# How to sort even/odd numbers separately in a list?

## Question:

I have list with both even and odd numbers. I want to sort the positions with even, odd distinct numbers together. It’s like:

``````s = [5, 6, 4, 7, 11, 14, 12, 1, 3]
#    5        7  11          1  3
#       6  4         14  12
#Now sort it
#    1        3  5          7  11
#       4  6        12  14
#   [1, 4, 6, 3, 5, 12, 14, 7, 11] -> result list
``````

I read two posts here and here but it different with my case.

Is there any way to sort it this way?

i do this, i think it could be done properly but it works:

``````s = [5, 6, 4, 7, 11, 14, 12, 1, 3]
odd = [i if i % 2 == 1 else " " for i in s]
even = [i if i % 2 == 0 else " " for i in s]

print(odd)
print(even)

for i in range(len(odd)):
for j in range(i + 1, len(odd)):
tmp = odd[j]
if tmp != " " and odd[i] != " " and odd[i] > tmp:
odd[j] = odd[i]
odd[i] = tmp

for i in range(len(even)):
for j in range(i + 1, len(even)):
tmp = even[j]
if tmp != " " and even[i] != " " and even[i] > tmp:
even[j] = even[i]
even[i] = tmp

print(odd)
print(even)

final = [odd[i] if odd[i] != " " else even[i] for i in range(len(odd))]

print(final)
``````

first i separate odd / even number, but keeping initial index in lists.
After that i sort the list, still keeping inital index.

Finally, i could merge the two sorted lists.

Output: `[1, 4, 6, 3, 5, 12, 14, 7, 11]`

Edit:
Another option is to make 2 consecutive sort:
The first one on odd numbers, second one on even:

``````s = [5, 6, 4, 7, 11, 14, 12, 1, 3]

for i in range(len(s)):
for j in range(i + 1, len(s)):
tmp = s[j]
if tmp % 2 == 1 and s[i] % 2 == 1 and s[i] > tmp:
s[j] = s[i]
s[i] = tmp

for i in range(len(s)):
for j in range(i + 1, len(s)):
tmp = s[j]
if tmp % 2 == 0 and s[i] % 2 == 0 and s[i] > tmp:
s[j] = s[i]
s[i] = tmp

print(s)
``````

I suggest the following algorithm:

• (1) Build a list with the odd values, and a list with the even values;
• (2) Sort these two lists separately;
• (3) Build a new list by iterating on the original list, picking the next even number when the original list has an even number, picking the next odd number when the original list has an odd number.

I encourage you to try implementing it yourself first, without looking at the code below.

``````def sorted_oddsevens(seq):
# (1) Build a list with the odd values, and a list with the even values
evens = [x for x in seq if x % 2 == 0]
odds = [x for x in seq if x % 2 != 0]
# (2) Sort these two lists separately
evens.sort()
odds.sort()
# (3) Build a new list by iterating on the original list
#     picking the next even number when the original list has an even number
#     picking the next odd number when the original list has an odd number
iter_evens = iter(evens)
iter_odds = iter(odds)
return [(next(iter_evens) if x % 2 == 0 else next(iter_odds)) for x in seq]

sortedlist = sorted_oddsevens([5, 6, 4, 7, 11, 14, 12, 1, 3])
print(sortedlist)
# [1, 4, 6, 3, 5, 12, 14, 7, 11]
``````

The compact version, doing steps 1 and 2 on the same line:

``````def sorted_oddsevens(seq):
iter_evens = iter(sorted(x for x in seq if x % 2 == 0))
iter_odds = iter(sorted(x for x in seq if x % 2 != 0))
return [(next(iter_evens) if x % 2 == 0 else next(iter_odds)) for x in seq]
``````

A different version by Tomerikoo, inverting steps 1 and 2:

``````def sorted_oddsevens(seq):
sorted_seq = sorted(seq)
evens = (x for x in sorted_seq if x % 2 == 0)
odds = (x for x in sorted_seq if x % 2 != 0)
return [(next(evens) if x % 2 == 0 else next(odds)) for x in seq]
``````

You can try a variation of selection sort so that we only consider elements with the same parity when looking for a minimal element:

``````a = [5, 6, 4, 7, 11, 14, 12, 1, 3]
N = len(a)

for m in range(N):
bit = a[m] & 1

k = m
for j in range(m + 1, N):
if a[j] & 1 == bit and a[j] < a[k]:
k = j

a[m], a[k] = a[k], a[m]
``````

Another, faster, but less memory-efficient idea is to

• sort the list
• separate evens and odds from the sorted list
• for each item in the original list, add an element from evens or odds
``````buf = [[], []]
res = []

for x in sorted(a):
buf[x % 2].append(x)

for x in a:
res.append(buf[x % 2].pop(0))
``````

I think the way you want to sort can be done with the help of this function.

``````def sorting_even_odd(s):
pos_list = []
evens = []
odds = []
for i in range(len(s)):
if s[i]%2==0:
pos_list.append(0)
evens.append(s[i])
else:
pos_list.append(1)
odds.append(s[i])

evens.sort()
odds.sort()
e=0
o=0
for i in range(len(s)):
if pos_list[i]==0:
s[i]=evens[e]
e=e+1
else:
s[i]=odds[o]
o+=1

s = [5, 6, 4, 7, 11, 14, 12, 1, 3]
sorting_even_odd(s)
print(s)
``````
``````output -> [1, 4, 6, 3, 5, 12, 14, 7, 11]
``````

One pass to separate, one pass to bring back together:

``````s = [5, 6, 4, 7, 11, 14, 12, 1, 3]

a = [], []
for x in s:
a[x%2].append(x)
a = [b.sort() or iter(b) for b in a]
s = [next(a[x%2]) for x in s]

print(s)
``````

Or using one of my favorite sorting algorithms:

``````s = [5, 6, 4, 7, 11, 14, 12, 1, 3]

for i in range(len(s)):
for j in range(len(s)):
if s[i] < s[j] and s[i]%2 == s[j]%2:
s[i], s[j] = s[j], s[i]

print(s)
``````

Or a funky `more-itertools` solution, first splitting at the even numbers and sorting them, then doing the same with the odds:

``````from more_itertools import split_at, flatten

s = [5, 6, 4, 7, 11, 14, 12, 1, 3]

for i in range(2):
s = list(split_at(s, lambda x: x%2 == i, keep_separator=True))
s[1::2] = sorted(s[1::2])
s = list(flatten(s))

print(s)
``````
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