quick way to determine the array is irdered?

[Roman Mashak]

Hello, All!

Is there an easy way to determine that array e.g. int X[N] contains ordered
items (for example, ascending), except running loop with comparison of
items?

It would be good to provide me with some useful link

Thanks!

With best regards, Roman Mashak. E-mail: (e-mail address removed)

 

[Clark S. Cox III]

Hello, All!

Is there an easy way to determine that array e.g. int X[N] contains
ordered items (for example, ascending), except running loop with
comparison of items?

No.

 

[Jack Klein]

Hello, All!

Is there an easy way to determine that array e.g. int X[N] contains ordered
items (for example, ascending), except running loop with comparison of
items?

It would be good to provide me with some useful link

Thanks!

With best regards, Roman Mashak. E-mail: (e-mail address removed)

There are other ways, but not likely to be quicker.

You could use a second array of the same type and size (defined or
allocated) and copy the first array into the second. Then you could
sort the second with qsort() or a sort function you write yourself.

Then you could compare the two arrays element by element and if you
find a difference, the first array was not ordered.

But as I said, not likely to be quicker.

 

[r.devaraj]

i guess, one simple way ( rather than going for quick sort ) of doing
it at o(n) is,(if ur goal is just to check ascending or descending
property)

Check the difference between the consecutive elements, for the whole
array.

The array is ascending or descending, depending on the difference..

(more blah-blah:

if an array is in descending order, an element will be always greater
than its successor... just check this for whole array.. and the other
way for asceding order..)


Regards,
Devaraj Rangasamy

 

[r.devaraj]

but do remember that you should scan the whole array, any how.. ;>



eager to know possible further optimizations..

 

[Emmanuel Delahaye]

Roman Mashak wrote on 18/06/05 :

Is there an easy way to determine that array e.g. int X[N] contains ordered
items (for example, ascending), except running loop with comparison of items?

if (X[0] == searched)
else if X[1] == searched <...>
else if X[2] == searched <...>
else if X[3] == searched <...>
<...>
else if x[N-1] == searched <...>

If you don't like this code, use ... a loop ... !

In real world, the loop is more or less hidden by some lookup functions
such as the standard qsort()/bsearch() couple or some handmade
functions...

--
Emmanuel
The C-FAQ: http://www.eskimo.com/~scs/C-faq/faq.html
The C-library: http://www.dinkumware.com/refxc.html

I once asked an expert COBOL programmer, how to
declare local variables in COBOL, the reply was:
"what is a local variable?"

 

[Malcolm]

Is there an easy way to determine that array e.g. int X[N] contains
ordered items (for example, ascending), except running loop with
comparison of items?

It would be good to provide me with some useful link

No way of doing what you wnat in less than O(N) time.

However if you know the propeties of your array you can do a "good enough"
test by taking the start, the end, the middle, and the second and third
quartiles. The chance of these being in order by chance is relatively low.
(5!, or 1 in 120) In addition if the middle is very approximately the mean
of the middle three, and you know the distribution is either unform or with
a symmetrical central peak, then it is pretty certain that the array is
ordered.
What the test won't detect is slight deviations from orderedness, for
instnace by swapping one pair of elements. These could be malicious or they
could be because ordering is not random. However the chance of them arising
from a random distribution is vanishingly small.

 

[Lawrence Kirby]

Hello, All!

Is there an easy way to determine that array e.g. int X[N] contains ordered
items (for example, ascending), except running loop with comparison of
items?

Note that comp.lang.c is for discussing the C programming language itself,
a good place to discuss algorithms is comp.programming.

If you know nothing about the array then you probably won't do better than
this, you clearly need to access evenry element of the array to determine
this, any element you don't access might be out of order and your
algorithm couldn't detect it. OTOH it might be better to deal with this
when you build the array, e.g. build it ordered or test ordering while you
build it. In that case the determining step becomes trivial.

Lawrence

 

[Joe Wright]

Hello, All!

Is there an easy way to determine that array e.g. int X[N] contains ordered
items (for example, ascending), except running loop with comparison of
items?

It would be good to provide me with some useful link

No. You have to check. What will you do if, after checking, X is not
ordered? Will you then order it? If so, don't check at all, simply order
the array with a simple insertion sort. If X were already ordered only
checking takes place.

 

[Jean-Claude Arbaut]

Le 18/06/2005 17:01, dans (e-mail address removed), « Joe Wright »

Hello, All!

