Compute the product of the next n elements in matrix

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I would like to compute the product of the next n adjacent elements of a matrix. The number n of elements to be multiplied should be given in function's input. For example for this input I should compute the product of the 3 consecutive elements, starting from 1.
[product, ind] = max_product([1 2 2 1 3 1],3);
This gives (4,4,6,3).
Is there any practical way to do it? Now I do this using
for ii=1:(length(v)-2)
p=prod(v(ii:ii+n-1));
where v is the input vector and n is the number of elements to be multiplied.
Depending whether n is odd or even or length(v) is odd or even, I get sometimes right answer but sometimes the following error
Index exceeds matrix dimensions.
Error in max_product (line 6)
p=prod(v(ii:ii+n-1));.
Is there any correct general way to do it?
  6 Comments

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Answers (6)

Walter Roberson
Walter Roberson on 4 Sep 2016
hint: cumprod divided by cumprod
  4 Comments
John D'Errico
John D'Errico on 4 Sep 2016
Edited: John D'Errico on 4 Sep 2016
Of course, if the vector is long with elements that are larger than 1, expect this to overflow and turn the result into inf, then when you divide, you have inf/inf, so nans will result.
Or, if the elements are less than 1, then you will get underflows, which become zero. Then 0/0 is also NaN.
As well, even for some cases where overflow does not result, you may experience some loss of precision in the least significant bits, if the intermediate products exceed 2^53-1.
But for short vectors with reasonable numbers in them, this will work.

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Matt J
Matt J on 4 Sep 2016
Edited: Matt J on 4 Sep 2016
function prodout = max_product(A,n)
prodout = exp( conv(log(A),ones(1,n),'valid') );
if isreal(A), prodout=real(prodout); end
end
Just to be clear, this will handle input with zeros and negatives, e.g.,
>> prodout=max_product([0 2 2 1 3 -1], 3)
prodout =
0 4.0000 6.0000 -3.0000
  2 Comments
Matt J
Matt J on 4 Sep 2016
Edited: Matt J on 5 Sep 2016
Thanks, John, but as far as (1) is concerned, note that I gave an example showing it works with non-positive values as well. As for (2), I think you could modify the code to detect integer input and post-round the result in that case.

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Andrei Bobrov
Andrei Bobrov on 4 Sep 2016
Edited: Andrei Bobrov on 4 Sep 2016
[prodout, ind] = max_product(A,n)
ii = 1:numel(A);
ind = hankel(ii(1:n),ii(n:end));
prodout = prod(A(ind));
end
or just
max_product = @(A,n)prod(hankel(A(1:n),A(n:end)));

Steven Lord
Steven Lord on 19 Feb 2018
I believe you want to use the movprod function introduced in release R2017a.

Srishti Saha
Srishti Saha on 11 Mar 2018
This should work. has been tested and refined:
function B = maxproduct(A,n)
% After checking that we do not have to return an empty array, we initialize a row vector % for remembering a product, home row and column, and one of four direction codes.
[r,c] = size(A);
if n>r && n>c
B = []; % cannot be solved
return
end
L = [-Inf,0,0,0]; % [product, home-row, home-col, direction]
for i=1:r
for j=1:c-n+1
L = check(A(i,j:j+n-1),[i,j,1],L); % row, right case
end
end
for i=1:r-n+1
for j=1:c
L = check(A(i:i+n-1,j),[i,j,2],L); % column, down case
end
end
for i=1:r-n+1
for j=1:c-n+1
S=A(i:i+n-1,j:j+n-1);
L = check(diag(S),[i,j,3],L); % diagonal, down case
L = check(diag(flip(S,2)),[i,j,4],L); % reverse diagonal, down case
end
end
i=L(2); j=L(3); % reconstruct coordinates
switch L(4)
case 1, B = [ones(n,1)*i,(j:j+n-1)'];
case 2, B = [(i:i+n-1)',ones(n,1)*j];
case 3, B = [(i:i+n-1)',(j:j+n-1)'];
case 4, B = [(i:i+n-1)',(j+n-1:-1:j)'];
end
end
function L = check(V,d,L)
p = prod(V);
if p>L(1) % if new product larger than any previous
L = [p,d]; % then update product, home and direction
end
end

michio
michio on 4 Sep 2016
In the spirit of avoiding for-loops...
x = 1:10; n = 3 % Example
N = length(x);
index = zeros(N-n+1,n);
index(:,1) = 1:N-n+1';
index(:,2) = index(:,1) + 1;
index(:,3) = index(:,1) + 2;
prod(x(index),2)
  2 Comments
michio
michio on 4 Sep 2016
Oops. You are exactly correct. The above script works as intended only when n = 3.

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