Finding longest consecutive numbers in array

Hello everyone,

This is somewhat of a silly question but I can't seem to figure it out. If I have an array of numbers what I want is to find the location of my longest consecutive numbers for example:

my arrary = [ 1999 2000 2001 2003 2004 2005 2006 2007];

I want my output to be = [ 4 5 6 7 8]; because that's the location of my longest consecutive numbers (2003-2007). I tried to find where difference is equal to 1 but then the result is [1 1 1 0 1 1 1 1] , it doesn't take the position of the last number which is 2007 here and even if I fix that problem also I'd still have to find the locations for the longest consecutive one's for the second scenario.

I could use some help in this,

Thank You!!!

Answer 1

[Stephen23]

V = [1999,2000,2001,2003,2004,2005,2006,2007];
D = diff([0;diff(V(:))==1;0]);
B = find(D>0);
E = find(D<0);
[~,idx] = max(E-B);
idy = B(idx):E(idx)

idy = 1×5

4 5 6 7 8

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[John Doe]

Thank You!!!!

[Martina]

Note that this code does not work well in some cases (maybe when the first number is not part of a sequence of consecutive numbers):

V = [1999,2001,2003,2004,2005,2006,2007];
D = diff(diff(V)==1);
B = find([true,D>0]);
E = find([D<0,true])+1;
[~,idx] = max(E-B);
B(idx):E(idx)

ans = 1×7

1 2 3 4 5 6 7

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[Stephen23]

@Martina: well spotted! I made a few changes, please check it!

longestConsecutive([2001,2002,2003,2004,2005,2006,2007,2009,2010]) % 1:7

ans = 1×7

1 2 3 4 5 6 7

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longestConsecutive([1999,2001,2003,2004,2005,2006,2007,2009,2010]) % 3:7

ans = 1×5

3 4 5 6 7

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longestConsecutive([1999,2001,2003,2004,2005,2006,2007,2008,2009]) % 3:9

ans = 1×7

3 4 5 6 7 8 9

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longestConsecutive([2001,2002,2003,2004,2005,2006,2007,2008,2009]) % 1:9

ans = 1×9

1 2 3 4 5 6 7 8 9

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longestConsecutive([2001,2003,2005,2007]) % none

ans =

  1×0 empty double row vector

function idy = longestConsecutive(inp)
D = diff([0;diff(inp(:))==1;0]);
B = find(D>0);
E = find(D<0);
[~,idx] = max(E-B);
idy = B(idx):E(idx);
end

As an alternative you could use something from FEX, for example

https://www.mathworks.com/matlabcentral/fileexchange/41813-runlength

on the DIFF() of the vector.

[Martina]

Thank you so much!! <3