[Tutor] Question regarding character conversion usage.

[Kent Johnson]

With apologies to the original author who is not here to defend himself and 
did not ask for the critique, I have to say that this is poorly written code.

The use of string concatenation (+) to build up a string is a 
well-documented bad practice. The preferred way to build up a string is to 
create a list of components and join the finished list into a single 
string. The use of an index to iterate over a string is unnecessary. The loop
>     newstr = ""
>     for ix in xrange(len(str)):
>         newstr = newstr + chr(a2eG[ord(str[ix])])

would be better written as
buf = []
for c in str:
   buf.append(chr(a2eG[ord(c)]))
newstr = ''.join(buf)

or even better
newstr = ''.join( [chr(a2eG[ord(c)]) for c in str] )

The call to chr could be eliminated by converting the arrays to hold 
characters instead of ints.

Better yet, use the built-in string.translate() function and do everything 
in one call. Leaving the definitions of a2eG and e2aG alone, the 
definitions of AsciiToEbcdic and EbcdicToAscii can be replaced with this:

a2eG = ''.join(map(chr, a2eG))  # Convert a2eG to a single string
e2aG = ''.join(map(chr, e2aG))  # ditto

def AsciiToEbcdic(str):
     '''Return the ASCII string str in EBCDIC form.
     '''
     global a2eG
     try:
         return str.translate(a2eG)
     except AttributeError:
         raise "Bad data", "Expected a string argument"

def EbcdicToAscii(str):
     global e2aG
     try:
         return str.translate(e2aG)
     except AttributeError:
         raise "Bad data", "Expected a string argument"

I also took out the test for string type and just catch the exception you 
will get if it is not.

This version runs the Test() method 30 times faster than the original. The 
difference would most likely be more with a longer string as the 
performance of repeated string concatenation is O(n^2)

Short on tolerance tonight :-)
Kent


At 04:25 PM 8/18/2004 -0500, Steve Robb wrote:
>a2eG = [
>       0,  1,  2,  3, 55, 45, 46, 47, 22,  5, 37, 11, 12, 13, 14, 15,
>      16, 17, 18, 19, 60, 61, 50, 38, 24, 25, 63, 39, 28, 29, 30, 31,
>      64, 79,127,123, 91,108, 80,125, 77, 93, 92, 78,107, 96, 75, 97,
>     240,241,242,243,244,245,246,247,248,249,122, 94, 76,126,110,111,
>     124,193,194,195,196,197,198,199,200,201,209,210,211,212,213,214,
>     215,216,217,226,227,228,229,230,231,232,233, 74,224, 90, 95,109,
>     121,129,130,131,132,133,134,135,136,137,145,146,147,148,149,150,
>     151,152,153,162,163,164,165,166,167,168,169,192,106,208,161,  7,
>      32, 33, 34, 35, 36, 21,  6, 23, 40, 41, 42, 43, 44,  9, 10, 27,
>      48, 49, 26, 51, 52, 53, 54,  8, 56, 57, 58, 59,  4, 20, 62,225,
>      65, 66, 67, 68, 69, 70, 71, 72, 73, 81, 82, 83, 84, 85, 86, 87,
>      88, 89, 98, 99,100,101,102,103,104,105,112,113,114,115,116,117,
>     118,119,120,128,138,139,140,141,142,143,144,154,155,156,157,158,
>     159,160,170,171,172,173,174,175,176,177,178,179,180,181,182,183,
>     184,185,186,187,188,189,190,191,202,203,204,205,206,207,218,219,
>     220,221,222,223,234,235,236,237,238,239,250,251,252,253,254,255
>     ]
>
>e2aG = [
>       0,  1,  2,  3,156,  9,134,127,151,141,142, 11, 12, 13, 14, 15,
>      16, 17, 18, 19,157,133,  8,135, 24, 25,146,143, 28, 29, 30, 31,
>     128,129,130,131,132, 10, 23, 27,136,137,138,139,140,  5,  6,  7,
>     144,145, 22,147,148,149,150,  4,152,153,154,155, 20, 21,158, 26,
>      32,160,161,162,163,164,165,166,167,168, 91, 46, 60, 40, 43, 33,
>      38,169,170,171,172,173,174,175,176,177, 93, 36, 42, 41, 59, 94,
>      45, 47,178,179,180,181,182,183,184,185,124, 44, 37, 95, 62, 63,
>     186,187,188,189,190,191,192,193,194, 96, 58, 35, 64, 39, 61, 34,
>     195, 97, 98, 99,100,101,102,103,104,105,196,197,198,199,200,201,
>     202,106,107,108,109,110,111,112,113,114,203,204,205,206,207,208,
>     209,126,115,116,117,118,119,120,121,122,210,211,212,213,214,215,
>     216,217,218,219,220,221,222,223,224,225,226,227,228,229,230,231,
>     123, 65, 66, 67, 68, 69, 70, 71, 72, 73,232,233,234,235,236,237,
>     125, 74, 75, 76, 77, 78, 79, 80, 81, 82,238,239,240,241,242,243,
>      92,159, 83, 84, 85, 86, 87, 88, 89, 90,244,245,246,247,248,249,
>      48, 49, 50, 51, 52, 53, 54, 55, 56, 57,250,251,252,253,254,255
>]
>
>def AsciiToEbcdic(str):
>     '''Return the ASCII string str in EBCDIC form.
>     '''
>     global a2eG
>     if type(str) != type(""):
>         raise "Bad data", "Expected a string argument"
>     if len(str) == 0:  return str
>     newstr = ""
>     for ix in xrange(len(str)):
>         newstr = newstr + chr(a2eG[ord(str[ix])])
>     return newstr
>
>def EbcdicToAscii(str):
>     global e2aG
>     if type(str) != type(""):
>         raise "Bad data", "Expected a string argument"
>     if len(str) == 0:  return str
>     newstr = ""
>     for ix in xrange(len(str)):
>         newstr = newstr + chr(e2aG[ord(str[ix])])
>     return newstr
>
>def Test():
>     str = "The dog jumped over the lazy brown fox in 1.234567890 seconds"
>     str1 = EbcdicToAscii(AsciiToEbcdic(str))
>     if str != str1:
>         raise "Test failed"
>
>if __name__ == '__main__':
>     Test()
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