How to check for incrementing elements in a list

Question

I am trying to check if 2 elements in a list are incrementing by 1.

For instance, in a list like this [1,3,4,5,6,7,9,11,12], my code should return a new list like this [3,4,5,6,11,12]

This is my code:

same = [1,3,4,5,6,7,9,11,12]
new_datalist = []
index = 0
for k in same:
    try:
        new_index = index+1
        if k+1 == same[new_index]:
            new_datalist.append(k)
            new_datalist.append(k+1)
        index += 1
    except:
        pass

new_datalist

But it is returning this - [3, 4, 4, 5, 5, 6, 6, 7, 11, 12]

How do I solve this?

Just to explain further:

Let's look at it like this, elements in the list represents the monthly salary of a worker. I want to check if the worker receives half-salary for 2 consecutive months. For example, a list like this [3,4,5,6,7,8,1,44] would return a list like [3,4,5,6,7,8]. This means the worker receives half-salary for months 3&4, 5&6 and 7&8

You can [Remove consecutive duplicates](https://stackoverflow.com/questions/5738901/removing-elements-that-have-consecutive-duplicates) from your result. — DYZ, May 08 '20 at 20:42

Patrick Artner · Accepted Answer · 2020-05-08T21:10:15.570

1

You need to fix the algorythm - currently you are adding twice.

You can use zip() to solve it:

same = [1,3,4,5,6,7,9,11,12]

# create pairs:  [ (1,3),(3,4),(4,5),(5,6),(7,9),(9,11),(11,12),(12,12) ]
z = zip(same, same[1:]+same[-1:])

r =[]
# take pairs into a,b
for a,b in z:
    if r and r[-1] == a:  # if we added a already, skip it
        continue          # last run added 3 and 4, now it checks 4 and 5: skip
    if a==b-1:            # if a is b-1 add both
        r.append(a)
        r.append(b)

print(r)  # [3, 4, 5, 6, 11, 12]

To look for stringently raising numbers, use:

same = [1,3,4,5,6,7,9,11,12]

a = iter(same)
b = iter(same)
next(b)
r = []
while a and b:
    aa = next(a)
    try:
        bb = next(b)
        if aa == bb-1:
            r.append(aa)
        elif r and r[-1] == aa-1:
            r.append(aa)
    except StopIteration:
        if bb-1== r[-1]:
            r.append(bb)
        break

print(r) # [3, 4, 5, 6, 7, 11, 12]

edited May 08 '20 at 21:10

answered May 08 '20 at 20:40

Patrick Artner

50,409
9
43
69

7 is not supposed to be part of the results. Something like this - [3,4,5,6,11,12]. – shekwo May 08 '20 at 20:49
Let's look at it like this, elements in the list represents the monthly salary of a worker. I want to check if the worker receives half-salary for 2 consecutive months. For example, a list like this [3,4,5,6,7,8,1,44] would return a list like [3,4,5,6,7,8]. This means the worker receives half-salary for months 3&4, 5&6 and 7&8 – shekwo May 08 '20 at 20:58
Thank you! Life saver! – shekwo May 08 '20 at 21:14

Colebasaur · Answer 2 · 2020-05-08T21:14:17.607

0

Here.

same = [1,3,4,5,6,7,9,11,12]
output = []
i = 0
while i < len(same) - 1:
    if same[i] + 1 == same[i + 1]:
        output.extend([same[i], same[i + 1]])
        i += 2
    else:
        i += 1
print(output)

edited May 08 '20 at 21:14

answered May 08 '20 at 20:34

Colebasaur

11
2

7 is not supposed to be part of the results. Something like this - [3,4,5,6,11,12]. – shekwo May 08 '20 at 20:52
If 7 is not included, then the 12 should not be included. Unless you want to treat the last seen consecutive number as a special case. – Colebasaur May 08 '20 at 20:53
For example, a list like this [3,4,5,6,7,8,1,44] would return a list like [3,4,5,6,7,8] – shekwo May 08 '20 at 20:53
Let's look at it like this, elements in the list represents the monthly salary of a worker. I want to check if the worker receives half-salary for 2 consecutive months. For example, a list like this [3,4,5,6,7,8,1,44] would return a list like [3,4,5,6,7,8]. This means the worker receives half-salary for months 3&4, 5&6 and 7&8 – shekwo May 08 '20 at 20:58
I understand the problem. My solution works for this second example... My issue is, 6 and 7 are consecutive, but you are not including 7 for some reason. Do you not see this? – Colebasaur May 08 '20 at 21:01
These are random numbers in the list. It can be ```[3,100,200,201,5,2]``` – shekwo May 08 '20 at 21:02
The key is just to get the pairs – shekwo May 08 '20 at 21:03

How to check for incrementing elements in a list

2 Answers2