# accessing and modifying matrix element

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!

• ReplacePart[A, a -> 1]? Nov 29 '21 at 14:00
• ReplacePart[A, a -> 1] will create a new matrix that is what you want. You can assign that to A.
– Alan
Nov 29 '21 at 14:01
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• SparseArray[{2, 3} -> 1, {4, 9}]; % // MatrixForm Nov 29 '21 at 14:12
• Try: A[[Sequence @@ a]] = 1 Nov 29 '21 at 16:22

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


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

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


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

• What info? I just want to change line 3 in the example code I've given above. My problem: I have a matrix and a pair and I want to change the entry of the matrix corresponding to my pair. Nov 29 '21 at 16:05
• Cool, thanks! I guess this is slightly more readable. Nov 29 '21 at 16:17
• Thanks for the accept. @PeterMüller. The idea behind editing is that your post should be self-contained so that future visitors to the page should be able to read and understand the question without having to sift through the comments. See you later.
– Syed
Nov 29 '21 at 16:21