One other question

Hey everyone, I just asked a few hw questions on here but I also have an exam question I'd like some help with, Its basically like this;

We need to write a program where we input a matrix and it will tell us how far into the matrix the first number out of sequence is located, so for instance if i put in:

(1,2,3,5,6) it will return 4 because the 4th spot is the first number that is not in sequence, or another ex:

(5,6,7,8,9,10,6) it will return 7 because the 7th number is not in order.

Once again, you can answer here or email me, hurry though my exams in an hour ahhh

[Andrew Newell]

@Mike, could you give this question a more meaningful title? An example is "How do I find a number out of sequence?"

Answer 1

[Andrew Newell]

Assuming your input is really a vector (as in your two examples), try using

diff(diff(vector))

and think about the output.

Answer 2

[Walter Roberson]

Something like,

find( (A(1) : A(1)+length(A) - 1) ~= A )

[Mike]

how would I put this into a function formula. I had something like:

function = sequence(a)

n=0

n=1+n

while a(m+1)-a(m)=x;

findx~=1

end

but it was not working obviously

[Mike]

aka i'm good at understanding kinda what i need to do, just terrible at telling the computer how to do that

[Walter Roberson]

function wrongidx = sequence(a)

wrongidx = find( (a(1) : a(1)+length(a) - 1) ~= a, 1 );

end

[Andrew Newell]

I hope the sequence isn't [2 4 6 8 9]!

[Walter Roberson]

@Andrew: see http://www.mathworks.com/matlabcentral/answers/3262-how-can-i-retrieve-common-patterns-from-a-file#comment_7867

[Andrew Newell]

That Mathematical Recreations article is great!

Answer 3

[Andrew Newell]

I think a reasonable interpretation of the problem is that it is an arithmetic sequence and has to be established at the beginning before any deviations occur (so there must be at least 3 elements in the sequence first). A generalization of Walter's function to this case is then:

function wrongidx = sequence(a)
vdiff = diff(a(1:3));
if diff(vdiff)~=0,
  error('There must be a sequence of at least 3 elements to start.')
end
wrongidx = find( (a(1) : vdiff(1):a(1)+vdiff(1)*(length(a) - 1)) ~= a, 1 );