code to find th square root of a number

CODY21 · 11-03-2008, 02:53 AM

I'm a newbie and I'm writing a program preform Pythagoras’ theorem but I cant work out the code to square a number.
If anyone out there knows it please help me.
Thanks

Akkernight · 11-03-2008, 03:02 AM

Ehh... I think it's in math.h header, and I think it was something like... sqroot(), but probably not, try searching "Square Root in C++" or something on Google

EVOEx · 11-03-2008, 05:29 AM

It's sqrt for doubles, sqrtf for a double and sqrtl for a long double...
And they reside in the "cmath" header.

matsp · 11-03-2008, 05:30 AM

Quote:

Originally Posted by EVOEx

It's sqrt for doubles, sqrtf for a double and sqrtl for a long double...
And they reside in the "cmath" header.

Correction: sqrtf() would be the function for float.

Although in C++ I think there are wrappers to handle float, double and long double under a single name.

--
Mats

C_ntua · 11-03-2008, 05:31 AM

Code:

template<class T)
class T mySqrt(T x, double precision)
{
    if (x <= 0) return 0;
    T r = x;
    while (x - x/r > precision)
        r = (r + x/r) / 2;
    return r;
}

or for obvious reasons just use the ready sqrt() in math.h which handles double, long double and float http://www.cplusplus.com/reference/c...math/sqrt.html

abachler · 11-03-2008, 05:44 AM

or...

Code:

double Number;
 
__asm {
   FLD     Number
   FSQRT Number
   FSTP   Number
   }

which is far faster than calling the sqrt() function

master5001 · 11-03-2008, 08:02 PM

I find the hypot() function useful. By the way, depending on what you are doing, its usually optimal to NOT square root your result and just always work with (x^2). It makes collision detection algorithms work oh so much faster.

Saimadhav · 11-05-2008, 04:44 AM

both the functions are in cmath header file.
to find the squre root

Code:

cout<<sqrt(variable);

to find the square

Code:

cout<<pow(a,2);

g4j31a5 · 11-05-2008, 11:30 AM

Sorry, a little OOT. After some thorough browsing, I found this snippet:

Code:

float InvSqrt (float x)
{
    float xhalf = 0.5f*x;
    int i = *(int*)&x;
    i = 0x5f3759df - (i>>1);
    x = *(float*)&i;
    x = x*(1.5f - xhalf*x*x);
    return x;
}

FYI, it is Quake's inverse square root. But I sure can't understand it clearly. Especially the " i = 0x5f3759df - (i>>1);" part. I think it's much faster than the cmath's version.

EDIT: My mistake. You needed square and square root. Not inverse square root.

BobS0327 · 11-05-2008, 11:52 AM

Quote:

But I sure can't understand it clearly. Especially the " i = 0x5f3759df - (i>>1);"

0x5F3759DF

Raigne · 11-05-2008, 12:56 PM

That was very insightful, and surprising to me.

master5001 · 11-05-2008, 01:02 PM

Very interesting, Bob. Where on earth did you dig that up from?

BobS0327 · 11-05-2008, 01:15 PM

Quote:

Originally Posted by master5001

Very interesting, Bob. Where on earth did you dig that up from?

From Chris Lomont's home page. Unfortunately, the link to his papers from his home page has some isues at times. So, I linked directly to the PDF.

**CornedBee** · 11-05-2008, 01:36 PM

Quote:

Originally Posted by abachler

or...

Code:

double Number;
 
__asm {
   FLD     Number
   FSQRT Number
   FSTP   Number
   }

which is far faster than calling the sqrt() function

And far less portable. Not to mention that, with the right switches (e.g. -ffast-math in GCC), your compiler might just optimize the sqrt call to the FSQRT instruction anyway. Which wouldn't break the compiler's register management code like the ASM snippet does. Or, in a loop, the compiler might parallelize a sqrt algorithm into a SIMD implementation with four times the throughput of the x87 (and a non-braindead architecture).

g4j31a5 · 11-05-2008, 08:51 PM

Quote:

Originally Posted by BobS0327

0x5F3759DF

Thanks.

