Question

I worked all afternoon on a simple thing but cannot seem to get it right for some reason : how to turn a list into a matrix of given width.

Example :
I got a list such as

[1, 3, 5, 7, 6, 8, 9, 0]

and want to create a matrix such as

[[1, 3],
 [5, 7],
 [6, 8],
 [9, 0]]

through a predicate

list2matrix/3 : list2matrix(List, Size_of_Rows, Matrix).

In this example used like :

list2matrix([1, 3, 5, 7, 6, 8, 9, 0], 2, Matrix).

The predicate should fail if the length of the list is not a multiple of the size of the rows.

I decided not to post my work since I think I got it so wrong that it would not help me to get correction on it ;(

Thanks by advance if you can propose any leads about how to deal with such a problem.

Answer 1

You can divide the problem in two parts.
The first building block would be to build a row of N elements. That is to take the input list and split it in two lists, one will have exactly N elements (the row) and the other is the remaining of the input list.

The second building block would be to build the matrix which is made of rows.

list_to_matrix([], _, []).
list_to_matrix(List, Size, [Row|Matrix]):-
  list_to_matrix_row(List, Size, Row, Tail),
  list_to_matrix(Tail, Size, Matrix).

list_to_matrix_row(Tail, 0, [], Tail).
list_to_matrix_row([Item|List], Size, [Item|Row], Tail):-
  NSize is Size-1,
  list_to_matrix_row(List, NSize, Row, Tail).