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u"n Rank Conjunction
Applies the verb u to each cell in turn of an array y, or to corresponding cells of x and y . The "parts" are called n-cells, the operand n determining the size of the n-cell.
For other uses of (") see:
- Constant Verb (m"n)
- Apply a Different Verb on Each Cell (gerund"n)
- Copy Rank (u"v) (m"v)
(") is the most-often-used modifier in J. It corresponds to the simplest form of looping in other languages.
u"n creates a new verb whose rank is n. This new verb applies u to the n-cells (for the given operand n) of its argument y, and collects the results from each cell into a single result.
] y=: i. 2 3 0 1 2 3 4 5 u=: < NB. u is the primitive verb: Box u y NB. Apply Box (<) to the whole of y +-----+ |0 1 2| |3 4 5| +-----+ u"1 y NB. Apply Box (<) to each 1-cell (i.e. each row (=list)) of y +-----+-----+ |0 1 2|3 4 5| +-----+-----+ u"0 y NB. Apply Box (<) to each 0-cell (i.e. each atom) of y +-+-+-+ |0|1|2| +-+-+-+ |3|4|5| +-+-+-+
Applying a Different Verb on Each Cell
If u is a gerund, it specifies a sequence of verbs. The first verb is applied to the first cell, the second verb to the second cell, and so on, wrapping around if there are more cells than verbs in the gerund.
+:`*:`%:"0 (2 3 4) NB. double the first, square the second, square-root the third 4 9 2
Common uses
1. Sum a matrix (an array of rank 2) across rather than down.
] y=: i.2 2 0 1 2 3 +/ y NB. sum down 2 4 +/"1 y NB. sum across 1 5
Verb: +/"1 applies the verb +/ to the 1-cells (i.e. the lists) of y (a list of lists). This sums each list.
2. Select items 1 and 2 from each row of a table y
] y =: i. 3 4
0 1 2 3
4 5 6 7
8 9 10 11
1 2 { y NB. Select items 1 and 2 - not what we want (treated as rows 1 and 2)
4 5 6 7
8 9 10 11
1 2 {"1 1 y NB. Select items 1 and 2 from each row; i.e. the entire columns 1 and 2
1 2
5 6
9 10
Note: n can be a list as well as an atom. {"1 1 specifies 1-cells for x and y in turn.
3. Rotate each row of an array y by a different amount
]y =: 4 $ ,: 'a b c d ' a b c d a b c d a b c d a b c d 1 0 _1 0 |."0 1 y b c d a a b c d a b c d a b c d
The verb |. is applied between 0-cells of x and 1-cells of y . In other words, each atom of x tells how much to rotate the corresponding row of y.
Note: n can be a list as well as an atom. The phrase {"0 1 specifies 0-cells for x and 1-cells for y in turn.
4. Guide execution of a verb to desired cells
Cards =. '6789TJQKA' ,"0"1 0 (ucp '♠♡♢♣') NB. Join (atoms of) (lists of x with atoms of y) <"2 Cards NB. Box by suit ┌──┬──┬──┬──┐ │6♠│6♡│6♢│6♣│ │7♠│7♡│7♢│7♣│ │8♠│8♡│8♢│8♣│ │9♠│9♡│9♢│9♣│ │T♠│T♡│T♢│T♣│ │J♠│J♡│J♢│J♣│ │Q♠│Q♡│Q♢│Q♣│ │K♠│K♡│K♢│K♣│ │A♠│A♡│A♢│A♣│ └──┴──┴──┴──┘
Note: x v"0 y is equivalent to x v"0 0 y
Note: (,) append has a rank of _ _
More Information
1. Important point: u"n does not change the rank of u.
Every verb has a rank of its own that can never be changed at call-time. u"n creates a new verb that is executed on each n-cell of the operands. But the actual rank of u doesn't get suppressed. It comes into play when u is executed on these n-cell(s).
2. Because u"n causes u to be executed multiple times, it resembles looping in other languages.
Multiple application of u"n correspond to nested loops, as shown by "More Examples" 1 below.
3. Every verb has both monadic rank and dyadic rank, with the appropriate rank being selected depending on whether the verb is executed as a monad or as a dyad.
The monadic rank is a single number; the dyadic rank is two numbers, one for x and one for y.
You may think of your verb as just a monad or just a dyad, but nothing prevents someone from trying to execute it with either valence.
(But sometimes J may signal an execution error.)
u"n always specifies both the monadic and dyadic ranks. n itself may be an atom or a list of up to 3 numbers; the ranks will be set as follows:
The ranks specified by n n Monadic rank Dyadic rank left right n n n n n0 n1 n1 n0 n1 n0 n1 n2 n0 n1 n2
4. u"n may specify a negative value in n. u is still executed on n-cells, but with n negative the rank of the cell depends on the rank of the argument.
