Store buffer in DataView then extract with Float64Array not working

Question

I want to write a buffer into an array and then access that array with the Float64Array view. This seems to be a lot more difficult than it should. I have used the mozilla documentation but still having problems....

I have tried multiple variants of below. Does anyone know why this is not working? At the end I should be able to print out the first float which is 4.

//64 bit floating point numbers with 8 bytes each in hex
//first  float64 bytes : 4 = 0x4010000000000000
//second float64 bytes : 5 = 0x4014000000000000

//2 x 8 bytes in a buffer with each hex number
var buff = new Buffer("40100000000000004014000000000000", "hex");
var ab = new ArrayBuffer(buff.length);
var view = new DataView(ab);

for (var i = 0; i < buff.length; ++i) {
    view[i] = buff[i];
}

console.log(new Float64Array(ab).length);//prints 2
console.log(new Float64Array(ab)[0]);// SHOULD print '4' but prints 0

userNaN · Accepted Answer · 2015-07-05 20:25:06Z

1

Probs due to endianness. Using node Buffer API:

var buff = new Buffer("40100000000000004014000000000000", "hex");
buff.readDoubleLE(0) // 2.0553e-320
buff.readDoubleBE(0) // 4

And if we reverse buff and read from the 8th byte:

var buffRev = new Buffer("00000000000014400000000000001040", "hex")
buff.readDoubleLE(8) // 4
buff.readDoubleBE(8) // 2.0553e-320

Feel free to correct/extend if you know more.

answered Jul 5, 2015 at 20:25

userNaN

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Exitos · Accepted Answer · 2015-07-06 09:52:21Z

0

I ended up realising that really the data needed to be read 'INTO' the view in correct chunks. In the example I provided this means two reads of 8 bytes. With each read needing to be populate one item in the Float64Array. Surprised this is not easier and I can't just populate the ArrayBuffer with the entire underlying buffer.

Anyway this is what I ended up with...

var buff = new Buffer("40100000000000004014000000000000", "hex");
var ab = new ArrayBuffer(buff.length);
var view = new Float64Array(ab);

var viewIndex = 0;
for (var bufferIndex=0;bufferIndex<buff.length;bufferIndex=bufferIndex+8)            {

    view[viewIndex] = buff.readDoubleLE(bufferIndex);
    viewIndex++;
}

answered Jul 6, 2015 at 9:52

Exitos

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