Function composition in J

Question

This is a pretty simple question but I can't seem to find an answer anywhere -- to map a list of numbers to their percentages of the sum of the list values (e.g., 1 2 2 -> 0.2 0.4 0.4), you can write the function

func =: %+/

but just writing %+/ numbers where numbers is a list of numbers doesn't work -- why is this? Why do you need to put parentheses around the function composition?

Answer 1

J evaluates every expression from right to left, unless it sees a parenthesis (in which case it evaluates the expression in the parenthesis - right to left - and then continues leftwards).

Examples:

1 - 2 - 3 - 4 - 5
3                  NB. because it's: 1 - (2 - (3 - (4 - 5)))

%+/ 1 2 3
0.166667           NB. because it's: % (+/ 1 2 3) -> % (1 + (2 + 3))

(%+)/ 1 2 3
1.5                NB. because it's: 1 (%+) (2 (%+) 3)

Also note that adverbs don't split. i.e. / can't stand on its own.

Answer 2

There are two rules relevant: 1. Expressions in J do not yield to associativity. 2. When J sees a verb, it implicitly adds a parentheses around it.

 %+/ 1 2 2 = % (+/ 1 2 2)!= (%+/) 1 2 2 and 
 func 1 2 2 =  (%+/) 1 2 2 = 1 2 2  % (+/1 2 2) = 0.2 0.4 0.4, which is a hook.  

Answer 3

The answers to the following two FAQs on the J Wiki should help explain why this is the case.

• Sentence Train
• Tacit Train