Fixing a 4 nested for loops in Python

Question

So i'm trying to implement the agglomerative clustering algorithm and to check the distances between each cluster i use this:

a, b = None, None
c = max
for i in range(len(map)-1):
    for n in range(len(map[i])):
        for j in range(i+1, len(map)):
            for m in range(len(map[j])):
                //dist is distance func.
                d = dist(map[i][n], map[j][m])
                if c > d:
                    a, b, c = i, j, d
print(a, ' ', b)
return a, b

map looks like this: { 0: [[1,2,3], [2,2,2]], 1: [[3,3,3]], 2: [[4,4,4], [5,5,5]] }

What I expect from this is for each row item to compare with every row/col of every other row. So something like this:

comparisons: [1,2,3] and [3,3,3], [1,2,3] and [4,4,4], [1,2,3] and [5,5,5], [2,2,2] and [3,3,3] and so on

When I run this it only works 1 time and fails any subsequent try after at line 6 with KeyError.
I suspect that the problem is either here or in merging clusters.

Check out [itertools](https://docs.python.org/3/library/itertools.html)! — noumenal, Jul 15 '17 at 19:29

Hendrik Makait · Accepted Answer · 2017-07-15T19:16:44.647

If map is a dict of values, you have a general problem with your indexing:

for m in range(len(map[j])):

You use range() to create numerical indices. However, what you need j to be in this example is a valid key of the dictionary map.

EDIT: That is - of course - assuming that you did not use 0-based incremented integers as the key of map, in which cause you might as well have gone with a list. In general you seem to be relying on the ordering provided in a list or OrderedDict (or dict in Python3.6+ as an implementation detail). See for j in range(i+1, len(map)): as a good example. Therefore I would advise using a list.

EDIT 2: Alternatively, create a list of the map.keys() and use it to index the map:

a, b = None, None
c = max
keys = list(map.keys())
for i in range(len(map)-1):
    for n in range(len(map[keys[i]])):
        for j in range(i+1, len(map)):
            for m in range(len(map[keys[j]])):
                #dist is distance func.
                d = dist(map[keys[i]][n], map[keys[j]][m])
                if c > d:
                    a, b, c = i, j, d
print(a, ' ', b)
return a, b

pooya · Answer 2 · 2017-07-15T19:23:47.340

0

Before accessing to map[j] check is it valid or not like:

if j in map.keys():
     #whatever

or put it in try/except:

try:
   #...
except KeyError:
   #....

Edit: its better to use for loop like this:

for i in map.keys():
    #.....

edited Jul 15 '17 at 19:23

answered Jul 15 '17 at 19:03

pooya

155
6

so you have to check map[i] to. generally , when ever you want to access dictionary by key, you should check if the key exist or not OR put it on try/except . – pooya Jul 15 '17 at 19:20

Fixing a 4 nested for loops in Python

2 Answers2