Is there an easy way to determine that array e.g. int X[N] contains ordered
items (for example, ascending), except running loop with comparison of
items?

It would be good to provide me with some useful link

No. You have to check. What will you do if, after checking, X is not
ordered? Will you then order it? If so, don't check at all, simply order
the array with a simple insertion sort. If X were already ordered only
checking takes place.

And if it was not, insertion sort is a snail, bad advice I think.

 

[Jean-Claude Arbaut]

Le 18/06/2005 17:04, dans BEDA0490.56DC%[email protected],

Le 18/06/2005 17:01, dans (e-mail address removed), « Joe Wright »

Hello, All!

Is there an easy way to determine that array e.g. int X[N] contains ordered
items (for example, ascending), except running loop with comparison of
items?

It would be good to provide me with some useful link

No. You have to check. What will you do if, after checking, X is not
ordered? Will you then order it? If so, don't check at all, simply order
the array with a simple insertion sort. If X were already ordered only
checking takes place.

And if it was not, insertion sort is a snail, bad advice I think.

BTW, doing a check and sorting with heapsort is still better
asymptotically

 

[Mac]

Hello, All!

Is there an easy way to determine that array e.g. int X[N] contains ordered
items (for example, ascending), except running loop with comparison of
items?

It would be good to provide me with some useful link

Thanks!

With best regards, Roman Mashak. E-mail: (e-mail address removed)

Maybe you can throw the array into a struct along with a flag indicating
whether the array is sorted. Of course it will then be necessary to keep
the flag up to date in all cases.

struct data
{ int *X;
int is_sorted;
};

--Mac

 

[Christian Bau]

Le 18/06/2005 17:01, dans (e-mail address removed), « Joe Wright »

Hello, All!

Is there an easy way to determine that array e.g. int X[N] contains ordered
items (for example, ascending), except running loop with comparison of
items?

It would be good to provide me with some useful link

No. You have to check. What will you do if, after checking, X is not
ordered? Will you then order it? If so, don't check at all, simply order
the array with a simple insertion sort. If X were already ordered only
checking takes place.

And if it was not, insertion sort is a snail, bad advice I think.

Should run in O (k*N) if k elements in an initially sorted array of N
elements have been changed. Kind of hard to beat for small k.

 

[Joe Wright]

Le 18/06/2005 17:04, dans BEDA0490.56DC%[email protected],

Le 18/06/2005 17:01, dans (e-mail address removed), « Joe Wright »

Roman Mashak wrote:

Hello, All!

Is there an easy way to determine that array e.g. int X[N] contains ordered
items (for example, ascending), except running loop with comparison of
items?

It would be good to provide me with some useful link


No. You have to check. What will you do if, after checking, X is not
ordered? Will you then order it? If so, don't check at all, simply order
the array with a simple insertion sort. If X were already ordered only
checking takes place.

And if it was not, insertion sort is a snail, bad advice I think.

You think? I expect that 'int X[N]' is maintained in order. Calling an
insertion sort on X when X is already ordered is fast as lightning.

BTW, doing a check and sorting with heapsort is still better
asymptotically

My point was to sort without checking, knowing the result is an ordered
array. The choice of which sort algorithm is up to the user.

 

[SM Ryan]

# >>> Is there an easy way to determine that array e.g. int X[N] contains ordered
# >>> items (for example, ascending), except running loop with comparison of
# >>> items?

Sheesh, the answer is there is no library function, but it's easy to code
with one loop.
int increasing = 1,i;
for (i=1; i<N && increasing; i++) increasing = X[i-1]<X;
will check for strictly increasing. For increasing, use X[i-1]<=X.
You can use > and >= for strictly decreasing and decreasing. To
check monotonictity
int monotonic = 1,cc = 0,i;
for (i=1; i<N && cc==0; i++) cc = X[i-1]-X;
if (cc<0) {
for (; i<N && monotonic; i++) monotonic = X[i-1]< /*<=*/ X;
}else if (cc>0) {
for (; i<N && monotonic; i++) monotonic = X[i-1]> /*>=*/ X;
}
For N=0 and N=1, X is reported as (strictly) monotonic, increasing and decreasing.

If there are no other constraints on X besides being integers, you need N-1
binary comparison. Picture it as a binary tree with N leaves: no matter how
you arrange the tree, you will always have N-1 internal nodes.

On a parallel machines you can divide the tree in halves and halves again
recursively and thus complete it in lg N steps, but you will need N/2
processors in parallel.