# No.25 Perl Sample (code key - 73)
# Learning Perl: count log probability （対数確率を計算する）
# Tomonori Nagano <tnagano@gc.cuny.edu>
# Last Update: January 18, 2008
#
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$probA = 50/1000 ;				# supporse word A appeard 50 times in a 1000-word corpus
$probB = 10/1000 ;				# supporse word B appeard 10 times in a 1000-word corpus

print "Probability of A (what it usually looks): $probA\n" ;	# prob of A is 0.05
print "Probability of B (what it usually looks): $probB\n" ;	# prob of B is 0.01

printf "Probability of A (what precisely Perl gets): %20.20f\n",$probA ;	# this should be 0.05
printf "Probability of B (what precisely Perl gets): %20.20f\n",$probB ;	# this should be 0.01

print "Independent prob of A*B (the value on surface): ".$probA*$probB."\n" ;	# the surface value
printf "Independent prob of A*B (the precise value): %30.30f\n",$probA*$probB ;	# the real value

$logProbA = log(50/1000) ;	# taking log probability
$logProbB = log(10/1000) ;	# taking log probability

print "Independent log prob of A*B: ".(-1*($logProbA+$logProbB))."\n" ;	# compute log probability