Jacques Gaudin - 1 year ago 59

Python Question

Given a constant, a sorted list of floats and a list of 2-element list of floats,

`CONST = 1.`

lst1 = [1.2, 2.4, 3.1] #sorted

lst2 = [[2.0, 0.9], [3.1, 1.5], [1.0, 3.0], [2.5, 2.0]]

I construct a new list with the pairs

`[CONST, a]`

`lst1`

I need to "project"

`list2`

`[CONST,a]`

`lst2`

So the result with the example given would be:

`[[6.1, 1.2], [3.5, 2.4], [2.0, 3.1]]`

So far I have something like:

`from itertools import groupby`

from bisect import bisect

from operator import itemgetter

for t in lst2:

i = bisect(lst1, t[1])

bounds = lst1[i-1:i+1] if i else [lst1[0]]

t[1] = min(bounds, key=lambda x: abs(x-t[1]))

lst2 += [[CONST, a] for a in lst1]

lst2 = sorted(lst2, key=itemgetter(1))

res = [[sum([t[0] for t in group]), keys] for keys, group in groupby(lst2, itemgetter(1))]

but the lists (esp.

`lst1`

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Answer Source

I think I found an intersting way of doing it: a generator of generators ! Python is great ! This is about twice as fast as the previous code.

```
CONST = 1.
lst1 = [1.2, 2.4, 3.1] #sorted
lst2 = [[2.0, 0.9], [3.1, 1.5], [1.0, 3.0], [2.5, 2.0]]
def gen_generator(lst1):
#Create iterators to form consecutive pairs
it1_1, it1_2 = iter(lst1), islice(lst1+[float('inf')], 1, None)
#Create iterator over items to insert and initialize p
gen_generator.it2 = iter(sorted(lst2, key=itemgetter(1)))
gen_generator.p=next(gen_generator.it2)
for t1, t2 in zip(it1_1, it1_2):
#Calculate the mid point of the pair t1, t2
sup = (t1+t2)/2 if t2 != float('inf') else float('inf')
#Create a generator for each item in lst1
def generator():
#All the previously inserted items have disappeared
#So yield any p such that p[1] < sup
while gen_generator.p is not None and gen_generator.p[1] < sup:
yield gen_generator.p[0]
#Move to next or None
gen_generator.p = next(gen_generator.it2, None)
#Add the constant
yield CONST
yield generator
res = [[sum(gen()), t] for gen, t in zip(gen_generator(lst1), lst1)]
return res
```

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