list comparison vs integer comparison, which is more efficient?

Terry Reedy tjreedy at udel.edu
Sun Jan 4 08:10:41 CET 2015


On 1/3/2015 6:19 PM, austin aigbe wrote:

> I am currently implementing the LTE physical layer in Python (ver 2.7.7).
> For the qpsk, 16qam and 64qam modulation I would like to know which is more efficient to use, between an integer comparison and a list comparison:
>
> Integer comparison: bit_pair as an integer value before comparison
>
>      # QPSK - TS 36.211 V12.2.0, section 7.1.2, Table 7.1.2-1
>      def mp_qpsk(self):
>          r = []
>          for i in range(self.nbits/2):
>              bit_pair = (self.sbits[i*2] << 1) | self.sbits[i*2+1]
>              if bit_pair == 0:
>                  r.append(complex(1/math.sqrt(2),1/math.sqrt(2)))
>              elif bit_pair == 1:
>                  r.append(complex(1/math.sqrt(2),-1/math.sqrt(2)))
>              elif bit_pair == 2:
>                  r.append(complex(-1/math.sqrt(2),1/math.sqrt(2)))
>              elif bit_pair == 3:
>                  r.append(complex(-1/math.sqrt(2),-1/math.sqrt(2)))
>          return r
>
> List comparison: bit_pair as a list before comparison
>
>      # QPSK - TS 36.211 V12.2.0, section 7.1.2, Table 7.1.2-1
>      def mp_qpsk(self):
>          r = []
>          for i in range(self.nbits/2):
>              bit_pair = self.sbits[i*2:i*2+2]
>              if bit_pair == [0,0]:
>                  r.append()
>              elif bit_pair == [0,1]:
>                  r.append(complex(1/math.sqrt(2),-1/math.sqrt(2)))
>              elif bit_pair == [1,0]:
>                  r.append(complex(-1/math.sqrt(2),1/math.sqrt(2)))
>              elif bit_pair == [1,1]:
>                  r.append(complex(-1/math.sqrt(2),-1/math.sqrt(2)))
>          return r

Wrong question.  If you are worried about efficiency, factor out all 
repeated calculation of constants and eliminate the multiple comparisons.

sbits = self.sbits
a = 1.0 / math.sqrt(2)
b = -a
points = (complex(a,a), complex(a,b), complex(b,a), complex(b,b))
     complex(math.sqrt(2),1/math.sqrt(2))
def mp_qpsk(self):
     r = [points[sbits[i]*2 + sbits[i+1]]
         for i in range(0, self.nbits, 2)]
     return r

-- 
Terry Jan Reedy




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