Question

I need some help converting the following nested for loop into a list comprehension.

adj_edges = []
for a in edges:
    adj = []
    for b in edges:
        if b[0] < a[0] and b[1] >= a[0] and b[1] <= a[1]:
            adj.append(b)
    adj_edges.append((a[0], adj))

where edges is a list of lists like this [[0, 200], [200, 400]]
I’ve used list comprehensions before but i don’t know why im having trouble with this one.

Answer 1

here it is:

adj_edges = [(a[0], [b for b in edges if b[0] < a[0] and b[1] >= a[0] and b[1] <= a[1]]) for a in edges]

this is an interactive session showing the two alternative approaches:

>>> edges = [[0, 200], [200, 400]]
>>> adj_edges = []
>>> for a in edges:
...     adj = []
...     for b in edges:
...         if b[0] < a[0] and b[1] >= a[0] and b[1] <= a[1]:
...             adj.append(b)
...     adj_edges.append((a[0], adj))
... 
>>> adj_edges
[(0, []), (200, [[0, 200]])]
>>> [(a[0], [b for b in edges if b[0] < a[0] and b[1] >= a[0] and b[1] <= a[1]]) for a in edges]
[(0, []), (200, [[0, 200]])]