Checking loop in Python

Question

I have this question about loops. Imagine You have this data array:

list = [1,2,3,4,5,6,7,8,9,8,7,6,5,4,3,2,1]

How would You write a loop which checks if the previous number is lower, and the next after the checked one (Condition looks like this [5,6,5]). So the loop will get to number 9 and print it or save it, whatever.

jpp · Accepted Answer · 2018-06-05 13:08:05Z

5

Using next with a generator expression:

lst = [1,2,3,4,5,6,7,8,9,8,7,6,5,4,3,2,1]

res = next(j for i, j, k in zip(lst, lst[1:], lst[2:]) if i < j and i == k)

If you need all such numbers, use a list comprehension instead:

res = [j for i, j, k in zip(lst, lst[1:], lst[2:]) if i < j and i == k]

If you need a condition that will show all numbers that are higher than their previous and next ones:

lst = [1,2,3,4,3,2,3,1,2,1,2,3,4,5,6,7,8,6]

res = [j for i, j, k in zip(lst, lst[1:], lst[2:]) if i < j > k]

[4, 3, 2, 8] is printed.

Explanation

You can iterate the list with shifted versions of itself via zip.
For each triplet, test your two conditions.
Use next to extract such triplets; if no such triplet exists, you will meet StopIteration error.
Never name a variable after a built-in, e.g. use lst instead of list.

edited Jun 5, 2018 at 13:08

answered Jun 5, 2018 at 12:05

jpp

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1 Comment

Hiddenguy Over a year ago

Thank You so much for that explanation. Well, my problem is much more complex. Can I ask You to check my other thread? Maybe You'll have some ideas for that problem. stackoverflow.com/questions/50647094/…

RoadRunner · Accepted Answer · 2018-06-05 12:26:39Z

1

You could just write a simple loop that checks the previous number is less than the current number, and the next number is equal to the previous number:

lst = [1,2,3,4,5,6,7,8,9,8,7,6,5,4,3,2,1]

for i in range(len(lst)):
    if lst[i-1] < lst[i] and lst[i-1] == lst[i+1]:
        print(lst[i])

# 9

answered Jun 5, 2018 at 12:26

RoadRunner

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