Arrays in J programming Language

Question

How does one do array accesses in the J programming language? For example, using C++ as my pseudocode language:

int M [100];  // declare an array called M
int j = 5;  //index into the array
int y = 10;  //value to store or load from the array

M[j] = y;  // store y into the array

y = M[j];  // load y from the array

What would these sorts of array accesses look like in idiomatic J?

Lethia

RosettaCode might be a better forum for this type of comparison. – Chrono

Answer 1

The literal (but still pretty idiomatic) way to write this in J would be as follows.

m =: 100 $ 0   NB. This means create a 1d array consisting of 100 zeros.
j =: 5
y =: 10

With that initialization out of the way, now we're ready for the meat of the answer, which consists of two different usages of the } adverb ("Item Amend" and "Amend").

m =: y j } m

Putting two arguments to the left of the } causes J to replace the jth element of the right hand argument m with the value y. NOTE: we had to assign the result back in to m because the result of y j } m was simply to compute a new array which incorporated the change that you requested using the } verb.

y =: j } m

Putting only one argument to the left of the } causes J to excerpt the jth element of m and return it. In this case, we set y to the result.

Cavit

Note that the amend above will be executed "in place". J recognizes that the new array is being assigned to the same name and only writes the changes to the new array. See the jsoftware.com/jwiki/Essays/In-Place%20Operations . In my experience the dyadic verb From jsoftware.com/help/dictionary/d520.htm {is more often used to retrieve items from an array. – Chrono

Very good point, @Tikkanz. And thanks for the corrections in your edit. – Cavit

OMG! I had no idea amend } had the single left argument feature! I just use the regular take {... – Dulcinea

@MPelletier, @Tikkanz's correction that } is technically an adverb helped me internalize an important clarification here. Namely, in my examples above, the actual verb ends up being (j }), which when applied dyadically results in an amended array and when applied monadically does the same thing as Take. – Cavit

The appearance of the phrase "two arguments to the left" telegraphs that misunderstanding. J syntax never involves two arguments to a single side. When arguments apply to a dyadic verb, or conjunction, it's an infix form with one argument on each side. In this case j is an argument to the adverb and y an argument to the resulting verb. – Ferrand