Theano（6）：Theano条件语句，IfElse vs Switch

Theano条件操作：IfElse与Switch的比较

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#Theano #Theano条件语句 #IfElse #Switch

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本文探讨了Theano库中用于条件控制的IfElse和Switch操作。IfElse是一个布尔条件操作，而Switch是张量条件操作，更通用。IfElse会计算两个变量，而Switch则根据条件惰性评估。尽管目前不默认使用优化的linker，但未来可能会改变。

http://deeplearning.net/software/theano/tutorial/conditions.html

IfElse vs Switch

Both ops build a condition over symbolic variables.
IfElse takes a boolean condition and two variables as inputs.
Switch takes a tensor as condition and two variables as inputs. switch is an elementwise operation and is thus more general thanifelse.
Whereas switch evaluates both output variables, ifelse is lazy and only evaluates one variable with respect to the condition.

ifelse的使用：

>>> from theano.ifelse import ifelse
>>> a,b=T.dscalars('a','b')
>>> x=T.dscalar('x')
>>> y=T.dscalar('y')
>>> z=ifelse(T.lt(a,b),x+y,x-y)
>>> ifelseFun=theano.function([a,b,x,y],z,mode=theano.Mode(linker='vm'))
>>> AltB=ifelseFun(1,2,3,4)
>>> print AltB
7.0
>>> BltA=ifelseFun(2,1,3,4)
>>> print BltA
-1.0

ifelse和switch比较：

from theano import tensor as T
from theano.ifelse import ifelse
import theano, time, numpy

a,b = T.scalars('a', 'b')
x,y = T.matrices('x', 'y')

z_switch = T.switch(T.lt(a, b), T.mean(x), T.mean(y))
z_lazy = ifelse(T.lt(a, b), T.mean(x), T.mean(y))

f_switch = theano.function([a, b, x, y], z_switch,
                           mode=theano.Mode(linker='vm'))
f_lazyifelse = theano.function([a, b, x, y], z_lazy,
                               mode=theano.Mode(linker='vm'))

val1 = 0.
val2 = 1.
big_mat1 = numpy.ones((10000, 1000))
big_mat2 = numpy.ones((10000, 1000))

n_times = 10

tic = time.clock()
for i in range(n_times):
    f_switch(val1, val2, big_mat1, big_mat2)
print('time spent evaluating both values %f sec' % (time.clock() - tic))

tic = time.clock()
for i in range(n_times):
    f_lazyifelse(val1, val2, big_mat1, big_mat2)
print('time spent evaluating one value %f sec' % (time.clock() - tic))

IfElse op （只需要计算一个变量）花费的时间大约是 Switch op （两个都需要）的一半。

注意：Unless linker='vm' or linker='cvm' are used, ifelse will compute both variables and take the same computation time as switch. Although the linker is not currently set by default to cvm, it will be in the near future.