float to byte (JavaScript)

Question

In function is transmited double.

function writebyte(id, data)
{
    data = Math.floor(data);
    buf[id] = String.fromCharCode(data & (255));
}

It works ok for 0-127 values. But with negative or > 127 works wrong.

128 = 0xC2 0x80 (should be 0x80 1 byte)
-1  = 0xC3 0xBF (should be 0xFF 1 byte)

So I think problem is function String.fromCharCode with parameter 128++ or negative.

Is there any way writing bytes to array directly without String.fromCharCode?

It looks like you are wanting to be able to represent numbers greater than +127 with 8 bits yet also represent negative numbers with twos complement binary with the MSB acting as -128. You can't do both... — El Ronnoco
– El Ronnoco, Commented Dec 6, 2011 at 13:46
I dont understand. Why cant I do both? -1 == 255 == 0xFF -2 == 254 == 0xFE. buf is sent by websocket protocol i just want that if i transmit -1 or 255 to this function first byte would be 0xFF (for example) — Demion
– Demion, Commented Dec 6, 2011 at 14:04
Well conventionally the most-significant bit is made negative (ie -128) giving a range of -128 to 127 for a signed bit. I'm not sure if you are using a different system? — El Ronnoco
– El Ronnoco, Commented Dec 6, 2011 at 14:16
You are right. So signed range -128 to 127. Unsigned 0 to 255. But this doesnt solve my question. I want transmit in function -1 and in data[id] should be 0xFF or I transmit 255 and in data[id] should 0xFF and so on. — Demion
– Demion, Commented Dec 6, 2011 at 14:21

Community · Accepted Answer · 2017-05-23 11:46:01Z

1

In this answer you will find JavaScript code that will convert from (hex) bytes into a double. [JS doesn't have "floats"]

Reversing the process is left as an exercise for the reader...

edited May 23, 2017 at 11:46

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answered Dec 6, 2011 at 13:48

Alnitak

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7 Comments

hugomg Over a year ago

-1; Byte operations in JS convert the double to an intermediate integer representation. They do not operate on the actual floating point representation.

Alnitak Over a year ago

@missingno you're missing the point - by manipulation of the original floating point number it is possible to calculate the sign, mantissa, and exponent, and from there work out the correct IEE754 representation thereof. The code I previously posted does that in reverse.

Demion Over a year ago

Doesnt Math.floor convert float to integer?

Alnitak Over a year ago

@Demion no, not really, it just removes the fractional component. In any event, can you clarify your question - it now appears that you're really only talking about small integer values, and not doubles at all!

Demion Over a year ago

Yes i work only with -128 .. 127 or with 0..255. But another (not mine) code transmits double to this function.

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