Python Remove array items inside for loop

Question

I am trying to remove some items inside an array based on the distance from each other.

I have 104 items in an array with tuples:

points = [(910, 28), (914, 29), (919, 30), (915, 32), (766, 73), (777, 75), (768, 80), (1227, 117), (1224, 119), (1228, 120), (1224, 121), (1228, 122), (1221, 124), (1218, 126), (486, 147), (482, 150), (474, 153), (905, 182), (908, 184), (904, 186), (901, 187), (913, 188), (909, 190), (903, 193), (187, 213), (186, 214), (189, 215), (611, 262), (617, 264), (619, 265), (610, 268), (1231, 272), (1228, 274), (1228, 276), (1232, 278), (1223, 282), (486, 306), (477, 309), (463, 312), (470, 313), (486, 315), (473, 319), (764, 376), (773, 379), (770, 383), (795, 386), (778, 390), (631, 412), (626, 414), (624, 416), (626, 418), (1218, 434), (1217, 435), (1218, 436), (1219, 437), (1220, 438), (1222, 439), (1225, 440), (1226, 442), (480, 460), (478, 463), (1071, 466), (1062, 468), (1067, 469), (1072, 470), (339, 485), (343, 487), (345, 489), (346, 490), (350, 492), (343, 495), (352, 497), (930, 505), (929, 508), (929, 513), (199, 535), (197, 537), (203, 539), (201, 542), (771, 547), (774, 547), (773, 548), (772, 549), (776, 549), (776, 550), (629, 576), (628, 579), (631, 580), (625, 583), (1237, 586), (1218, 590), (1226, 593), (1223, 595), (1227, 599), (639, 732), (643, 733), (643, 734), (204, 875), (209, 877), (210, 879), (210, 880), (210, 882), (210, 884), (204, 887)]

and my code is as follows:

print len(points)
w = 152
h = 157
for pt in points:
    for fpt in points:
        if pt == fpt:
            continue
        else:
            distX = abs(pt[0] - fpt[0])
            distY = abs(pt[1] - fpt[1])
            dist = distX + distY
            if distX < w / 2 and distY < h / 2:
                points.remove(fpt)

print len(points)

When I run it, it removes lots of items and it leaves me with only 33, but it is not correct. If I run it again it leaves me the correct number which is 22.

I am sure that I did something wrong, can anyone help me locate and correct my mistake?

Are you sure the points remaining should be 22? According to current logic, seems like all the points are to be removed? — Anand S Kumar
– Anand S Kumar, Commented Jun 29, 2015 at 9:13
The logic is that I want from a cluster of points to get only one of them. I don't care which one so that it will be easier... — bkbilly
– bkbilly, Commented Jun 29, 2015 at 9:21

Community · Accepted Answer · 2017-05-23 12:30:02Z

2

Removing list items in place while iterating the list is not good practice. For instance,

if points has only 3 elements [a, b, c], 
a is adjacent to b, 
b is adjacent to c, 
but a is not adjacent to c, 
Correct result is [a] after the algorithm runs. But with your algorithm,
after the first iteration, b is removed; points = [a, c]
c can never be removed, since a and c are not adjacent.

What you are looking for is a Union-find-set, here is a implementation: http://code.activestate.com/recipes/215912-union-find-data-structure/

And here is a more naive implementation of Union-Find, for your reference: A set union find algorithm

edited May 23, 2017 at 12:30

CommunityBot

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answered Jun 29, 2015 at 10:25

NeoWang

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1 Comment

bkbilly Over a year ago

I knew that it wasn't a good idea to remove items while iterating, but I didn't know any other solution... I just thought of storring all close items to a double list and then pick one of them. If it works i'll tell you! Thanks for clarifying what I did wrong!

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