# little/big endian conversions

Bill Baxter dnewsgroup at billbaxter.com
Tue Apr 8 13:56:38 PDT 2008

```You don't necessarily need to put it into a byte array.  Just cast:

void swap(ref byte a, ref byte b) {
byte tmp; tmp=a; a=b; b=tmp;
}
float f;
byte[4] fbytes = (cast(byte*)&f)[0..4];
swap(fbytes[0],fbytes[3]);
swap(fbytes[1],fbytes[2]);
float fswapped = *(cast(float*)fbytes.ptr);

Or instead of casts you can use a union.

union FC { float f;      ubyte[4] c; }
FC fs;
fs.f = f;
swap(fs.c[0],fs.c[3]);
swap(fs.c[1],fs.c[2]);
float fswapped = fs.f;

--bb

lurker wrote:
> so i need to put the float/double into an byte array and just swap?
>
> Regan Heath Wrote:
>
>> Bill Baxter wrote:
>>> Regan Heath wrote:
>>>> lurker wrote:
>>>>> does anybody know how to convert float and doubles to little/big endian?
>>>> This is a guess but if you read:
>>>> http://en.wikipedia.org/wiki/IEEE_754
>>>>
>>>> You'll see the internal representation of a float, given that and a
>>>> little guess work I've come up with:
>>>>
>>>> import std.stdio;
>>>>
>>>> int extractSign(float f)
>>>> {
>>>>     return (*(cast(int*)&f) & 0x80000000) ? -1 : 1;
>>>> }
>>>>
>>>> ubyte extractExp(float f)
>>>> {
>>>>     return (*(cast(int*)&f) << 1) & 0xFF000000;
>>>> }
>>>>
>>>> int extractFraction(float f)
>>>> {
>>>>     return *(cast(int*)&f) & 0x007FFFFF;
>>>> }
>>>>
>>>> void main()
>>>> {
>>>>     float f = -1.25f;
>>>>         auto   sign     = extractSign(f);
>>>>     auto   exp      = extractExp(f);
>>>>     auto   fraction = extractFraction(f);
>>>>         writefln(f);
>>>>     writefln(sign);
>>>>     writefln(exp);
>>>>     writefln(fraction);   }
>>>>
>>>> which will extract the various parts of a float.
>>>>
>>>> Now, I have no idea how they might change on a big/little endian
>>>> system but I suspect each part would have it's byte order swapped.  In
>>>> which case, byte order swapping the extracted parts then re-assembling
>>>> might give you a byte order swapped float.
>>>>
>>>> Like I said, I'm guessing.
>>>>
>>>> What you want is 2 systems with different ordering and then you want
>>>> to dump the content of the float like this:
>>>>
>>>> writefln("%032b", *(cast(int*)&f));
>>>>
>>>> then compare.
>>>>
>>>> Regan
>>>
>>> It doesn't matter that it's in IEEE 745 format.  You just swap the bytes
>>> like it was any old kind of data.
>> Doh, for some reason I dismissed that as too simple.
>>
>> Regan
>
```