What I really need to do is to export a floating point number to C with no precision loss.
I did this in python:
import math
import struct
x = math.sqrt(2)
print struct.unpack('ii', struct.pack('d', x))
# prints (1719614413, 1073127582)
And in C I try this:
#include <math.h>
#include <stdio.h>
int main(void)
{
unsigned long long x[2] = {1719614413, 1073127582};
long long lx;
double xf;
lx = (x[0] << 32) | x[1];
xf = (double)lx;
printf("%lf\n", xf);
return 0;
}
But in C I get:
7385687666638364672.000000 and not sqrt(2).
What am I missing?
Thanks.
The Python code appears to work. The problem is in the C code: you have the long long filled out right, but then you convert the integer value directly into floating point, rather than reinterpreting the bytes as a double. If you throw some pointers/addressing at it it works:
jkugelman$ cat float.c
#include <stdio.h>
int main(void)
{
unsigned long x[2] = {1719614413, 1073127582};
double d = *(double *) x;
printf("%f\n", d);
return 0;
}
jkugelman$ gcc -o float float.c
jkugelman$ ./float
1.414214
Notice also that the format specifier for double (and for float) is %f, not %lf. %lf is for long double.
If you're targeting a little-endian architecture,
>>> s = struct.pack('<d', x)
>>> ''.join('%.2x' % ord(c) for c in s)
'cd3b7f669ea0f63f'
if big-endian, use '>d' instead of <d. In either case, this gives you a hex string as you're asking for in the question title's, and of course C code can interpret it; I'm not sure what those two ints have to do with a "hex string".
repr() is your friend.
C:\junk\es2>type es2.c
#include <stdio.h>
#include <math.h>
#include <assert.h>
int main(int argc, char** argv) {
double expected, actual;
int nconv;
expected = sqrt(2.0);
printf("expected: %20.17g\n", expected);
actual = -666.666;
nconv = scanf("%lf", &actual);
assert(nconv == 1);
printf("actual: %20.17g\n", actual);
assert(actual == expected);
return 0;
}
C:\junk\es2>gcc es2.c
C:\junk\es2>\python26\python -c "import math; print repr(math.sqrt(2.0))" | a
expected: 1.4142135623730951
actual: 1.4142135623730951
C:\junk\es2>
