Jsoftware recently released a free J app for the iPhone. Search for “jsoftware” in Apple’s app store and you will land right on it. There are many excellent free iPhone apps, I have half-a-dozen on my iPhone, but this little jewel sets a new standard for power in your palm.

Let’s start with the good news; this is not a crippled version of J. It’s the same high-caliber interpreter that J programmers have used on Windows, Linux and Mac machines for years. Anything this app’s big desktop brothers can do this little app can do. You won’t see the same blistering desktop speed but I think you’ll be pleasantly surprised at how fast this app munches numbers. I have run desktop J since the 1990’s and J on the iPhone 4 matches or beats what I was seeing on laptops ten years ago. This is the real J deal.

Now for a few caveats — this J app is not a complete J development environment. To meet Apple’s restrictive app store rules C-style APIs, JAL addons, GUI tools and third-party libraries like opengl and lapack are not included. With this app you get the J interpreter and a few of the most useful J addons like plot. Despite these limitations this J app is probably the most powerful, purely local, calculator available for the iPhone.

Don’t believe me? Turn off Wi-Fi and set your iPhone to Airplane mode: Airplane mode cuts off phone and internet access.  Now try computing the following on your favorite iPhone calculator.

NB. generate one million random numbers and average them
(+/ % #) ? 1000000#10000

NB. generate a 50 50 random matrix, invert it and multiply with the
NB. original - rounding to the nearest 0.0001 to form an identity matrix
round=: [ * [: <. 0.5 + %~
matrix=: ? 50 50 $ 10000
invmat=: %. matrix
identity=: 0.0001 round matrix +/ . * invmat

NB. sum of the diagonal elements of matrix (identity) is 50
50 = +/ (<0 1) |: identity

NB. multiply two polynomials with complex number coefficients
polyprod=:  +//.@(*/)

NB. complex number coefficients - AjB is J's notation for A + Bi
poly0=: 2j5 3j7 0 1
poly1=: 1j2 0 3j7 0 0 2
poly0 polyprod poly1

NB. prime factorization table of 50 random integers less than one billion
(<"0 nums) ,: -.&0 &.> <"1 q: nums=.50?1e9

The J app blows through these examples on my iPhone 4 in a few seconds. Of course this is only a tiny taste of what J on the iPhone is capable of. I have managed to run 1000+ line J scripts on this app. The only desktop J code I have not been able to run depends on external compiled libraries like regex.

Cell phones are powerful little computers and it’s gratifying to finally see software that can focus that power on something other than Angry Birds.  If you’re interested in learning an array oriented functional programming language or if you’re already familiar with J and want to pack serious computational heat then this app is for you!

J running on the iPhone