6 回答
The trouble with these:
(f t)[x]
x⌷f t
x⊃f t
is that both t and f get evaluated.
If you want to short-circuit the thing, you can use guards:
{x:t ⋄ f}
This is equivalent to
if (x) {
return t;
}
f;
in a C-like language.
Yes, there are APL programmers on SO (but not many!).
I think the answer is that there is no standard on this.
For a scalar solution, I use "pick":
x⊃f t
While for a Boolean array I use indexing as you do above:
f t[x]
I always use index origin zero, so there is no need to add 1, and the parens are not needed.
If these are not simple enough, I think you have to cover them with a function named "if". That will also let you put the true and false in the perhaps more natural ordering of t f.
An old, old idiom which did something like C's ternary operator ? : and returned a result was the following:
r←⍎(¯3 3)[x=42]↑'6×8 ⋄ 6×7'
Note that this is written for origin 0 and the parens around the -3 3 are there for clarity.
x=42 evaluates to zero or one, depending on this answer we choose -3 or 3, and thus select and execute either the first 3 elements ("6x8") or last 3 elements ("6x7") of the string. The diamond ⋄ is just there for decoration.
Needless to say, one would probably not code this way if one had :if :else avaiable, though the control structure form would not return a result.
The APL expression:
(1+x=0)⌷y z
should be the C language equivalent for
x?y:z
And all the others
(1+x>0)⌷y z
for
x<=0?y:z
Etc. In general if a b c are expressions of respective languages the APL expression:
(1+~a)⌷b c
It should be equivalent to the C language:
a?b:c