Finding missing number in a sequence

Question

I need to find one missing number, from a sequence of numbers

such as

4 8 12 16 __ 24.

I need to find the missing number. How would I do that programmatically?

the numbers and the missing placement are not static, so they should be able to change.

Hope it is possible.

It is impossible because there are infinite number of ways you can build your logic. What you can do is to limit it to only several methods of building sequences which will make it a trivial loop-through.. — sashkello
– sashkello, Commented Jul 12, 2013 at 0:43
I'd also say this is a math.stackoverflow question because there is no particular programming aspect here. — sashkello
– sashkello, Commented Jul 12, 2013 at 0:45
@Simon Whitehead: I think the OP wants the program to find the missing number of an arbitrary series of numbers, (if I understood correctly, that is). — pcnThird
– pcnThird, Commented Jul 12, 2013 at 0:51

sa_ddam213 · Accepted Answer · 2013-07-12 02:03:57Z

2

You could use some silly linq like this silly example :)

var numbers = new List<int>{4, 8, 12, 16, 24, 28, 36};
int first = numbers.First();
int last = numbers.Last();

var missing = Enumerable.Range(first, last).Where(n => n % first == 0).Except(numbers);


Returns:
20
32

-Bracing self for downvotes-

edited Jul 12, 2013 at 2:03

answered Jul 12, 2013 at 1:40

sa_ddam213

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2 Comments

neilb Over a year ago

this assumes that you'll that your first element is equal to the difference between all the elements in the list.

canhazbits Over a year ago

In an interview (which is where this problem is usually seen), you would probably be told to make a more general solution, i.e. not using Linq. Also it would need to work with negative and positive values.

fcuesta · Accepted Answer · 2013-07-12 01:08:37Z

1

If you know that it is always an Arithmetic Progression, you can use the formula:

an = a1 + (n - 1) * d

being a1 the first element, d the difference between 2 elements and n the position to calculate, in your case:

an = 4 + (5 - 1) * 4 = 20

Check this: https://en.wikipedia.org/wiki/Arithmetic_progression

answered Jul 12, 2013 at 1:08

fcuesta

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