Convert a little-endian hex string to a number

[Peter Watkins]

I'd like to convert a little endian hex string:

'00020000020000FC'

to a number.

I was going to use

struct.unpack('>q', theString)

but this is meant for a string where the bytes in the string correspond to bytes in the number.

I think that something like the opposite of the hex() function would work perfect as long as it supported little-endian conversion.

The file I'm reading is an ASCII file that looks like this:

010FA3456FF
A4F5ABB3DE
...

That is the actual ASCII text there. It's really a waste of space to store data this way but that's what I have to read.

Any ideas?

[bartonc]

I'd like to convert a little endian hex string:

'00020000020000FC'

to a number.

I was going to use

struct.unpack('>q', theString)

but this is meant for a string where the bytes in the string correspond to bytes in the number.

I think that something like the opposite of the hex() function would work perfect as long as it supported little-endian conversion.

The file I'm reading is an ASCII file that looks like this:

010FA3456FF
A4F5ABB3DE
...

That is the actual ASCII text there. It's really a waste of space to store data this way but that's what I have to read.

Any ideas?

There is support for differing byte arrangements in the ctypes module, but I haven't looked into that yet. Off the top of my head:
We know that this is not what you are looking for:
>>> int('00020000020000FC', 16)
562949986975996L

But perhaps something like this could work for you:
>>> aLittleEndianHex = '00020000020000FC'
>>> aBigEndianList = []
>>> for i in range(0, len(aLittleEndianHex), 2):
... aBigEndianList.insert(0, aLittleEndianHex[i:i+2])
...
>>> aBigEndianList
['FC', '00', '00', '02', '00', '00', '02', '00']
>>>
Now you can use struct or what ever you want to.

[bartonc]

By the way, welcome to the Python Forum on TheScripts. Thank you for your input on the matlab question. I like your style!
I hope you stick around,
Barton

[bartonc]

There is support for differing byte arrangements in the ctypes module, but I haven't looked into that yet. Off the top of my head:
We know that this is not what you are looking for:
>>> int('00020000020000FC', 16)
562949986975996L

But perhaps something like this could work for you:
>>> aLittleEndianHex = '00020000020000FC'
>>> aBigEndianList = []
>>> for i in range(0, len(aLittleEndianHex), 2):
... aBigEndianList.insert(0, aLittleEndianHex[i:i+2])
...
>>> aBigEndianList
['FC', '00', '00', '02', '00', '00', '02', '00']
>>>
Now you can use struct or what ever you want to.

perhaps a string?

>>> aBigEndianStr = ""
>>> for byteStr in aBigEndianList:
... aBigEndianStr += byteStr
...
>>> aBigEndianStr
'FC00000200000200'
>>>

[Peter Watkins]

Thanks guys! I used both ideas except that I concatenated the list of strings using S.join(sequence)

Here's the function I came up with:

    def hexStrEndianSwap(self, theString):
        """Rearranges character-couples in a little endian hex string to
        convert it into a big endian hex string and vice-versa. i.e. 'A3F2'
        is converted to 'F2A3'
 
        @param theString: The string to swap character-couples in
        @return: A hex string with swapped character-couples. -1 on error."""
 
        # We can't swap character couples in a string that has an odd number
        # of characters.
        if len(theString)%2 != 0:
            return -1
 
        # Swap the couples
        swapList = []
        for i in range(0, len(theString), 2):
            swapList.insert(0, theString[i:i+2])
 
        # Combine everything into one string. Don't use a delimeter.
        return ''.join(swapList)
 

[bartonc]

Thanks guys! I used both ideas except that I concatenated the list of strings using S.join(sequence)

Here's the function I came up with:

    def hexStrEndianSwap(self, theString):
        """Rearranges character-couples in a little endian hex string to
        convert it into a big endian hex string and vice-versa. i.e. 'A3F2'
        is converted to 'F2A3'
 
        @param theString: The string to swap character-couples in
        @return: A hex string with swapped character-couples. -1 on error."""
 
        # We can't swap character couples in a string that has an odd number
        # of characters.
        if len(theString)%2 != 0:
            return -1
 
        # Swap the couples
        swapList = []
        for i in range(0, len(theString), 2):
            swapList.insert(0, theString[i:i+2])
 
        # Combine everything into one string. Don't use a delimeter.
        return ''.join(swapList)
 

"".join() - I don't know how I missed that one. One of the sweetest tricks in the book.

Thanks for the update. It really helps others when you post back with your solutions. Keep posting,
Barton