Access indices of variables with a list comprehension

Question

I've defined these six points in a coordinate system:

P1 = (0,0)
P2 = (0,-2)
P3 = (4,-2)
P4 = (4,0)
P5 = (4,2)
P6 = (2,1)

Now, I'd like to make a list of all the x-coordinates with a for loop reading the indices.

Something like this:

[P[i+1][0] for i in range(6)]

to get the result [0, 0, 4, 4, 4, 0]. How do I make Python read the P[i+1] as P1, P2, P3...?

Answer 1

To use your P[i+1] indexing I would suggest a dict where the keys are indices in range [1, 7) and the values are your points.

# P = {1: (0,0), 2: (0,-2), 3: (4,-2) ...}
P = dict(zip(range(1,7), [(0,0), (0,-2), (4,-2), (4,0), (4,2), (0,2)]))

x_coord = [P[i+1][0] for i in range(6)]
print(x_coord)

However if you only want the x coordinate of each tuple you could go for something like this:

points = [(0,0), (0,-2), (4,-2), (4,0), (4,2), (0,2)]
x_coords = ...
print(x_coords)

You could go for either this:

x_coords = [x for x,_ in points] 

or:

x_coords = list(map(lambda x:x[0], points))

Output:

[0, 0, 4, 4, 4, 0]

Answer 2

You want Python to

read the P[i+1] as P1, P2, P3...

So, the main point is using f"P{i+1}" to produce P1, P2, .... Then, I suggest you use these as keys to a dictionary. This way, your code works even when the number of points is changed.

points = {'P1': (0,0), 'P2': (0,-2), 'P3': (4,-2), 'P4': (4,0), 'P5': (4,2), 'P6': (2,1)}

Assume the number of points is 6:

indices = range(1, 7)

Now you can have a list of X's:

Print([points[f"P{index}"][0] for index in indices])

Output:

[0, 0, 4, 4, 4, 2]