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POSingle precision big endian float values to Python's float (double precision, big endian)
 

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  1. POSingle precision big endian float values to Python's float (double precision, big endian)
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    <p>I need to receive hex encoded single precision big endian float values coming from an Arduino over a serial line (RS-232). How do convert them to Python's float which are big endians with double precision?</p> <p>The Arduino send something like "8192323E" and in Python I would like to have 0.174387. I found "<a href="https://stackoverflow.com/questions/1592158/python-convert-hex-to-float">Convert hex to float</a>" but it seems that all of them don't work for single precision floats.</p> <p>From the linked page, this looks promising:</p> <pre><code>from ctypes import * def convert(s): i = int(s, 16) # convert from hex to a Python int cp = pointer(c_int(i)) # make this into a c integer fp = cast(cp, POINTER(c_float)) # cast the int pointer to a float pointer return fp.contents.value # dereference the pointer, get the float </code></pre> <p>But it still doesn't work with my single precision floats.</p> <p>In Java (<a href="http://en.wikipedia.org/wiki/Processing_%28programming_language%29" rel="nofollow noreferrer">Processing</a>) I've been able to do that:</p> <pre><code>float decodeFloat(String inString) { byte [] inData = new byte[4]; inString = inString.substring(2, 10); // discard the leading "f:" inData[0] = (byte) unhex(inString.substring(0, 2)); inData[1] = (byte) unhex(inString.substring(2, 4)); inData[2] = (byte) unhex(inString.substring(4, 6)); inData[3] = (byte) unhex(inString.substring(6, 8)); int intbits = (inData[3] &lt;&lt; 24) | ((inData[2] &amp; 0xff) &lt;&lt; 16) | ((inData[1] &amp; 0xff) &lt;&lt; 8) | (inData[0] &amp; 0xff); //unhex(inString.substring(0, 8)); return Float.intBitsToFloat(intbits); } </code></pre> <p>For your reference, this is the C code running on the Arduino implementing the hex encoding.</p> <pre><code>void serialFloatPrint(float f) { byte * b = (byte *) &amp;f; Serial.print("f:"); for(int i=0; i&lt;4; i++) { byte b1 = (b[i] &gt;&gt; 4) &amp; 0x0f; byte b2 = (b[i] &amp; 0x0f); char c1 = (b1 &lt; 10) ? ('0' + b1) : 'A' + b1 - 10; char c2 = (b2 &lt; 10) ? ('0' + b2) : 'A' + b2 - 10; Serial.print(c1); Serial.print(c2); } } </code></pre>
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    1. USFabio Varesano
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    1. COCan you give some examples of how floats are decoded by the ctypes code? My first suspicion would be byte order. You may need to swap bytes to get the decoding right.
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      1. USGintautas Miliauskas
    2. COOn the wire it flows: f:FFFF7F7F f:FFFF7FFF f:8192323E f:00000000 While the ctypes code reads: 'FFFF7F7F' nan 'FFFF7FFF' nan '8192323E' -5.37040237597e-38 '00000000' 0.0
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      1. USFabio Varesano
    3. COSince the Arduino processor is little-endian (AFAICT) the C code running on it doing the hex encoding is processing low bytes first -- so when all four bytes are concatenated together in the order produced they will be backwards to the normal way multi-byte hex (and multi-digit decimal) numbers are written and interpreted. This is consistent with and why the Java decoder code "works" (see my answer).
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      1. USmartineau
 

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