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i have a 16 bit variable(unsigned short).
now i want a best algorithm (function) which will do the following.
1.if contiguous bits at LSB are high then the function should return 1.
for example, consider the following bit patterns
0000000001111111
or
0000111111111111
or
0000000000000001
2.if continuous bits at MSB are high then the function should return 2.
for example,
1110000000000000 and so on.
3.other wise it should return 0 for the examoples such as
1010000000000011
0000011101010011 and so on
thanks in advance.
any more clarifications required, please get back.
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Test the int with this constant variables if you want to not use a for loop with SHIFTs and bit-comparing.
To return 1:
1111111111111110
1111111111111100
1111111111111000
...
1100000000000000
1000000000000000
to return 2:
0000000000000001
0000000000000011
...
0111111111111111
else return 0
Russell
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There might be some nifty bit processing, but this should work with no more than about 6 comparisons (32 bits - 62 options)
8-bit version:
int fn(int x)
{
switch (x)
case 0x01:
case 0x03:
case 0x07:
case 0x0F:
case 0x1F:
case 0x3F:
case 0x7F:
return 1;
case 0x80:
case 0xC0:
case 0xE0:
case 0xF0:
case 0xF8:
case 0xFC:
case 0xFE:
return 1;
default:
return 0;
}
I think compilers are quite good at optimizing this sort of code, but I have never tested it so I'd be interested in comments.
Peter
"Until the invention of the computer, the machine gun was the device that enabled humans to make the most mistakes in the smallest amount of time."
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thanks,
the above logic is simple and works as of now.
but the ultimate goal is to design a function in the following prototype.
int GetWordStatus(int word,int word length)
{
//code here
}
if the word length is certainly 8 or 16 or 32, then teh above logic is appropriate.
but the checking has to be done on the first specified number of bits. i.e. wordlength.
hope i dont need to give any example,
if required, i can.
thanku.
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You should be able to extend this - do something like:
const int hi1 = {0xFF, 0xFE, 0xFC, 0xF8, 0xF0, 0xE0, 0xC0, 0x80, 0};
int GetWordStatus(int word, int wordlength)
{
ASSERT(wordLength <= 8 && wordLength >= 0);
int x = word & ~hi1[wordLength];
switch (x)
{
case 0x01:
case 0x03:
case 0x07:
case 0x0F:
case 0x1F:
case 0x3F:
case 0x7F:
return 1;
}
x = word | hi1[wordLength];
switch (x)
{
case 0x80:
case 0xC0:
case 0xE0:
case 0xF0:
case 0xF8:
case 0xFC:
case 0xFE:
return 2;
}
return 0;
}
It may be a bit inefficient for small wordlengths, if it is a problem you should spend some time and make it better.
-- modified at 6:57 Monday 3rd September, 2007
The reason I am suggesting a switch statement rather than a loop is that I thought the reason that in C/C++ the case values had to be constants was so that the compiler could order them and implement the switch as a simple binary search, so for N case statements there would be only log2(N) comparisons. I can't easily find a reference for this now - does anyone know if this happens?
Peter
"Until the invention of the computer, the machine gun was the device that enabled humans to make the most mistakes in the smallest amount of time."
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cp9876 wrote: I can't easily find a reference for this now - does anyone know if this happens?
Try to look the disassembly window opening it to the code you suggested ... 
I think that this shortcut is applyed from the compiler only if the number of case is greather then a specified number ... and only if you turn on that optimizations that is probally related with faster applications, but not applyed in small exe file dimension.
Russell
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In this case I think that the better solution is a general solution...so a loop like
tmpword=word;
for(i=0;i<wordlength;i++){
if(tmpword == 1) return 1;
if(tmpword & 1 ) tmpword >>=1;
else break;
}
tmpword=word;
for(i=0;i<wordlength;i++){
if(tmpword == 0x80) return 2;
if(tmpword & 0x80 ) tmpword <<=1;
else break;
}
return 0;
or somethig similar to this
(I haven't tested the code...it is only an idea of what I'm meaning above... )
Russell
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try this vb code:
Private Function GetWordStatus(ByVal word As Integer) As Integer
Dim significantLength As Integer = (word & "").Length
'Calculate significant Length of number
Dim temp As Integer
temp = (2 ^ significantLength) - 1
If Convert.ToInt64(word, 2) = temp Then
Return 1
Else
Dim cntr As Integer
temp = 2 ^ (significantLength - 1)
cntr = (significantLength - 1)
While cntr > 1
If Convert.ToInt64(word, 2) = temp Then
Return 2
Else
temp += 2 ^ (cntr - 1)
End If
cntr = cntr - 1
End While
End If
' Return 0
End Function
Its not the best but an alternative
Any systematic work reflects its significance for a long time. So let's discuss the best...
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Hi,
it is much simpler than that.
one spec is missing: what if input is zero ?
THEORY (assume unsigned integers)
1.
if all zeroes followed by all ones, then one more becomes a power of two
2.
if all ones followed by all zeroes, then bitwise complement reverts to first case
3.
a power of two has just one bit set; it can be tested in one line:
if (( x & (-x) )==x) then either x = zero or x = power of 2 !!!
CODE
if (x==0) return whatever you think is appropriate
if ((x&(-x))==x) return 1;
x=~x;
if ((x&(-x)==x) return 2;
return 3;
Luc Pattyn [Forum Guidelines] [My Articles]
this weeks tips:
- make Visual display line numbers: Tools/Options/TextEditor/...
- show exceptions with ToString() to see all information
- before you ask a question here, search CodeProject, then Google
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You won
Russell
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I knew there would be a clever bit twiddling answer to this - very neat!
