嵌套列表理解，其中内部循环范围取决于外部循环

Question

I am trying to represent the following as a list comprehension:

L = []
for x in range(n):
    for y in range(x):
        L.append( (x, y) )

I have done nested list comprehension in the more typical matrix scenario where the inner loop range is not dependent on the outer loop.

I have considered there may be solutions in itertools , using product() or chain() but have been unsuccessful there as well.

Answer 1

Remember to wrap the x, y in parentheses this is the only slight caveat that if omitted leads to a SyntaxError .

Other than that, the translation is pretty straightforward; the order of the for s inside the comprehension is similar to that with the nested statements:

n = 5
[(x, y) for x in range(n) for y in range(x)]

Yields similar results to its nested loop counterpart:

[(1, 0),
 (2, 0),
 (2, 1),
 (3, 0),
 (3, 1),
 (3, 2),
 (4, 0),
 (4, 1),
 (4, 2),
 (4, 3)]

Answer 2

Below is the example to convert your code to list comprehension.

>>> n = 10
>>> [ (x,y) for x in range(n) for y in range(x)]
[(1, 0), (2, 0), (2, 1), (3, 0), (3, 1), (3, 2), (4, 0), (4, 1), (4, 2), (4, 3), (5, 0), (5, 1), (5, 2), (5, 3), (5, 4), (6, 0), (6, 1), (6, 2), (6, 3), (6, 4), (6, 5), (7, 0), (7, 1), (7, 2), (7, 3), (7, 4), (7, 5), (7, 6), (8, 0), (8, 1), (8, 2), (8, 3), (8, 4), (8, 5), (8, 6), (8, 7), (9, 0), (9, 1), (9, 2), (9, 3), (9, 4), (9, 5), (9, 6), (9, 7), (9, 8)]

Alternatively, you achieve the same result using itertools library (sharing it just for your knowledge info, but is not recommended for this problem statement):

>>> import itertools
>>> list(itertools.chain.from_iterable(([(list(itertools.product([x], range(x)))) for x in range(n)])))
[(1, 0), (2, 0), (2, 1), (3, 0), (3, 1), (3, 2), (4, 0), (4, 1), (4, 2), (4, 3), (5, 0), (5, 1), (5, 2), (5, 3), (5, 4), (6, 0), (6, 1), (6, 2), (6, 3), (6, 4), (6, 5), (7, 0), (7, 1), (7, 2), (7, 3), (7, 4), (7, 5), (7, 6), (8, 0), (8, 1), (8, 2), (8, 3), (8, 4), (8, 5), (8, 6), (8, 7), (9, 0), (9, 1), (9, 2), (9, 3), (9, 4), (9, 5), (9, 6), (9, 7), (9, 8)]

Answer 3

List comprehensions are designed to make a straightforward translation of that loop possible:

[ (x,y) for x in range(3) for y in range(x) ]

Is that not what you wanted?