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Raistlin
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 Posted: Sun Sep 28, 2003 6:00 am Post subject: Comparing 2 unequal arrays... |
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Hey, for c++ does any body have a way to compare two arrays? Say one
has the numbers 1-5000, the other has *most* of the numbers from
1-5000, is there a way to display the numbers that are in the 1st
array, but not in the 2nd array? Or load those numbers in a 3rd array?
Thats the simple version...there are a couple other problems: ex:
the 10th value of the 2nd array corresponds to the 12th value of the
first.
- you cant simply make a for loop and compare each individual one...
The arrays are different sizes...one having 5000 values, the other
like 4700ish.
Any ideas? I was thinking of something like writing the smaller array
into a file, and then having something read the values of the first
array, and if it doesnt find it in the file, then it sends it to a
diff file or something...but I have NO IDEA how to code that...
Any ideas/thoughts/etc would be very welcome =)
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Conrad Weyns
Guest
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| Posted: Sun Sep 28, 2003 7:29 am Post subject: Re: Comparing 2 unequal arrays... |
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"Raistlin" <IceSword_Web (AT) hotmail (DOT) com> wrote
| Quote: | Hey, for c++ does any body have a way to compare two arrays? Say one
has the numbers 1-5000, the other has *most* of the numbers from
1-5000, is there a way to display the numbers that are in the 1st
array, but not in the 2nd array? Or load those numbers in a 3rd array?
Thats the simple version...there are a couple other problems: ex:
the 10th value of the 2nd array corresponds to the 12th value of the
first.
- you cant simply make a for loop and compare each individual one...
The arrays are different sizes...one having 5000 values, the other
like 4700ish.
Any ideas? I was thinking of something like writing the smaller array
into a file, and then having something read the values of the first
array, and if it doesnt find it in the file, then it sends it to a
diff file or something...but I have NO IDEA how to code that...
Any ideas/thoughts/etc would be very welcome =)
|
Play with the following:
---- main.cpp ----
#include <iostream>
#include <map>
int main()
{
using namespace std;
int a[] = { 1, 3, 5, 600, 600, 123, 145 };
int b[] = { 3, 3, 1, 600, 5000, 145 };
map<int, pair m;
for (int i = 0; i < sizeof(a) / sizeof(a[0]); ++i)
{
pair
p.first = true;
}
for (int i = 0; i < sizeof(b) / sizeof(b[0]); ++i)
{
pair
p.second = true;
}
for (map<int, pair::const_iterator cit = m.begin(); cit !=
m.end(); ++cit)
{
int key = cit->first;
pair<bool, bool> const& p = cit->second;
cout << key << " " << p.first << " " << p.second << endl;
}
return 0;
}
/* Output:
1 1 1
3 1 1
5 1 0
123 1 0
145 1 1
600 1 1
5000 0 1
thus:
1 is in a and b
3 is in a and b
5 is in a not b
....
5000 is not in a but in b
etc etc
*/
--------------------
h.t.h
Conrad Weyns
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Mattia Belletti
Guest
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| Posted: Sun Sep 28, 2003 10:50 am Post subject: Re: Comparing 2 unequal arrays... |
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Well, this is not inherently C++, it's more a general algorithm question.
I add another way over that suggested by Conrad: sort the two arrays (I
know qsort C method, but surely there's something equivalent in C++
standard libraries), and then you can simply run with two pointers over
the two arrays: when the value pointed by one is greater that the
other, increase by one the other (pointer math...) - whenever the two
pointers swap role (the one which before pointed a number greater now
points a number lesser than the other) it means that the value given by
the pointer which has remained fixed isn't present in the other array;
if they are the same, you've found an exact match, you can print it, and
then increase both.
This is very C-like, but it should work.
Time needed:
sorting (n lg n) + comparisons (n) = n lg n. I think it's the best you can
sort out, if the arrays are unordered.
--
/**
* Mattia Belletti - Undergraduate student @ cs.unibo.it
* ICQ: 33292311 - email: [email]mbellett (AT) cs (DOT) unibo.it[/email]
* IRC: BluShine - site(s): http://cs.unibo.it/~mbellett
* Linux registered user 299762 @ Linux registered machine 213003
*/
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Ivan Vecerina
Guest
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| Posted: Sun Sep 28, 2003 11:16 am Post subject: Re: Comparing 2 unequal arrays... |
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"Raistlin" <IceSword_Web (AT) hotmail (DOT) com> wrote
| Quote: | Hey, for c++ does any body have a way to compare two arrays? Say one
has the numbers 1-5000, the other has *most* of the numbers from
1-5000, is there a way to display the numbers that are in the 1st
array, but not in the 2nd array? Or load those numbers in a 3rd array?
Thats the simple version...there are a couple other problems: ex:
the 10th value of the 2nd array corresponds to the 12th value of the
first.
- you cant simply make a for loop and compare each individual one...
The arrays are different sizes...one having 5000 values, the other
like 4700ish.
Any ideas? I was thinking of something like writing the smaller array
into a file, and then having something read the values of the first
array, and if it doesnt find it in the file, then it sends it to a
diff file or something...but I have NO IDEA how to code that...
|
Sorting both sequences first is the most efficient approach.
The standard C++ algorithm std::set_difference outputs the
'difference' of two sorted sequences (that is, items of the first
one that are not present in the second one).
