How to stop sage graph generating iterator after N iterations?

Question

I want to create N planar graphs using sagemath’s graph.planar_graphs iterator. The problem is that this function return all possible planar graphs given a certain number of vertices. What I want to do is something like:

while len(gen) < N:
    gen = list(graphs.planar_graphs(50))

So in the end, I want a list with the first N generated planar graphs instead of all of them (since thats many orders of magnitudes more graphs and computationally very expensive).

Asked By: LucTuc

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Source

Answer 1

One nice thing about iterators is you can grab
as many or as few elements as you want.

Iterator for all planar graphs of order 50:

sage: planar_50 = graphs.planar_graphs(50)

Grab the first four:

sage: planar_50_4 = [next(planar_50) for _ in range(4)]

Check:

sage: planar_50_4
[Graph on 50 vertices,
 Graph on 50 vertices,
 Graph on 50 vertices,
 Graph on 50 vertices]

Turn that into a function:

def planar_graphs_order_k_first_n(k, n):
    pg = graphs.planar_graphs(k)
    return [next(pg) for _ in range(n)]

Use it:

sage: planar_graphs_order_k_first_n(50, 4)
[Graph on 50 vertices,
 Graph on 50 vertices,
 Graph on 50 vertices,
 Graph on 50 vertices]

Answered By: Samuel Lelièvre

How to stop sage graph generating iterator after N iterations?

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