Compare previous and next values when looping through a list in Python

Question

I should compare the previous, actual and next values in a list. This lines of code here underneath doesn't fully works.

The results of the code are:

5 4 6
8 10 11
20 18 19

so I miss values like

9 8 10

and values like

8 10 11

are simply wrong.

What am I missing in the enumerate method?

list=[1,2,3,4,5,4,6,7,9,8,10,11,14,15,16,20,18,19]

for i,n in enumerate (list):

    if (i + 1 < len( list ) and i - 1 >= 0):

        a = str( list[i - 1] )
        b = str( n )
        c = str( list[i + 1] )

        if a>b<c:
            print (a,b,c)

Use: a>b and b < c and why do you need to convert them from int to str? — Amit Vikram Singh
– Amit Vikram Singh, Commented Apr 20, 2021 at 14:14
Don't call your lists list because that makes them hide python's inbuilt list class — pho
– pho, Commented Apr 20, 2021 at 14:26

user2390182 · Accepted Answer · 2021-04-20 14:25:27Z

2

After the str conversion, it holds that "10" < "8" because strings are compared lexicographically. So you can simply omit those:

a = list[i - 1]
b = n 
c = list[i + 1]

You could also use zip with 1-offset slices to simplify this sort of "iterate neighbors" pattern:

lst = [1,2,3,4,5,4,6,7,9,8,10,11,14,15,16,20,18,19]  # don't shadow built-in `list`
for a, b, c in zip(lst, lst[1:], lst[2:]):
    if a > b < c:
        print(a,b,c)

5 4 6
9 8 10
20 18 19

answered Apr 20, 2021 at 14:25

user2390182

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1 Comment

P C Over a year ago

Thanks Schwobaseggl, interesting use of zip function, where can i find more info about this uses of zip?Regards

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