Re: Floating point exception

[Jan-Benedict Glaw <jbglaw@lug-owl.de>]

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On Sat, 2004-09-11 12:39:13 +0100, Ankit Jain <ankitjain1580@yahoo.com>
wrote in message <20040911113913.86435.qmail@web52904.mail.yahoo.com>:
> #include<stdlib.h>
> #include<string.h>
> #include<complex.h>
> #include<errno.h>
> #include<time.h>
> #include<stdio.h>
> #define NX 4 	 	 
> #define NY 4
> #define N 2*NX*NY		//N is the size of 2 D DFT

Braces missing

> #include <math.h>

You'd first finish all includes, then add your own defines.

> #define num 2
> #define SWAP(a,b) tempr=(a);(a)=(b);(b)=tempr

This may cause subtle errors. Consider

	if (something)
		SWAP (x, y);

Only the part "tempr=(a);" would be executed conditionally, what is not
exactly what you intended. I suggest using:

#define SWAP(a,b)		\
	do {			\
		tempr = (a);	\
		(a) = (b);	\
		(b) = tempr;	\
	} while (0)

> void fft(float data[], unsigned long nn[], int ndim,
> int isign)
> {
> int idim;
> unsigned long
> i1,i2,i3,i2rev,i3rev,ip1,ip2,ip3,ifp1,ifp2;
> unsigned long ibit,k1,k2,n,nprev,nrem,ntot;
> float tempi,tempr;
> double theta,wi,wpi,wpr,wr,wtemp; //Double precision
> for trigonometric recur-rences.
> for (ntot=1,idim=1;idim<=ndim;idim++)    //Compute
> total number of complex val-ues.
> 	ntot *= nn[idim];
> nprev=1;
> 
> for (idim=ndim;idim>=1;idim--) 
> {					// Main loop over the dimensions.
> 	n=nn[idim];
> 	nrem=ntot/(n*nprev);

In your example code, n may become zero, so you devide by zero, which
isn't legal.

MfG, JBG

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