CODEWITHSUNDEEP
matrixprologidentity
Saul Garcia
Saul Garcia

Reputation: 940

Creating an Identity Matrix in Prolog

I have to write the predicate identity/2 which receives a number n, and makes an identity matrix [n x n].

Example:

identity(3,I).
I = [[1, 0, 0], [0, 1, 0], [0, 0, 1]];

For this one I do not even know how to start. At least an insight on how to build a simple list of n elements could provide me a good starting point! Thank you!

Upvotes: 1

Views: 313

Answers (1)

Daniel Lyons
Daniel Lyons

Reputation: 22803

Well, the first thing you need to do is worry about your base case, which I'll just give you:

identity(1, [[1]]).

Now you need to make it work inductively for the rest.

Personally, I would write some helper predicates, like this to produce a list of zeroes:

zeroes(0, []).
zeroes(N, [0|Rest]) :- succ(N0, N), zeroes(N0, Rest).

You can also generate lists of arbitrary size using length/2:

?- length(X, 3).
X = [_G1563, _G1566, _G1569].

Other predicates that might be helpful to you: nth1/3:

?- length(X, 3), nth1(1, X, foo), nth1(3, X, bar).
X = [foo, _G1658, bar].

And don't forget you can prepend to a list just by using [X|Rest]. :) Good luck!

Upvotes: 3

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