red_snake1@hotmail.com (Goran) wrote in
news:f1eb43b6.0308230715.1995674a@posting.google.c om:
[color=blue]
> Hi!
>
> I need to convert from a unsigned char array to a float. I don't think
> i get the right results in the program below.
>
> unsigned char array1[4] = { 0xde, 0xc2, 0x44, 0x23}; //I'm not sure in
> what order the data is stored so i try both ways.
> unsigned char array2[4] = { 0x23, 0x44, 0xc2, 0xde};
>
> float *pfloat1, *pfloat2;
>
> pfloat1 = (float *)array1;
> pfloat2 = (float *)array2;
>
> printf("pfloat1 = %f, pfloat2 = %f\n", pfloat1, pfloat2);[/color]
As Artie pointed out, these need to be floats, not pointers
to floats. Also, pfloat1 and pfloat2 might not be correctly
aligned for a float.
[color=blue]
> The result here is:
> pfloat1 = 0.000000, pfloat2 = -6999176012340658176.000000[/color]
But if you fix the above, then these look OK for IEEE
single precision, although you have a bogus amount of
precision for pfloat2, and not enough for pfloat1. If
you use %g you should see results something like
pfloat1 = 1.06664e-17, pfloat2 = -6.99918e+18
[color=blue]
> I know this data is stored in single-precision floatingpoint number.
> Calculating this with the formula: 1.mantissa * 2^(exp-127) gets the
> following results:[/color]
No it doesn't.
[color=blue]
> -6614457784713468934.9861376 when using array1
> -79.784837 when using array2.
>
> Anyone know why i get so bad results?[/color]
I would suggest that you have made in mistake in your
calculations. Here is some code that does it for you:
$ type ieee2float.c
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <float.h>
#include <assert.h>
double decode_ieee_single(const void *v, int natural_order)
{
const unsigned char *data = v;
int s, e;
unsigned long src;
long f;
double value;
if (natural_order) {
src = ((unsigned long)data[0] << 24) |
((unsigned long)data[1] << 16) |
((unsigned long)data[2] << 8) |
((unsigned long)data[3]);
}
else {
src = ((unsigned long)data[3] << 24) |
((unsigned long)data[2] << 16) |
((unsigned long)data[1] << 8) |
((unsigned long)data[0]);
}
s = (src & 0x80000000UL) >> 31;
e = (src & 0x7F800000UL) >> 23;
f = (src & 0x007FFFFFUL);
if (e == 255 && f != 0) {
/* NaN (Not a Number) */
value = DBL_MAX;
}
else if (e == 255 && f == 0 && s == 1) {
/* Negative infinity */
value = -DBL_MAX;
}
else if (e == 255 && f == 0 && s == 0) {
/* Positive infinity */
value = DBL_MAX;
}
else if (e > 0 && e < 255) {
/* Normal number */
f += 0x00800000UL;
if (s) f = -f;
value = ldexp(f, e - 150);
}
else if (e == 0 && f != 0) {
/* Denormal number */
if (s) f = -f;
value = ldexp(f, -149);
}
else if (e == 0 && f == 0 && s == 1) {
/* Negative zero */
value = 0;
}
else if (e == 0 && f == 0 && s == 0) {
/* Positive zero */
value = 0;
}
else {
/* Never happens */
printf("s = %d, e = %d, f = %lu\n", s, e, f);
assert(!"Woops, unhandled case in decode_ieee_single()");
}
return value;
}
int main(void)
{
unsigned char f[4] = {0xde, 0xc2, 0x44, 0x23};
printf("0x%02X%02X%02X%02X as an IEEE float is %f\n",
f[0], f[1], f[2], f[3], decode_ieee_single(f, 0));
printf("0x%02X%02X%02X%02X as an IEEE float is %f\n",
f[3], f[2], f[1], f[0], decode_ieee_single(f, 1));
return 0;
}
$ cc ieee2float
$ link ieee2float
$ run ieee2float
0xDEC24423 as an IEEE float is 0.000000
0x2344C2DE as an IEEE float is -6999176012340658200.000000
This code has not been exhastively tested. The above output
was generated on system not using IEEE floating point, but its
output agrees with your IEEE system.
Phil T
