Thread: The sum that does not equals the sum. gdb involved.

Hello everyone,

I'm having a very strange problem with my code. In an assignment, the right hand is a sum, that I verify using gdb. The left side does not take that value. Instead, it takes the rounded value. Given that the values are doubles, I wasn't expecting a round up.

The following code is what I run with gdb:

Code:

54	      for (Int i = 0; i < nvar; i++)
(gdb) 
55	        xx[i] = xcx[i] + dx[i];
(gdb) 
54	      for (Int i = 0; i < nvar; i++)
(gdb) print xcx[0]
$1 = 0.49999999998343619
(gdb) print dx[0]
$2 = 1.6563789318635447e-11
(gdb) print i
$3 = 0
(gdb) print xcx[0]+dx[0]
$4 = 0.49999999999999994
(gdb) print xx[0]
$5 = 0.5

Unfortunately the project is too big and has too many dependencies.
Thanks in advance.

Thanks for the quick reply.

But shouldn't gdb round it up too? Also, when I manually set the number to the sum using gdb, it works. And the rest of the program runs normally.

Code:

(gdb) print xx[0]
$1 = 0.5
(gdb) print xcx[0] + dx[0]
$2 = 0.49999999999999994
(gdb) print (double)(xcx[0] + dx[0])
$3 = 0.49999999999999994
(gdb) print (float)(xcx[0] + dx[0])
$4 = 0.49999997
(gdb) print (double)(xcx[0]) + (double)(dx[0])
$5 = 0.49999999999999994
(gdb) set xx[0]=xcx[0]+dx[0]
(gdb) print xx[0]
$6 = 0.49999999999999994

I'm programming an interior point method, and my upper bound in the first variable is 0.5. With this round up, the method fails, because it can't touch the bound. After setting xx[0], the method goes all the way to the end.

Also, if it is indeed a rounding problem, how can I prevent it?

I tried this:

Code:

for (Int i = 0; i < nvar; i++) {
  if (xcx[i] + dx[i] == bux[i])
    xx[i] = bux[i] - dciTiny;
  else
    xx[i] = xcx[i] + dx[i];
}

but it didn't work.

Thanks

Since I know that xcx[i] + dx[i] < bux[i], I decided to make small values of dx to be equal to zero. Thank you for the replies.