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90 lines
2.0 KiB
Python
90 lines
2.0 KiB
Python
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# A class which presents the reverse of a sequence without duplicating it.
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# From: "Steven D. Majewski" <sdm7g@elvis.med.virginia.edu>
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# It works on mutable or inmutable sequences.
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#
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# >>> for c in Rev( 'Hello World!' ) : sys.stdout.write( c )
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# ... else: sys.stdout.write( '\n' )
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# ...
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# !dlroW olleH
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#
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# The .forw is so you can use anonymous sequences in init, and still
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# keep a reference the forward sequence. )
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# If you give it a non-anonymous mutable sequence, the reverse sequence
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# will track the updated values. ( but not reassignment! - another
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# good reason to use anonymous values in creating the sequence to avoid
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# confusion. Maybe it should be change to copy input sequence to break
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# the connection completely ? )
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#
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# >>> nnn = range( 0, 3 )
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# >>> rnn = Rev( nnn )
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# >>> for n in rnn: print n
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# ...
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# 2
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# 1
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# 0
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# >>> for n in range( 4, 6 ): nnn.append( n ) # update nnn
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# ...
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# >>> for n in rnn: print n # prints reversed updated values
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# ...
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# 5
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# 4
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# 2
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# 1
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# 0
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# >>> nnn = nnn[1:-1]
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# >>> nnn
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# [1, 2, 4]
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# >>> for n in rnn: print n # prints reversed values of old nnn
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# ...
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# 5
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# 4
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# 2
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# 1
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# 0
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# >>>
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#
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# WH = Rev( 'Hello World!' )
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# print WH.forw, WH.back
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# nnn = Rev( range( 1, 10 ) )
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# print nnn.forw
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# print nnn
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#
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# produces output:
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#
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# Hello World! !dlroW olleH
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# [1, 2, 3, 4, 5, 6, 7, 8, 9]
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# [9, 8, 7, 6, 5, 4, 3, 2, 1]
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#
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# >>>rrr = Rev( nnn )
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# >>>rrr
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# <1, 2, 3, 4, 5, 6, 7, 8, 9>
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from string import joinfields
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class Rev:
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def __init__( self, seq ):
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self.forw = seq
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self.back = self
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def __len__( self ):
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return len( self.forw )
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def __getitem__( self, j ):
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return self.forw[ -( j + 1 ) ]
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def __repr__( self ):
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seq = self.forw
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if type(seq) == type( [] ) :
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wrap = '[]'
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sep = ', '
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elif type(seq) == type( () ) :
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wrap = '()'
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sep = ', '
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elif type(seq) == type( '' ) :
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wrap = ''
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sep = ''
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else:
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wrap = '<>'
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sep = ', '
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outstrs = []
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for item in self.back :
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outstrs.append( str( item ) )
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return wrap[:1] + joinfields( outstrs, sep ) + wrap[-1:]
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