Filtering a comprehension

We can apply filtering to a comprehension. Let's do it first with filter. Let's find all Pythagorean triples whose short sides are numbers smaller than 10. We obviously don't want to test a combination twice, and therefore we'll use a trick similar to the one we saw in the previous example:

# pythagorean.triple.py
from math import sqrt
# this will generate all possible pairs
mx = 10
triples = [(a, b, sqrt(a**2 + b**2))
    for a in range(1, mx) for b in range(a, mx)]
# this will filter out all non pythagorean triples
triples = list(
    filter(lambda triple: triple[2].is_integer(), triples))

print(triples)  # prints: [(3, 4, 5.0), (6, 8, 10.0)]

In the preceding code, we generated a list of three-tuples, triples. Each tuple contains two integer numbers (the legs), and the hypotenuse of the Pythagorean triangle whose legs are the first two numbers in the tuple. For example, when a is 3 and b is 4, the tuple will be (3, 4, 5.0), and when a is 5 and b is 7, the tuple will be (5, 7, 8.602325267042627).

After having all the triples done, we need to filter out all those that don't have a hypotenuse that is an integer number. In order to do this, we filter based on float_number.is_integer() being True. This means that of the two example tuples I showed you before, the one with 5.0 hypotenuse will be retained, while the one with the 8.602325267042627 hypotenuse will be discarded.

This is good, but I don't like that the triple has two integer numbers and a float. They are supposed to be all integers, so let's use map to fix this:

# pythagorean.triple.int.py
from math import sqrt
mx = 10
triples = [(a, b, sqrt(a**2 + b**2))
    for a in range(1, mx) for b in range(a, mx)]
triples = filter(lambda triple: triple[2].is_integer(), triples)
# this will make the third number in the tuples integer
triples = list(
    map(lambda triple: triple[:2] + (int(triple[2]), ), triples))

print(triples)  # prints: [(3, 4, 5), (6, 8, 10)]

Notice the step we added. We take each element in triples and we slice it, taking only the first two elements in it. Then, we concatenate the slice with a one-tuple, in which we put the integer version of that float number that we didn't like. Seems like a lot of work, right? Indeed it is. Let's see how to do all this with a list comprehension:

# pythagorean.triple.comprehension.py
from math import sqrt
# this step is the same as before
mx = 10
triples = [(a, b, sqrt(a**2 + b**2))
    for a in range(1, mx) for b in range(a, mx)]
# here we combine filter and map in one CLEAN list comprehension
triples = [(a, b, int(c)) for a, b, c in triples if c.is_integer()]
print(triples)  # prints: [(3, 4, 5), (6, 8, 10)]

I know. It's much better, isn't it? It's clean, readable, shorter. In other words, it's elegant.