11-03-2008, 02:53 AM	#1
CODY21 Registered User Join Date: Nov 2008 Posts: 1	code to find th square root of a number I'm a newbie and I'm writing a program preform Pythagoras’ theorem but I cant work out the code to square a number. If anyone out there knows it please help me. Thanks
CODY21 is offline

11-03-2008, 03:02 AM	#2
Akkernight Hail to the king, baby. Join Date: Oct 2008 Location: Faroe Islands Posts: 718	Ehh... I think it's in math.h header, and I think it was something like... sqroot(), but probably not, try searching "Square Root in C++" or something on Google __________________ Currently research OpenGL
Akkernight is offline

11-03-2008, 05:29 AM	#3
EVOEx Registered User Join Date: Oct 2008 Posts: 528	It's sqrt for doubles, sqrtf for a double and sqrtl for a long double... And they reside in the "cmath" header.
EVOEx is offline

11-03-2008, 05:31 AM	#5
C_ntua Registered User Join Date: Jun 2008 Posts: 1,274	Code: template<class T) class T mySqrt(T x, double precision) { if (x <= 0) return 0; T r = x; while (x - x/r > precision) r = (r + x/r) / 2; return r; } or for obvious reasons just use the ready sqrt() in math.h which handles double, long double and float http://www.cplusplus.com/reference/c...math/sqrt.html
C_ntua is offline

11-03-2008, 05:44 AM	#6
abachler Malum in se Join Date: Apr 2007 Posts: 3,188	or... Code: double Number; __asm { FLD Number FSQRT Number FSTP Number } which is far faster than calling the sqrt() function __________________ Until you can build a working general purpose reprogrammable computer out of basic components from radio shack, you are not fit to call yourself a programmer in my presence. This is cwhizard, signing off.
abachler is offline

11-03-2008, 08:02 PM	#7
master5001 Banned Join Date: Aug 2001 Location: Visalia, CA, USA Posts: 3,699	I find the hypot() function useful. By the way, depending on what you are doing, its usually optimal to NOT square root your result and just always work with (x^2). It makes collision detection algorithms work oh so much faster.
master5001 is offline

11-05-2008, 04:44 AM	#8
Saimadhav C++Pandit Join Date: Jul 2008 Posts: 49	both the functions are in cmath header file. to find the squre root Code: cout<<sqrt(variable); to find the square Code: cout<<pow(a,2); __________________ I would rather be hated for who I am than be loved for who I am not!
Saimadhav is offline

11-05-2008, 11:30 AM	#9
g4j31a5 In the Land of Diddly-Doo Join Date: Jul 2006 Posts: 381	Sorry, a little OOT. After some thorough browsing, I found this snippet: Code: float InvSqrt (float x) { float xhalf = 0.5fx; int i = (int)&x; i = 0x5f3759df - (i>>1); x = (float)&i; x = x(1.5f - xhalfxx); return x; } FYI, it is Quake's inverse square root. But I sure can't understand it clearly. Especially the " i = 0x5f3759df - (i>>1);" part. I think it's much faster than the cmath's version. EDIT: My mistake. You needed square and square root. Not inverse square root. __________________ ERROR: Brain not found. Please insert a new brain! “Do nothing which is of no use.” - Miyamoto Musashi.
g4j31a5 is offline

11-05-2008, 12:56 PM	#11
Raigne Registered User Join Date: Nov 2005 Posts: 634	That was very insightful, and surprising to me.
Raigne is offline

11-05-2008, 01:02 PM	#12
master5001 Banned Join Date: Aug 2001 Location: Visalia, CA, USA Posts: 3,699	Very interesting, Bob. Where on earth did you dig that up from?
master5001 is offline

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Pointer confusion	Blackroot	C++ Programming	11	09-12-2007 12:44 AM
Issue w/ Guess My Number Program	mkylman	C++ Programming	5	08-23-2007 01:31 AM
Finding the square root! Not Working!	Lah	C Programming	5	09-14-2003 07:28 PM
Square Root	Kyoto Oshiro	C++ Programming	5	09-05-2002 01:22 AM
can anyone find the problem in my code	ArseMan	C++ Programming	2	09-20-2001 09:02 PM