list comparison vs integer comparison, which is more efficient?
austin aigbe
eshikafe at gmail.com
Sun Jan 4 07:17:05 EST 2015
On Sunday, January 4, 2015 12:20:26 PM UTC+1, austin aigbe wrote:
> On Sunday, January 4, 2015 8:12:10 AM UTC+1, Terry Reedy wrote:
> > 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
>
> Cool. Thanks a lot.
Hi Terry,
No difference between the int and list comparison in terms of the number of calls(24) and time (0.004s). Main part is the repeated call to sqrt().
However, it took a shorter time (0.004s) with 24 function calls than your code (0.005s) which took just 13 function calls to execute.
Why is this?
Integer comparison profile result:
>>> p = pstats.Stats('lte_phy_mod.txt')
>>> p.strip_dirs().sort_stats(-1).print_stats()
Sun Jan 04 12:36:32 2015 lte_phy_mod.txt
24 function calls in 0.004 seconds
Ordered by: standard name
ncalls tottime percall cumtime percall filename:lineno(function)
1 0.004 0.004 0.004 0.004 lte_phy_layer.py:16(<module>)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:20(Scrambling)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:276(LayerMapping)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:278(Precoding)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:280(ResourceElementMapping)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:282(OFDMSignalGenerator)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:65(Modulation)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:71(__init__)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:87(mp_qpsk)
1 0.000 0.000 0.000 0.000 {len}
8 0.000 0.000 0.000 0.000 {math.sqrt}
4 0.000 0.000 0.000 0.000 {method 'append' of 'list' objects}
1 0.000 0.000 0.000 0.000 {method 'disable' of '_lsprof.Profiler' objects}
1 0.000 0.000 0.000 0.000 {range}
<pstats.Stats instance at 0x028F3F08>
>>>
List comparison:
>>> import pstats
>>> p = pstats.Stats('lte_phy_mod2.txt')
>>> p.strip_dirs().sort_stats(-1).print_stats()
Sun Jan 04 12:57:24 2015 lte_phy_mod2.txt
24 function calls in 0.004 seconds
Ordered by: standard name
ncalls tottime percall cumtime percall filename:lineno(function)
1 0.004 0.004 0.004 0.004 lte_phy_layer.py:16(<module>)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:20(Scrambling)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:276(LayerMapping)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:278(Precoding)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:280(ResourceElementMapping)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:282(OFDMSignalGenerator)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:65(Modulation)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:71(__init__)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:87(mp_qpsk)
1 0.000 0.000 0.000 0.000 {len}
8 0.000 0.000 0.000 0.000 {math.sqrt}
4 0.000 0.000 0.000 0.000 {method 'append' of 'list' objects}
1 0.000 0.000 0.000 0.000 {method 'disable' of '_lsprof.Profiler' objects}
1 0.000 0.000 0.000 0.000 {range}
<pstats.Stats instance at 0x025E3418>
>>>
Terry's code:
>>> import pstats
>>> p = pstats.Stats('lte_phy_mod3.txt')
>>> p.strip_dirs().sort_stats(-1).print_stats()
Sun Jan 04 13:04:51 2015 lte_phy_mod3.txt
13 function calls in 0.005 seconds
Ordered by: standard name
ncalls tottime percall cumtime percall filename:lineno(function)
1 0.004 0.004 0.005 0.005 lte_phy_layer.py:16(<module>)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:20(Scrambling)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:285(LayerMapping)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:287(Precoding)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:289(ResourceElementMapping)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:291(OFDMSignalGenerator)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:65(Modulation)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:71(__init__)
1 0.000 0.000 0.000 0.000 lte_phy_layer.py:87(mp_qpsk)
1 0.000 0.000 0.000 0.000 {len}
1 0.000 0.000 0.000 0.000 {math.sqrt}
1 0.000 0.000 0.000 0.000 {method 'disable' of '_lsprof.Profiler' objects}
1 0.000 0.000 0.000 0.000 {range}
<pstats.Stats instance at 0x02783418>
>>>
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