Nrthugu

Suppose you have

let a = array ((1,1),(2,2)) [((2,1),3),((1,2),2),((1,1),2),((2,2),3)]

Now I want that to multiply the 3 in the last tuple with some number. How can I do that?

If you wanted to multiply that by 5 it would be:

accum (*) a [((2,2),5)]

--     ^  ^   ^     ^

--     function to combine values

--        array to read

--            one of the indices to manipulated

--                  value to give to f for the associated index

The signature of this function reads

accum :: Ix i => (e -> a -> e) -> Array i e -> [(i, a)] -> Array i e

and its documentation says:

accum f takes an array and an association list and accumulates pairs
from the list into the array with the accumulating function f.

So calling accum f arrayValue listWithPairsOfIndicesAndValues will call f for every index given in listWithPairsOfIndicesAndValues providing the old value and the value from listWithPairsOfIndicesAndValues for this index and return a new array with all the positions mentioned in listWithPairsOfIndicesAndValues updated with the values returned by the respective calls to f.

There's two different ways to your goal:

1. 
   

   Using the incremental update:

   
   
   

   a // [((2,2),(3*7))]

   
   
   

   >>> array ((1,1),(2,2)) [((1,1),2),((1,2),2),((2,1),3),((2,2),21)]

   
   

(Instead of 3*7, you could use your own n and refer to that same location (a!(2,2)))

2. 
   

   Using the accumulation:

   
   
   

   accum (*) a [((2,2), 7)]

   
   
   

   >>> array ((1,1),(2,2)) [((1,1),2),((1,2),2),((2,1),3),((2,2),21)]

   
   

One could use a // [(2,2), n * (a ! (2,2))] to multiply the element with index (2,2) by n.

(There should also be some lens-y approach with a better syntax)