Iterating over 2 elements in a list but repeating the second element in the next iteration-CodePudding

How do I make a for loop or a list comprehension so that every iteration gives me two elements but the last value must be repeated.

l = [1,2,3,4,5,6]

for i,k in ???:
    print str(i), ' ', str(k), '=', str(i k)

Output:

1 2=3
2 3=5
3 4=7
4 5=9
...

CodePudding user response：

You can use zip with a one-off slice:

for i, k in zip(l, l[1:]):
    # ...

But in Python >= 3.10 you also have itertools.pairwise:

from itertools import pairwise

for i, k in pairwise(l):
    # ...

This is more space-efficient because it avoids the in-memory list produced by the slice. It can also be used with any iterator/iterable which helps avoid a lot of boilerplate code.

CodePudding user response：

You can use zip:

l = [1,2,3,4,5,6]
for i,k in zip(l, l[1:]):
    print(f'{i}   {k} = {i k}')

Output:

For more explanation:

>>> list(zip(l, l[1:]))
[(1, 2), (2, 3), (3, 4), (4, 5), (5, 6)]

CodePudding user response：

You can do this with standard indexing:

l = [1,2,3,4,5]
for i in range(1,len(l)):
    print( str(l[i-1]), ' ', str(l[i]), '=', str(l[i]   l[i-1]) )

Output:

CodePudding user response：

You can also do it like this :-

l = [1,2,3,4,5,6]
for i in l:
    print (i, ' ', i 1, '=',i i 1)