P-M - 9 months ago 43

Python Question

I have a network where each of my edges is labelled with a date. I want to now also label my vertices so that each vertex has a date assigned to it corresponding to the minimum date of all edges incident and emanating from it. Is there an inbuilt function to find this which would be faster than me looping over all vertices and then all edges for each vertex manually? In other words: I am after a function to find the minimum value of a given edge property for a given subset of edges.

My current code idea is:

`lowest_year = 2016`

for v in g.vertices():

for e in v.in_edges():

year = g.ep.year[e]

lowest_year = min(year,lowest_year)

for e in v.out_edges():

year = g.ep.year[e]

lowest_year = min(year,lowest_year)

g.vp.year[v]=lowest_year

lowest_year = 2016

Answer

There is hardly any solution that would not need to check all the edges to find the minimum.

You could however optimize your calls to `min`

by making a single call on the entire data instead of multiple calls and you also wouldn't need `lowest_year`

any longer:

```
from itertools import chain
for v in g.vertices():
g.vp.year[v] = min(map(g.ep.year.__getitem__, chain(v.in_edges(), v.out_edges())))
```

Methods `in_edges`

and `out_edges`

both returns lists which you can easily merge with the `+`

operator.

In a more general case, you would use `itertools.chain`

when oblivious of the types you're merging, but `+`

is better in this case since we know the items are lists.

Source (Stackoverflow)