accessing and modifying matrix element

Question

I basically want to do the following

A = Array[0 &, {4, 9}];
a = {2, 3};
A[[a[[1]], a[[2]]]] = 1;
A // MatrixForm

but with less cumbersome notation.(i.e. change line 3 to something nicer)!

I get the position as a pair (a list with 2 elements) and want to change the matrix entry at the position which that pair encodes. The pair is returned by the functions "RandomChoice" and "Position".

Thanks for the help!

Answer 1

This is my best interpretation of what the OP wants.

alist = RandomChoice[{"banana", "apple", "orange", "grapes"}, 9]

{"grapes", "orange", "orange", "banana", "orange", "orange", \
"grapes", "banana", "banana"}

Suppose the positions of "orange" have to be detected.

pos = Flatten@Position[alist, "orange"]

{2, 3, 5, 6}

A[[3, pos]] = 1
A // MatrixForm

$$\left( \begin{array}{ccccccccc} 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ 0 & 1 & 1 & 0 & 1 & 1 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ \end{array} \right)$$

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EDIT

Assuming that a pair of (valid) numbers is to be used to change the entry at that index to 13:

pos = {3, 8}  (* received from some function *)
A[[First@pos, Last@pos]] = 13
A // MatrixForm

$$\left( \begin{array}{ccccccccc} 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ 0 & 1 & 1 & 0 & 1 & 1 & 0 & 13 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ \end{array} \right)$$