QtScript float from byte values

David King

Hi,

I’m looking to place 4 values, into a 4-byte array, as follows:

var data = new Uint8Array(4);

data[0] = d0;
data[1] = d1;
data[2] = d2;
data[3] = d3;

However, the Qt Script debugger, errored as below:

Uncaught exception at foo.js:96: ReferenceError: Can't find variable: Uint8Array

I read that Uint8Array only debuted in 2017, so perhaps unsupported.
If so, is there a more basic way to create a 4-byte array, or 4-byte buffer,
which I can access bytewise ?

Instead of Uint8Array, I tried also ArrayBuffer, and char, but likewise errored.

I'm a newcomer to Qt and JavaScript, I'm sure I'm doing something comically wrong..

Ultimately, I'd like to do 'new Float32Array(new Uint8Array([d0, d1, d2, d3]).buffer)[0]',
to view my 4 bytes as an IEEE-74 32-bit float.

Best regards,

David

Leonardo

Why can't you just use a regular array?

var x = new Float32Array([1, 2, 3, 4])[0];
console.log(x)

VRonin

I don't think OP needs a Float32Array but rather to build a single 32bit float from 4 bytes that are d0, d1, d2 and d3.
In C++ it would be:

const char binaryData[4]={d0, d1, d2 ,d3};
float result;
std::memcpy(&result,binaryData,4);

I have no idea how to translate it to QtScript js

David King

Hi Leonardo,

First of, thanks very much for your post - I'm really impressed with how folks offer help on this forum, quite quickly

Hmm, first of I tried the code you posted there. But I get error as below:
Uncaught exception at C:/1/TouchPDT.js:49: ReferenceError: Can't find variable: Float32Array

I also tried removing the new, as I have some inkling it's now allowed in a .js, but I might be entirely wrong there !
But the editor complained thus:

Is this code syntax to initialise a 32-bit IEEE754 float, with the 4 bytes values that comprise the underlying representation of the float ?

You queried why I could not just use a regular array. From what I’ve read about JavaScript generic arrays,
they are a far more abstract concept, with an underlying memory storage model that includes for each element,
a forward pointer and back pointer, to allow array member iteration backwards and forwards, also a pointer to the actual data.
I might be completely wrong though :-)

Best regards,

David

Hi VRonin,

Likewise thank you.
Err, what does OP stand for ?
About your code, yes I'd do it that way in C / C++ too :-)
Or even, to avoid the memcpy need, as follows:

union
{
   char binaryData[4];
   float result;
}
foo;

foo.binaryData[0] = d0;
foo.binaryData[1] = d1;
foo.binaryData[2] = d2;
foo.binaryData[3] = d3;

console.log(foo.result);  // Show

Best regards,

David

mrjj

hi
OP = original Poster :)
Or "him who started the thread"