Error in my nested loop in iteration?

Question

consider

a = [1,2,3,4]

i = 0

j = 1

for i in range(len(a)):

       for j in range(len(a)):

          d = (a[i]-a[j])
          j = j + 1
          print i, j, d
       i = i + 1

Output

I am trying to iterate through my array so that I can only get numbers that are non-zero for d and that I do not go over the same i and j (ex: if i = 0,j=1 or i=1, j=0). Its just like doing a combination problem for which I am looking the number of pairs in my array and the d that goes for it.

Also, i=0 and j=0 doesn't do anything. Iterating through the for i in range(len(a)) starts out i as 0. And iterating for j in range(len(a)) starts j as 0. To iterate from j=1 to len(a), do: for j in range(1, len(a)). — Rushy Panchal
– Rushy Panchal, Commented Feb 21, 2013 at 18:20

Abhijit · Accepted Answer · 2013-02-21 18:20:10Z

1

I am trying to iterate through my array so that I can only get numbers that are non-zero for d

Unless this is a home work I would advise you to use itertools.combinations with a reverse sorted list or itertools.permutations for your problem

>>> list((a,b) for a,b in itertools.permutations(a, 2) if a > b)
[(2, 1), (3, 1), (3, 2), (4, 1), (4, 2), (4, 3)]
>>> list(itertools.combinations(sorted(a, reverse = True), 2))
[(4, 3), (4, 2), (4, 1), (3, 2), (3, 1), (2, 1)]

answered Feb 21, 2013 at 18:20

Abhijit

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Gijs · Accepted Answer · 2013-02-21 18:22:42Z

1

Just use permutations from itertools:

import itertools
a = [1,2,3,4]
for permutation in itertools.permutation(a, 2):
    print permutation

outputs

(1, 2)
(1, 3)
(1, 4)
(2, 1)
(2, 3)
...
...

If you also want the distances you can do

a = [1,2,3,4]
for permutation in itertools.permutation(a, 2):
    print permutation, permutation[1] - permutation[0]

(1, 2) 1
(1, 3) 2
(1, 4) 3
(2, 1) -1

edited Feb 21, 2013 at 18:22

answered Feb 21, 2013 at 18:17

Gijs

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1 Comment

user1821176 Over a year ago

Thanks! Actually using combinations from itertools was precisely what I was looking for. This led me to it though.

ATOzTOA · Accepted Answer · 2013-02-21 18:16:24Z

0

Try this:

a = [1,2,3,4]

i = 0

j = 1

for i in range(len(a)):

    for j in range(len(a)):

        d = (a[i]-a[j])
        j = j + 1
        if i != j and d != 0:
            print i, j, d
    i = i + 1

Output:

answered Feb 21, 2013 at 18:16

ATOzTOA

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