[Numpy-discussion] triangular matrix fill

[Robert Kern]

On Thu, May 22, 2008 at 9:07 PM, Charles R Harris
<charlesr.harris at gmail.com> wrote:
>
> On Thu, May 22, 2008 at 7:19 PM, Tom Waite <twaite at berkeley.edu> wrote:
>>
>> I have a question on filling a lower triangular matrix using numpy. This
>> is essentially having two loops and the inner loop upper limit is the
>> outer loop current index. In the inner loop I have a vector being
>> multiplied by a constant set in the outer loop. For a matrix N*N in size,
>> the C the code is:
>>
>> for(i = 0; i < N; ++i){
>>     for(j = 0; j < i; ++j){
>>         Matrix[i*N + j] = V1[i] * V2[j];
>>     }
>> }
>>
>
> You can use numpy.outer(V1,V2) and just ignore everything  on and above the
> diagonal.
>
> In [1]: x = arange(3)
>
> In [2]: y = arange(3,6)
>
> In [3]: outer(x,y)
> Out[3]:
> array([[ 0,  0,  0],
>        [ 3,  4,  5],
>        [ 6,  8, 10]])
>
> You can mask the upper part if you want:
>
> In [16]: outer(x,y)*fromfunction(lambda i,j: i>j, (3,3))
> Out[16]:
> array([[0, 0, 0],
>        [3, 0, 0],
>        [6, 8, 0]])
>
>  Or you could use fromfunction directly.

Or numpy.tril().

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless
enigma that is made terrible by our own mad attempt to interpret it as
though it had an underlying truth."
 -- Umberto Eco