6
$\begingroup$

Suppose that I have a data set data (input):

data = {{1, 1, 1}, {2, 1, 2}, {2, 2, 5}, {2, 3, 3}, {2, 4, 4}, {2, 6, 1}, {3,
2, 3}, {3, 3, 3}, {3, 4, 12}, {3, 6, 7}, {4, 4, 4}, {4, 6, 6}, {5, 
6, 1}}

and I want to build from it a matrix mat (output):

mat = {{1,0,0,0,0},{2,5,3,4,0,1},{0,3,3,12,0,7},{0,0,0,4,0,6},{0,0,0,0,0,1}}

where the rule of construction is the following:

the element {x,y,z} of data is mapped onto the element z of mat where z = mat[[x,y]] (i.e z is in the x-row and in the y-column of mat) and all the other elements are zeroes.

I think that that ArrayPad could work, but I dont see how quikly I can get it.

Thanks

$\endgroup$

2 Answers 2

11
$\begingroup$

This is precisely what SparseArray is good for:

SparseArray[data[[All, 1 ;; 2]] -> data[[All, 3]]]
$\endgroup$
2
  • $\begingroup$ Wow! Awesome. Great function, thank you very much! $\endgroup$ Dec 7, 2019 at 17:55
  • $\begingroup$ You're welcome. $\endgroup$ Dec 7, 2019 at 18:39
3
$\begingroup$

Another way

data = {{1, 1, 1}, {2, 1, 2}, {2, 2, 5}, {2, 3, 3}, {2, 4, 4}, {2, 6, 1}, {3, 2, 3}, {3, 3, 3}, {3, 4, 12}, {3, 6, 7}, {4, 4, 4}, {4, 6, 6}, {5, 6, 1}};

dims = Max /@ Transpose@data[[;; , ;; 2]]

mat = ConstantArray[0, dims];
(mat[[#1, #2]] = #3) & @@@ data;
mat

{5,6}

{{1, 0, 0, 0, 0, 0}, {2, 5, 3, 4, 0, 1}, {0, 3, 3, 12, 0, 7}, {0, 0, 0, 4, 0, 6}, {0, 0, 0, 0, 0, 1}}

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.