myfirsttime1 - 3 years ago 196
Python Question

# Python: Generating a symmetric array with list comprehension

I am trying to generate a matrix

`A == [f(i,j) for i,j in range(0,n)]`
. The matrix is symmetric
`(f(i,j) == f(j,i))`
and the diagonal elements are zero
`(f(i,i) == 0)`
.

Question: Is it possible to produce a list comprehension that generates this matrix but that only calls the function
`f(i,j)`
for
`i < j`
?

Not my question: Generate the symmetric matrix in some other way.

Possible solution: Make the call to
`f`
through an auxiliary function
`g`
that saves the value of
`f`
in an additional storage or returns the stored value.

Would it be possible to solve avoiding the additional storage?
I am not sure if this additional handicap implies that a reference to the list comprehension from itself (which I read doesn't exist in Python) is necessary, hopefully I am missing some other trick.

Assuming you really need the full matrix, in spite of its being symmetric, you could do this.

Create the matrix A first. (I know, this means you're not generating it in a list comprehension.) Here I define a 'dummy' function f and assume a value of 3 for n.

``````>>> n=3
>>> subs=((i,j) for i in range(n) for j in range(3) if i<=j)
>>> for i,j in subs:
...     i,j
...
(0, 0)
(0, 1)
(0, 2)
(1, 1)
(1, 2)
(2, 2)
>>> def f(i,j):
...     return
...
>>> def g(i,j):
...     if i==j:
...         A[i,i] = 0
...     else:
...         val=f(i,j)
...         A[i,j]=val
...         A[j,i]=val
``````

If I've understood you correctly, there's no need for extra storage. Call g in a list comprehension that exercises subs.

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