#include <vector>
#include <algorithm>
#include <iterator>
#include <iostream>
typedef std::vector<int> Items;
void printDiff(Items& a, Items& b)
{
std::sort( a.begin(), a.end() );
std::sort( b.begin(), b.end() );
std::set_difference
( a.begin(), a.end(), b.begin(), b.end()
, std::ostream_iterator<int>(std::cout,"n") );
}
Instead of printing the output to cout using ostream_iterator,
you could as well store it into a new collection:
Items diff;
std::set_difference
( a.begin(), a.end(), b.begin(), b.end()
, std::back_inserter(diff) );
// diff now contains the requested items.
I hope this helps,
Ivan
--
http://ivan.vecerina.com
http://www.brainbench.com <> Brainbench MVP for C++
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Conrad Weyns
Guest
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| Posted: Sun Sep 28, 2003 3:55 pm Post subject: Re: Comparing 2 unequal arrays... |
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"Ivan Vecerina" <ivecATmyrealboxDOTcom> wrote
| Quote: | "Raistlin" <IceSword_Web (AT) hotmail (DOT) com> wrote in message
news:c740351e.0309272200.74925e87 (AT) posting (DOT) google.com...
Hey, for c++ does any body have a way to compare two arrays? Say one
has the numbers 1-5000, the other has *most* of the numbers from
1-5000, is there a way to display the numbers that are in the 1st
array, but not in the 2nd array? Or load those numbers in a 3rd array?
Thats the simple version...there are a couple other problems: ex:
the 10th value of the 2nd array corresponds to the 12th value of the
first.
- you cant simply make a for loop and compare each individual one...
The arrays are different sizes...one having 5000 values, the other
like 4700ish.
Any ideas? I was thinking of something like writing the smaller array
into a file, and then having something read the values of the first
array, and if it doesnt find it in the file, then it sends it to a
diff file or something...but I have NO IDEA how to code that...
Sorting both sequences first is the most efficient approach.
The standard C++ algorithm std::set_difference outputs the
'difference' of two sorted sequences
|
The key here is "difference".
| Quote: | (that is, items of the first one that are not present in the second one).
|
I think this is only true if there are no duplicates in a collection else
you'll get the number of times one specific value occurs *more* often than
in the other sequence. Ehm, I should be able to phrase that better, so:
a[] = { 7, 7, 7 };
b[] = { 7 };
difference = { 7, 7 };
I think anyway..
Regards,
Conrad Weyns
| Quote: |
#include <vector
#include
#include
#include
typedef std::vector
void printDiff(Items& a, Items& b)
{
std::sort( a.begin(), a.end() );
std::sort( b.begin(), b.end() );
std::set_difference
( a.begin(), a.end(), b.begin(), b.end()
, std::ostream_iterator<int>(std::cout,"n") );
}
Instead of printing the output to cout using ostream_iterator,
you could as well store it into a new collection:
Items diff;
std::set_difference
( a.begin(), a.end(), b.begin(), b.end()
, std::back_inserter(diff) );
// diff now contains the requested items.
I hope this helps,
Ivan
--
http://ivan.vecerina.com
http://www.brainbench.com <> Brainbench MVP for C++
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Conrad Weyns
Guest
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| Posted: Sun Sep 28, 2003 4:04 pm Post subject: Re: Comparing 2 unequal arrays... |
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"Conrad Weyns" <weynsTAKEAWAY (AT) invalid (DOT) online.no> wrote
| Quote: |
"Ivan Vecerina" <ivecATmyrealboxDOTcom> wrote in message
news:3f76c1fd (AT) news (DOT) swissonline.ch...
"Raistlin" <IceSword_Web (AT) hotmail (DOT) com> wrote in message
news:c740351e.0309272200.74925e87 (AT) posting (DOT) google.com...
[] |
| Quote: | Sorting both sequences first is the most efficient approach.
The standard C++ algorithm std::set_difference outputs the
'difference' of two sorted sequences
The key here is "difference".
(that is, items of the first one that are not present in the second one).
|
Sorry! I see it now, that's exactly what you meant....
conrad weyns
| Quote: |
I think this is only true if there are no duplicates in a collection else
you'll get the number of times one specific value occurs *more* often than
in the other sequence. Ehm, I should be able to phrase that better, so:
a[] = { 7, 7, 7 };
b[] = { 7 };
difference = { 7, 7 };
I think anyway..
Regards,
Conrad Weyns
[] |
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Ron Natalie
Guest
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| Posted: Mon Sep 29, 2003 2:13 pm Post subject: Re: Comparing 2 unequal arrays... |
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"Raistlin" <IceSword_Web (AT) hotmail (DOT) com> wrote
| Quote: | Hey, for c++ does any body have a way to compare two arrays?
|
Well the equality operator doesn't really work on arrays at all (due to the misimplementation
of arrays in the language). There are various algorithms for doing element-wise comparisons.
See the Equal library function for simple equality tests.
| Quote: | Say one
has the numbers 1-5000, the other has *most* of the numbers from
1-5000, is there a way to display the numbers that are in the 1st
array, but not in the 2nd array? Or load those numbers in a 3rd array?
|
Are the arrays sorted? It makes a difference as to the algorithm.
| Quote: |
Thats the simple version...there are a couple other problems: ex:
the 10th value of the 2nd array corresponds to the 12th value of the
first.
- you cant simply make a for loop and compare each individual one...
|
Is the order of the elements found important? Are the numbers unique?
There are solutions, but we need a more conscise description of what you want.
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