Big O and algorithm to decide string A contains string B?
us...@yahoo.com
contains String B. For example, String A = "This is a test"; String B =
"is is". So it will return TRUE if String A includes two "i" and two
"s". The function should also handle if String A and B have huge
values, like two big dictionary.
What's the best approach to achieve this with the best performance?
what's the Big O then?
I am thinking to put A and B into two hashtable, like key = "i" and
value = "2" and so on. Then compare these two hashtable. But how to
compare two hashtables? Please advise.
Mark P
One simple possibility is to iterate through each position i within
string A and see if the substring A(i,i+length(B)) = B. Java and C++
have facilities to make this quite simple. The worst case runtime is
O(length(A)*length(B)) under reasonable assumptions about how string
comparisons are performed.
Perhaps one can do better if one is clever.
Siemel Naran
> I need to implement a function to return True/false whether String A
> contains String B. For example, String A = "This is a test"; String B =
> "is is". So it will return TRUE if String A includes two "i" and two
> "s". The function should also handle if String A and B have huge
> values, like two big dictionary.
Because String B = "is is" (note the space), should the function return true
if String A contains two "i", two "s", one space?
The first step would be to parse String B so that we know to expect 2 "i", 2
"s", 1 " ".
What's the number of distinct chars? Is it A-Z and a-z for a total of 52?
If the number of distinct chars is small, say 256 distinct chars, create an
array, such as unsigned expect[256], to represent the number of chars to
expect. expect['i'] would equal 2, expect['s'] would equal 2, expect[' ']
would equal 1, and all other expect elements would be zero. If the number
of distinct chars is large, then you could use a map<char_type, unsigned> or
hashtable.
Now step through every char in String A. Let the char in question be char
c. Decrement expect[c] by one, but if it is zero then don't decrement it.
Now check if all the elements in expect are zero. As an optimization, you
only need to do this check if you decremented expect[c].
To check if all the elements in expect are zero, you could for example
create another array zero, such as unsigned zero[256] = {0}, and use memcmp
to compare expect to zero. There are other ways, maybe platform specific
ways that might be faster.
Remember to deal with the special case that String B is the empty string, in
which case you can probably immediately return true.
> What's the best approach to achieve this with the best performance?
> what's the Big O then?
My algorithm is O(length(String A)) + O(length(String B)).
> I am thinking to put A and B into two hashtable, like key = "i" and
> value = "2" and so on. Then compare these two hashtable. But how to
> compare two hashtables? Please advise.
In principle it is doable, but putting all the chars of String A into a
hashtable might be rather expensive.
To compare two hashtables, you could iterate through the elements in the
first hash table, using a for_each(hash1.begin(), hash1.end()) structure, or
even for (Iter iter = hash1.begin(); iter != hash1.end(); ++iter). For each
element in hash1, look up the corresponding value in hash2, for example
hash2[iter->key]. Then check if the values are equal, for example
iter->value == hash2[iter->key].
kal...@poczta.onet.pl
should return true when string A contains string B. But your example
shows that it should return true when string A contains all characters
that are in string B. This is something very different. Which one is
your problem?
cheers,
Marcin Kalicinski
Gernot Frisch
<us...@yahoo.com> schrieb im Newsbeitrag
news:1111648920....@o13g2000cwo.googlegroups.com...
bool XY(const char* src, const char* seek)
{
std::map<char, int> counter;
std::map<char, int>::iterator it;
// Fill map with number of chars in dest
for(; *seek!='\0'; ++seek)
{
if(is_char_to_be_counted(*seek))
++counter[*seek];
}
// subtract the count of these chars on src
for(; *src!='\0'; ++src)
{
it = counter.find(*src);
if(it!=counter.end() && --it->second<0)
return false; // src has more 'x's than dest
}
// See if any of these has different count
for(it=counter.begin(); it!=counter.end(); ++it)
{
if(it->second) return false; // dest has more 'x's than src
}
return true;
}
this should so the trick quickly. If you really have chars, you can
replace the std::map with a simple array of 256 ints...
-Gernot
us...@yahoo.com
return TRUE if string A contains all characters
that are in string B. Or String A has what String B has. For example,
String B is "issi" and String A is "This is a test", the function will
return TRUE. So what if String A and B have big values, what's the best
algorithm and Big O to achieve this?
I am thinking creating two hashtable. For String B, key=i, value=2;
key=s, value=2. For String A, key=i, value=2; key=s, value=3, etc. Then
compare this two hashtable. So constructing these two hashtable will be
expensive but the Big O will be O(nlogn). Is it good?
Willem
) Acutally the problem is not about String Matching. The function will
) return TRUE if string A contains all characters
) that are in string B. Or String A has what String B has. For example,
) String B is "issi" and String A is "This is a test", the function will
) return TRUE. So what if String A and B have big values, what's the best
) algorithm and Big O to achieve this?
)
) I am thinking creating two hashtable. For String B, key=i, value=2;
) key=s, value=2. For String A, key=i, value=2; key=s, value=3, etc. Then
) compare this two hashtable. So constructing these two hashtable will be
) expensive but the Big O will be O(nlogn). Is it good?
Why a hashtable ? There are only 256 different characters, so you
can just make an array with 256 entries and count.
The BigO will be O(n).
SaSW, Willem
--
Disclaimer: I am in no way responsible for any of the statements
made in the above text. For all I know I might be
drugged or something..
No I'm not paranoid. You all think I'm paranoid, don't you !
#EOT
us...@yahoo.com
array using char as the index? array index is supposed to be int,
right? For example, array[0] = 'a' instead of array['a'] = 0. Sorry, I
am a newbie to algorithm...
Karl Heinz Buchegger
>
> Yes. An array with 256 entries should be enough. But how to create an
> array using char as the index? array index is supposed to be int,
> right?
a char is nothing more then a small integer in C and C++. Only
during input and output a char is treated differently.
> For example, array[0] = 'a' instead of array['a'] = 0. Sorry, I
> am a newbie to algorithm...
array['a'] = 0;
is fine. (Remember: a char is nothing more then an small integer. Its
value is the code number of the character on your system).
--
Karl Heinz Buchegger
kbuc...@gascad.at
msalters
char is an integral type, like short, int and long. They'll convert
without problems. i.e.
int frequency[ 1<<CHAR_BIT ] = {0}; // typically 256 entries
std::string A = foo();
for( int i=0;i!=A.size(); ++i )
++frequency[ A[i] ];
std::string B = bar();
for( int i=0;i!=B.size(); ++i )
if( frequency[ B[i] ]-- == 0 )
std::cout << "B not in A";
Obvious O(A.size+B.size) and you can't do better
in general.
HTH,
Michiel Salters
Roger Willcocks
news:1111747133.4...@l41g2000cwc.googlegroups.com...
std:string is represented as 'char's which can be signed or unsigned
depending on the platform, so you probably want e.g. ++frequency[A[i] &
0xff].
--
Roger
Pointless Harlows
"Willem" <wil...@stack.nl> wrote in message
news:slrnd46ebs....@toad.stack.nl...
You cross post this in a Jav newsgroup and have the cheek to suggest that
there are only 256 characters ! That's why I abandoned C and it's
derivatives... which simply assumed that nobody in China, Japan, Korea etc
would ever need to use a computer ;-)
Charles Richmond
computers in English!!! ;-)
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