Conditions

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 than ifelse.
  • Whereas switch evaluates both output variables, ifelse is lazy and only evaluates one variable with respect to the condition.

Example

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

In this example, the IfElse op spends less time (about half as much) than Switch since it computes only one variable out of the two.

$ python ifelse_switch.py
time spent evaluating both values 0.6700 sec
time spent evaluating one value 0.3500 sec

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.

There is no automatic optimization replacing a switch with a broadcasted scalar to an ifelse, as this is not always faster. See this ticket.