# Pythons unpack_From in Matlab

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Shivaputra Narke on 28 Feb 2017
Edited: Robert Snoeberger on 3 Mar 2017
Greetings!
I am receiving some data over UDP in form of array of integers. For example y= [ 67 36 73 43] this is the data received. The actual sensor value is 161.0.
I am not able to convert this received data to 161.0.
However when I have used Python's script using unpack_fom I was able to decode to correct value of 161.0
Python code : >>> pack('!f',161) 'C!\x00\x00' >>> unpack_from('!f','C!\x00\x00') (161.0,)
Let me know if anyone has better solution to decode value [67 36 73 43] to 161.0 float value

Robert Snoeberger on 2 Mar 2017
Edited: Robert Snoeberger on 3 Mar 2017
The function typecast [1] might do what you want. I'm going to give an example of how to use it but I need to make a few assumptions.
1. I'm assuming that the integers you received are uint8.
2. Since you have only 4 integers, the value 161.0 must be a single precision floating point number. I think that this is consistent with your use of 'f' in the pack and unpack_from functions.
Example:
To learn a little about typecast, I will start by converting the value 161.0 into an array of integers.
>> y = typecast(single(161.0), 'uint8')
y =
1×4 uint8 row vector
0 0 33 67
>>
Note that the value 67 is at the end instead of the beginning of the array. I can use the function swapbytes [2] to swap the byte ordering of the value 161.0 before the type cast. I think this byte swap is consistent with your use of '!' in the pack and unpack_from functions.
>> v = swapbytes(single(161.0));
>> sy = typecast(v, 'uint8')
sy =
1×4 uint8 row vector
67 33 0 0
>>
Now, to covert the data you received to a value ~161, just do these operations in reverse.
>> y = uint8([67 36 73 43]);
>> v = typecast(y, 'single');
>> swapbytes(v)
ans =
single
164.2858
>>
Edit: Fixed a typo in the display of sy. Original display was incorrect.

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