How to check consecutive number in list?

Question

This is my homework.

The problem is to find a way to check whether the items in a list are consecutive or not.

The following is code I've written:

def consecutive(var):
for x in range(2, len(var)):
    forward = var[x] - var[x-1]
    backward = var[x-1] - var[x-2]
if forward == backward:
    return True
else:
    return False

var = []
print 'Enter your number:'
while True:
    num = raw_input()
    if num == '':
        break
    var += [int(num)]

print consecutive(var)

If I input numbers like 1, 2, 3, 4, 5 then I will get True

If I input numbers like 2, 6, 3, 9, 7, 1, 4 then I'll get False

Here, I succeeded returning True or False values respectively.

But there're two questions that make me upset because if I use my code to solve the questions, I don't get the value that I want (it gives me an error)

First question: Is an empty list considered a consecutive list or not?

Second: Is a list that involves a single value considered a consecutive list or not?

Would you like to help me?

These are edge cases that should be defined in your homework. In my opinion however, an empty list or a list with one element should indeed be seen as having consecutive order, as by any construction they are ''sorted''. — miradulo
– miradulo, Commented Apr 16, 2016 at 18:03
I would say that a for a list to be consecutive, you must have at least two elements to make a comparison. — jtitusj
– jtitusj, Commented Apr 16, 2016 at 18:14
@JohnTitusJungao: while that's not a crazy perspective, it goes against convention. We call [] and [1] sorted because [] == sorted([]) and [1] == sorted([1]) even though there aren't two elements, because there are no elements which are unsorted. A similar logic should apply to consecutive elements, IMHO. — DSM
– DSM, Commented Apr 16, 2016 at 18:17
@DSM Although my initial thinking was to agree with you, now I'm not sure. This is obviously just semantics but if the question specified a list with consecutive elements, can you really attribute that back to sorting and sets? Or does that implicitly restrict your list to having two elements? (asking because I'm not sure :P) — miradulo
– miradulo, Commented Apr 16, 2016 at 18:19
@DonkeyKong: this really is just a convention, and the standard convention in math is to allow vacuous truth in definitions. — DSM
– DSM, Commented Apr 16, 2016 at 18:21

miradulo · Accepted Answer · 2016-04-16 18:26:12Z

2

By mathematical convention as discussed in the comments, it would be standard to consider both an empty list and a list with a single element as consecutive, should it not be specified in your homework more precisely.

By the logic of vacuous truth, were a list to not have consecutive order it would require enough elements (2) to break this condition of ordering. To handle this in your existing code, you could simply perform a check before your main check to ensure the base cases of an empty list and a list with one element return True.

answered Apr 16, 2016 at 18:26

miradulo

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