Vocabulary/Variables
Variables
J does not have "variables" as such. Instead it has the concepts of noun and pronoun, the latter being a name to which a noun happens to be assigned in the current namespace.
This isn't just a matter of J's quirky naming conventions. A pronoun is radically different from a (C) "variable". This difference is masked when using (=.) or (=:) in the J session, or in the body of an explicit verb. But as soon as you come to write your own tacit verb, it becomes clear just how different a pronoun is from a "variable".
C rests heavily on the notion of a "variable", which is essentially the old Assembler notion of a named memory location. The programmer can think of it as a named container into which a value can be placed. That value can be both retrieved and altered at any time. Thus if a given C expression contains the name of the container, e.g. Z, then as soon as the expression is executed, the token Z is replaced with its value, whatever it happens to be at the time.
It follows that Z can serve as a container for the result of a partial computation. This leads to a characteristic style of programming which can't be replicated in tacit J. Nor would you want to.
Z = 10 Z = 2 * (Z = Z+1) + Z
Note that a C "variable" is not at all like a mathematical variable. The latter is the symbol for an arbitrary member of some set. Thus if z is a mathematical variable (a member of the set Z of natural numbers, say), the proposition:
Let z=z+1
cannot be satisfied by a choice of z from Z. In other words the subset of Z to which z refers is empty.
Now, inside the body of an explicitly defined J verb, it is possible to translate the above C statements to J sentences thus:
z =. 10 z =. 2 * z + (z =. z+1) NB. a different order of execution
The results will be the same.
However what happens in J when these sentences are executed is this
- The noun (10) is assigned to the name z (which thereby becomes a pronoun)
- The phrase (z+1) is evaluated and the resulting noun is assigned to the name z (overwriting its existing value)
- The phrase (2 * z + z) is evaluated and the resulting noun is assigned to the name z.
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