<"_1 i. 3 NB. Operand rank 1; _1-cells are atoms +-+-+-+ |0|1|2| +-+-+-+ <"_1 i. 2 3 NB. Operand rank 2; _1-cells are lists +-----+-----+ |0 1 2|3 4 5| +-----+-----+
5. To see what the rank of a verb u is, execute u b. 0
+ b. 0 0 0 0 +"0 1 b. 0 1 0 1
6. For a more relaxed explanation of Rank, see Rank in a hurry, which contains links to more authoritative treatments of the concept of Rank and its uses.
More Examples
1. Create a Punnett square representing a dihybrid cross.
NB. First, a verb to create the result of a single breeding NB. x and y are alleles (R, r and A, a) cross =: dyad define alleles =. (>x) , (>y) NB. join the genotypes < alleles /: 'RrAa' i. alleles NB. sort into canonical order ) NB. Example: a single cross (<'Ra') cross (<'rA') +----+ |RrAa| +----+ NB. Define all the possible genotypes genotypes =: ;: 'RA Ra rA ra' NB. Create the Punnett square of all possible crosses genotypes cross"0 0"0 1 genotypes +----+----+----+----+ |RRAA|RRAa|RrAA|RrAa| +----+----+----+----+ |RRAa|RRaa|RrAa|Rraa| +----+----+----+----+ |RrAA|RrAa|rrAA|rrAa| +----+----+----+----+ |RrAa|Rraa|rrAa|rraa| +----+----+----+----+
Explanation of the verb cross"0 0"0 1 :
- The "0 1 means, Apply the verb cross"0 0 on each atom of x and each list of y. Since x and y are both lists, this means, Apply cross"0 0 4 times, with y each time equal to genotypes and x set to each atom of genotypes in turn.
- A single application of cross"0 0 will then have an atom for x and the list genotypes for y. The "0 0 means, Apply cross 4 times, with x each time equal to the atom, and y set to each atom of genotypes in turn.
- The result of each application of cross is a single box. Each use of "0 0 collects these into a list, which is its result.
- "0 1 collects these lists into a table, which is the final result.
Details
1. u can be a cyclic gerund as in the example given above. Because gerunds are nouns, and non-gerund noun values of u produce constant verbs, a cyclic gerund is recognized only when:
- the rank n is not all infinite
- u is a list of boxes (not an atom)
- u is a well-formed gerund.
Otherwise m"n is contrued as a constant verb.
2. The rank of a verb cannot actually be negative. If n is negative in u"n, the actual rank of u"n will be infinite, but u"n will apply u to n-cells of the argument.
<"_1 0 0 b. 0 NB. Actual rank _, but applied on _1-cells _ 0 0
3. In place of the operand (noun) n, you can use any verb v. This is equivalent to setting n equal to the list of ranks of verb v. See Copy Rank.
4. Coding u"n corresponds to writing a loop in other languages. The verb u will be executed multiple times, once for each cell of the arguments. This may be very inefficient. Consider the example:
10 #."0 1 i. 10000 6
The "0 1 instructs the interpreter to find the 0-cells of x (there is only one) and the 1-cells of y (there are 10000), and execute #. for each pair (the cell of x is repeated for each execution). The execution of #. requires converting x into an array of weights, which takes more time than applying the weights; this overhead is incurred 10000 times. Much better is
10 #. i. 10000 6
Because x #. y is defined with intrinsic rank 1 1, it knows to break y into 1-cells, and it knows to replicate an atomic x. There is minimal overhead.
If you time both sentences, however, you will see that they run at the same speed, because the interpreter cynically assumes you didn't mean what you wrote. The interpreter follows this rule: if u"n operates on the same cells of the argument(s) that u itself would, the rank conjunction is ignored. (read on to see the exception)
5. The suppression of a rank conjunction can rarely produce an incorrect result. Fill has to be involved:
]r =. 2 2 $ 1 2;3;4;5 +---+-+ |1 2|3| +---+-+ |4 |5| +---+-+ >"1 r 1 2 3 0 4 0 5 0
Even though > y was executed on the 4 boxes individually, this result is not right! If the bottom two boxes had been opened as a 1-cell of boxes, as the rank conjunction specified, they would have been joined together with no fill. To tell the interpreter than a rank conjunction must not be suppressed, specify the rank in floating-point:
>"1. r 1 2 3 0 4 5 0 0
This is the correct result.
6. All arithmetic primitives, and a few compounds, have Integrated Rank Support (IRS). Verbs with IRS have the rank loops built into the verb. For these verbs the cost of of using u"n is negligible.