Thread: Why does conditional (t == ZERO) always evaluate to false?

What's wrong with my conditional that causes it to always evaluates to false (execute the else block)?

Code:

if (t == ZERO) ...

In main, t is declared as realtype, and ZERO is defined using a macro RCONST:

Code:

#define ZERO  RCONST(0.0)
realtype reltol, abstol, t, tout;

realtype and RCONST are declared in a package. realtype​ is either float, double, or long double. In the current installation, realtype is a long double.

RCONST is to be used to for all floating point constants so that they are declared the same way as realtype:

Code:

#if defined(SUNDIALS_SINGLE_PRECISION)
...
#elif defined(SUNDIALS_DOUBLE_PRECISION)
...
#elif defined(SUNDIALS_EXTENDED_PRECISION)
typedef long double realtype;
# define RCONST(x) x##L
#endif

t's value is zero (I confirmed that in gdb). I've tried several forms of the conditional, but all have failed (actually, they all work correctly, but they all evaluate to false):

Code:

if (t == ZERO) ...
if (t == 0) ...
if (t == 0.0) ...
if (t == 0.0L) ...

All of those executed the else statement, even when t is zero. Pretty obviously the second one will not work right.

Code:

printf("%4.0Lf\t", t);
if (t == 0.0L) {
  printf("THEN");
} else {
  printf("ELSE");
}

Output:

0 ELSE ...
100 ELSE ...
200 ELSE ...
...

So, what's wrong with my conditional that causes it to always evaluates to false (execute the else block), even when t is zero?​ How do I make this work right?

Thanks.

Instead of

Code:

printf("%4.0Lf\t", t);

Try

Code:

printf("%Lf\t", t);

Also turn on all warnings your compiler can produce, and mind the warnings.

Are you telling me to print like that because it will give me a clue or because it's good programming? This is tabular output, so I can only use this to gather information. The result of changing the print is

0.000000 ELSE ...

which indicates that t is exactly zero.

Currently, I'm not getting any compiler warnings. I'm not completely sure what I'm doing, but I changed the make file to compile with option -O3, (that's a capital letter "O" three). I did not get any compiler warnings. Is that the most warnings (I'm using Linux and gcc) can produce?

Code:

/usr/local/mpi/bin/mpicc  -Wall -g -O3            -I/home/myname/sundials/instdir/include -I/usr/local/mpi/bin/../include -c FPU.c
/usr/local/mpi/bin/mpicc -o FPU FPU.o  -Wall -g -O3               -L/home/myname/sundials/instdir/lib -lsundials_cvode -lsundials_nvecparallel -lm  -L/usr/local/mpi/bin/../lib

Unfortunately, I'm still stumped.

Oops ... I meant

Code:

printf("%Lg", t);

with a 'g' rather than an 'f', sorry.

And, yes. That will print the number starting with the most significative digit.

Code:

#include <stdio.h>

int main(void) {
  long double x = 0.1;
  while (x > 1E-20) {
    printf("%4.0Lf %Lf %Lg\n", x, x, x);
    x /= 10;
  }
  return 0;
}

And the output is

0 0.100000 0.1
0 0.010000 0.01
0 0.001000 0.001
0 0.000100 0.0001
0 0.000010 1e-05
0 0.000001 1e-06
0 0.000000 1e-07
0 0.000000 1e-08
0 0.000000 1e-09
0 0.000000 1e-10
0 0.000000 1e-11
0 0.000000 1e-12
0 0.000000 1e-13
0 0.000000 1e-14
0 0.000000 1e-15
0 0.000000 1e-16
0 0.000000 1e-17
0 0.000000 1e-18
0 0.000000 1e-19
0 0.000000 1e-20

Yup, t was not equal to exactly zero. I'm not completely sure why (still a newbie!), but I have the code working now.
P.S. I like your posting style - give hints and let me find it. (It just feels better to find it yourself!)