Peter
"Until the invention of the computer, the machine gun was the device that enabled humans to make the most mistakes in the smallest amount of time."
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fn(WORD w)
{
WORD mark=0x8000;
if(w&mark)
{
mark >>= 1;
while( (w&mark) && mark )
mark >>= 1;
if(mark==0)//w must = 0xffff,return 1 or 2
return 2;//also can return 1
if(w&(mark-1))
return 0;
else
return 2;
}
else
{
mark >>= 1;
while( ((w&mark)==0) && mark )
mark >>= 1;
if( (w&(mark-1))==(mark-1) )
return 1;
else
return 0;
}
}
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Hi,
what about...
int GetWordStatus(int word,int word length)
{
if (word == 0)
return 0;
int nHigh = word / pow (2, word_length);
int nLow = word % pow (2, word_length);
if (nHigh > nLow)
return 1;
else if (nHigh < nLow)
return 2;
else
return 3;
}
Greetings.
--------
M.D.V.
If something has a solution... Why do we have to worry about?. If it has no solution... For what reason do we have to worry about?
Help me to understand what I'm saying, and I'll explain it better to you
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There are two sets
one set has ten thousands elements
other set has 30 element
Which loop would be the faster
for(i = 0; i < sizeoffirstset(10000); i++)
{
for (j=0; i < sizeofsecondset(20); j++)
{
if(set1[i] == set2[j])
//do some thing
}
}
for(i = 0; i < sizeofsecondset(20); i++)
{
for (j=0; i < sizeoffirstset(10000); j++)
{
if(set2[i] == set1[j])
//do some thing
}
}
Best Regards,
Mushq
Mushtaque Ahmed Nizamani
Software Engineer
Ultimus Pakistan
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Both are doing the same operations, just in a different order.
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Mushq wrote: sizeofsecondset(20)...sizeoffirstset(10000)
const UINT Sizeoffirstset =sizeoffirstset (10000);
const UINT Sizeofsecondset=sizeofsecondset(20);
for(i = 0; i < Sizeofsecondset; i++)
{
for (j=0; j < Sizeoffirstset; j++)
{
if(set2[i] == set1[j])
}
}
I'm looking for differences, I see only that the initialization j=0 is computed Sizeofsecondset times ... so let Sizeofsecondset be the smaller number.
Russell
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Mushq wrote: for(i = 0; i < sizeoffirstset(10000); i++)
{
for (j=0; i < sizeofsecondset(20); j++)
Ahahah
faster yes .... but to go where?
Russell
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First one is faster and because of internal smaller array.
run following code in VB:
Dim i, j As Integer
Dim strArr1(10000) As String
Dim strArr2(20) As String
For i = 0 To 10000
strArr1(i) = i & ""
Next
For i = 0 To 20
strArr2(i) = i * i * i & ""
Next
Response.Write(DateTime.Now.Millisecond.ToString())
For i = 0 To 10000
For j = 0 To 20
If (strArr1(i) = strArr2(j)) Then
Response.Write("
Hello" & strArr1(i) & strArr2(j))
End If
Next
Next
Response.Write(DateTime.Now.Millisecond.ToString())
For i = 0 To 20
For j = 0 To 10000
If (strArr2(i) = strArr1(j)) Then
Response.Write("
Hello " & strArr2(i) & strArr1(j))
End If
Next
Next
Response.Write(DateTime.Now.Millisecond.ToString())
Any systematic work reflects its significance for a long time. So let's discuss the best...
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Yes: finally the best way to find out who is the faster is use ... the clock
Russell
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Sure, measuring is good, smart measuring is even better.
if you have two alternatives that are likely to be almost equal, whichever you try first
will take longer, due to different starting conditions (maybe code needs to be JITted,
probably data needs to be loaded in memory and/or will be loaded in cache, etc).
Remedy: put everything in a for loop, and run a few passes; ignore the first pass,
take the average (or the best time!) of the remaining passes.
Luc Pattyn [Forum Guidelines] [My Articles]
this weeks tips:
- make Visual display line numbers: Tools/Options/TextEditor/...
- show exceptions with ToString() to see all information
- before you ask a question here, search CodeProject, then Google
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Thanks for reply.
rihdus wrote: First one is faster and because of internal smaller array.
can you please explain a bit more.
Best Regards,
Mushq
Mushtaque Ahmed Nizamani
Software Engineer
Ultimus Pakistan
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clock_t start,end;
start=clock();
//your code snippet here
end=clock();
//observe start=end here.
hope you got the point.
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If time is important you could almost certainly do better than either way. For example, instead doing a simple search 10000 times on a set of 20 things, spend a bit of time and either order them or hash them, so that the search that you have to do many times is much faster.
Peter
"Until the invention of the computer, the machine gun was the device that enabled humans to make the most mistakes in the smallest amount of time."
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Mushq wrote: for(i = 0; i < sizeoffirstset(10000); i++)
{
for (j=0; i < sizeofsecondset(20); j++)
{
if(set1[i] == set2[j])
//do some thing
}
}
another optimization:
for(i = 0; i < sizeoffirstset(10000); i++)
{
const double set1_i_ = set1[i];
for (j=0; j < sizeofsecondset(20); j++)
{
if(set1_i_ == set2[j])
}
}
In this way the program not needs to reload that value every time loooking into the array.
Hoping that the compiler is enough smart to better solution itself.
You can only check the time spend in every solutions and see who is the faster...
Important: do this time check in Release mode!!!
also look to the project features...I remember that there is some flags like smallest exe file dimension vs fastest run exe (that somethimes can be bigger)....but I don't remember the name of this flags.
Hope helps
Russell
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_Russell_ wrote: Important: do this time check in Release mode!!!
Could you please explain a bit what the difference is?
Thanks.
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