Conditionally combine or delete tuples in list

Question

I have a list of data in Python of the form [(x₀, f₀), …, (x_n, f_n)], where the tuple (x_i, f_i) represents the location and magnitude, respectively, of the i^th element. For example:

point_forces = [(0, 4), (3.5, 2), (0, -3.1), (4, 6), (2, 0), (3.5, -4)]

What is a good way to remove tuples where f_i = 0 and combine f_i + f_j of tuples where x_i = x_j, returning a result in the form [(x₀, f₀), …, (x_m, f_m)]? Continuing the example, this is what I want to get:

result = [(0, 0.9), (3.5, -2), (4, 6)]

The order in which these operations are applied or the order in which tuples appear in the resultant list does not matter to me as long as x_i ≠ x_j and f_i ≠ 0 for all i, j in [0, m].

Asked By: naterson

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Source

Answer 1

Try:

out = {}
for x, f in point_forces:
    if f != 0.0:
        out[x] = out.get(x, 0) + f

out = [(x, round(f, 2)) for x, f in out.items() if f != 0.0]  # if you want to keep the resulting tuples where f=0 then remove the if... part
print(out)

Prints:

[(0, 0.9), (3.5, -2), (4, 6)]

Answered By: Andrej Kesely

Answer 2

A solution using defaultdict:

d = defaultdict(float)
for k, v in point_forces:
    d[k] += v
    d[k] = round(d[k], 1)
result = [(k, d[k]) for k in d if d[k] != 0.]

(You can use Decimal for better floating point arithmetic)

Answered By: Jonathan Ciapetti

Conditionally combine or delete tuples in list

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