with suffix minors=:transuff^:2 NB. minors unboxed det=:-/ .* NB. determinant signs=:monad :'-_1+2*2|+/~i.#y.' NB. alternate 1,_1 cof=:signs * det every@<"2@minors NB. cofactors

### Reducing all but one

Rank again is at the heart of the matter, especially as typical experimental data is structured so that dimensions correspond to variables. However, J inherently structures higher rank data as lists, then lists of lists, lists of lists of lists and so on, which implies a nesting within dimensions which is not usually reflected in the ‘real world’ variables to which the dimensions correspond. For definiteness use `q` to demonstrate :

]q=:i.2 3 4 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

The standard definition of rank is:

rk=:#@$ NB. rank

`+/` inserts +'s between the list at the topmost level, thereby reducing rank by one, that is merging planes :

+/q NB. equivalent to +/"n q for n>2 12 14 16 18 20 22 24 26 28 30 32 34

Explicit rank conjunctions allows such reduction to take place at any level in the rank hierarchy :

(+/"1 q);(+/"2 q) NB. merge cols ; merge rows ┌────────┬───────────┐ │ 6 22 38│12 15 18 21│ │54 70 86│48 51 54 57│ └────────┴───────────┘

Progressive reduction to the level of a rank 1 object is obtained using a recursive verb :

sum=: sum@(+/)`] @.(=&1@rk) sum q

60 66 72 78

The above values are readily verifiable as the column sums of `q` . To find other such sums, say row sums, transpose the data so as to bring the corresponding dimension to the top level. This suggests a general verb which takes the list `i.n` and performs the a set of n possible shifts necessary to bring each item in turn into the leading position :

shifts=:|.each< NB. all distinct shifts of a list shifts i.rk q ┌─────┬─────┬─────┐ │0 1 2│1 2 0│2 0 1│ └─────┴─────┴─────┘

If the argument to shifts is a list generated by `i.`, the result is left arguments to transpose which provide all the restructured forms of `q` to which sum can be applied. This in turn is determined by the rank of the data matrix, so define

targs=:shifts@(i.@rk) NB. arguments for |: targs q ┌─────┬─────┬─────┐ │0 1 2│1 2 0│2 0 1│ └─────┴─────┴─────┘

The full set of transpositions to supply all possible sums by dimension is then

transposes=:targs |:each < NB. reqd. transposes transposes q ┌───────────┬─────┬────────┐ │ 0 1 2 3│ 0 12│ 0 4 8│ │ 4 5 6 7│ 1 13│12 16 20│ │ 8 9 10 11│ 2 14│ │ │ │ 3 15│ 1 5 9│ │12 13 14 15│ │13 17 21│ │16 17 18 19│ 4 16│ │ │20 21 22 23│ 5 17│ 2 6 10│ │ │ 6 18│14 18 22│ │ │ 7 19│ │ │ │ │ 3 7 11│ │ │ 8 20│15 19 23│ │ │ 9 21│ │ │ │10 22│ │ │ │11 23│ │ └───────────┴─────┴────────┘

These values are readily verifiable by summing the columns in the boxes above :

sum each@transposes q ┌───────────┬──────┬─────────┐ │60 66 72 78│66 210│60 92 124│ └───────────┴──────┴─────────┘

To give these sums in dimension order, that is so that the `$each` of the result matches the shape of `q`, write

allsums=:1&|.@(sum each@transposes) allsums q ┌──────┬─────────┬───────────┐ │66 210│60 92 124│60 66 72 78│ └──────┴─────────┴───────────┘

To summarise this section :

rk=:#@$ NB. rank sum=: sum@(+/)`] @.(=&1@rk) NB. sum down to rank 1 shifts=:|.each < NB. all shifts of i.n targs=:shifts@(i.@rk) NB. arguments for |: transposes=:targs |:each < NB. reqd. transpositions allsums=:1&|.@(sum each@transposes)

For comparison, here is a one-liner picked from the J Software forum some years ago which performs the same function by using the power conjunction `^:` to apply `+/` the requisite number of times with transpositions required between steps as in the version above :

mt=:(<@(+/^:(<:@rk)@:|:)"0 _~i.@rk) mt q ┌──────┬─────────┬───────────┐ │66 210│60 92 124│60 66 72 78│ └──────┴─────────┴───────────┘

### Subtotaling

It can be useful to be able to append such reductions to the original data as in :

total=:,+/ NB. append totals for leading dimension sub=:3 :0 i=.0 [ r=.total y while.(i<<:rk y)do.r=.total"i r [ i=.i+1 end. ) sub q 0 1 2 3 6 4 5 6 7 22 8 9 10 11 38 12 15 18 21 66 12 13 14 15 54 16 17 18 19 70 20 21 22 23 86 48 51 54 57 210 12 14 16 18 60 20 22 24 26 92 28 30 32 34 124 60 66 72 78 276

Multi-statement lines using `[` as a separator as in sub allow something approaching the succinctness of tacit definition. However the individual statements are executed from the right since `[` itself is just another verb. It is easy to remember that it is `[` rather than `]` which is the separator, since, for example, it is `0` to the left which is assigned to `i` in the first line of sub. As far as the J interpreter is concerned it is really only one line which is executed; multiple lines are essentially just an orthographic device.

### Code Summary

minors=: transuff^:2 NB. minors unboxed transuff=: 1&(|:\.)"2 NB. transpose with suffix det=: -/ .* NB. determinant cof=: signs * det every@<"2@minors NB. cofactors signs=: monad :'-_1+2*2|+/~i.#y.' NB. alternate 1,_1 transposes=: targs |:each < NB. reqd. transpositions targs=: shifts@(i.@rk) NB. arguments for |: shifts=: |.each < NB. all shifts of i.n rk=: #@$ NB. rank allsums=: 1&|.@(sum each@transposes) sum=: sum@(+/)`] @.(=&1@rk) NB. sum down to rank 1 total=: ,+/ NB. append totals for leading dimension sub=: 3 :0 i=: 0 [ r=: total y while.(i<<:rk y)do.r=: total"i r [ i=: i+1 end. )