J is my calculator tags: pl

April 30, 2020.

I used to use node (javascript) as my CLI calculator. (I know, I know.) After realizing how silly I was for using such an inexpressive language, I tried to learn calc and bc. The next few I went through were:

python
ghci (Haskell)
R
octave (matlab equivalent)
and finally, J

I don’t have a good reason to settle with J. Maybe I’ll change it soon. But here’s my review of J, collected slowly over a few months of usage:

Pros:

Hassle-free rationals: write 1%2r127+3r160 instead of 1/((2/127)+(3/160)), and get 20320r701 ( $\frac{20320}{701}$ ) as a result.
No need to repeat an operator. Write */12 52 65 13 42 rather than 12*52*65*13*42
Built-in polynomial evaluation at multiple points. Write _4 2 1 1 p. 1 2 3 to evaluate x³+x²+2x-4 at x={1,2,3}.
The idiom for convolution (polynomial multiplication) is +//. @: (*/) (take outer product */ and sum +/ the antidiagonals /.)
Any user defined function is an operator just like +
Hooks and forks make some expressions incredibly easy to write. Examples:
- (+/%#) 2.252 2.284 evaluates to the mean of the list: 2.268
- 2.268 (-,+) 0.016 evaluates to the interval 2.252 2.284
- (f,g,h) x evaluates multiple functions on the same data x (say, mean median range)
- recent example for me: (13.342-7.666)%%:+/18.140 14.378(*:@[*%)578 379 evaluates $\frac{13.342-7.666}{\sqrt{\frac{18.140^2}{578}+\frac{14.378^2}{379}}}$

Cons:

You can only share with other J users (not like you’re going to share one-off calculations though)
Need to get used to how every operator associates right without exception
Have to understand how J evaluates things in order to refactor expressions
Configuration is not obvious: e.g. (9!:11)4 sets displayed floating point precision to 4.
Jagged or non-homogenous arrays requires you to learn the messy art of value boxing/unboxing
Working in hex (or any base) is a hassle if you want hex output. You can input hex with 16bdeadbeef for example, but all output is in decimal (unless you pass it through the hex-printing (4+#@$) }. 2&(3!:3) or use the printf addon).
It’s hard to find the right operator for the job if you don’t know it by heart. It’s just like in mathematics. Languages like J suffer from lack of discoverability.

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