If name is the same as a or b, the whole operation is done in-place. This form does not create the intermediate result from dyad. There's nothing wrong with that, but J has a little doodad that is faster and uses less space, as long as you want to assign the result to a name. You could simply writeĪnd have the answer. Suppose you have two arrays a and b and a Boolean list m, and you want to create a composite list from a and b using each item of m to select the corresponding item of either a (if the item of m is 0) or b (if 1). You can specify a fill atom, but if you do you must bond x to # rather than giving it as a left operand:ġ 1 0 0 1^:_1!.'x' 'abc' abxxc Choose From Lists Item-By-Item: monad m} X #^:_1 y creates an array with the items of y in the positions corresponding to nonzero items of the Boolean vector x, and fills in the other items. :^:_1 y converts y from a list of boxed strings to a single character string with spaces between the boxed strings. This is a good way to take all items of y if x is 1, or no items if x is 0. The fill atom is the usual 0, ' ', or a: depending on the type of y, but the fit conjunction !.f may be used to specify f as the fill:įinally, a scalar x is replicated to the length of y. If an item of x is complex, the imaginary part tells how many cells of fill to insert after making the copies of the item of y. The items of x actually tell how many copies of the corresponding item of y to include in the result:īoolean x, used for simple selection, is a special case. X # y does not require that x be a Boolean list. I will discuss some of them briefly here. To keep my discussion from wandering too far afield I left out a number of useful features of J. Special Verb-Forms Used in Tacit Definitions.Operations Inside Boxes: u L: n, u S: n.Choose From Lists Item-By-Item: monad m}.Displaying Tabular Data: the Grid Control.Factors and Primes: Monad p:, Monad and Dyad q:.Extended Integers, Rational Numbers, and x:.Monad I.-Indexes of the 1s in a Boolean Vector.Passing an Executable Sentence: Monad ".Passing the Definition Of a Verb: 128!:2 (Apply).Extracting Variable-Length Fields Using ^: and.Apply On Specified Partitions: Dyad u.Asynchronous Sockets and socket_handler.Format Data For Printing: Monad And Dyad ":.Treating a File as a Noun: Mapped Files.Shining a Light: The J Performance Monitor.Use Large Verb-Ranks! and Integrated Rank Support.Compounds Recognized by the Interpreter.Apply Under Transformation: u&.v and u&.:v.What really happens during m :n and verb define.Power/If/DoWhile Conjunction u^:n and u^:v.Making a Dyad Into a Monad: u&n and m&v.Making a Monad Into a Dyad: The Verbs.Boxing As an Equivalent For Structures In C.When Dyad Frames Differ: Operand Agreement.Verb Execution-How Rank Is Used (Dyads).Controlling Verb Execution By Specifying a Rank.Verb Execution-How Rank Is Used (Monads).Valence of Verbs (Binary and Unary Operators).
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