# what is happening here

G

#### George George

i have some perl code

while (\$high - \$low > 0.001) { # precision
# calculate the sum of all normalized scores
my \$sum = Pn * Pn * exp(\$lambda * \$match) * 4
+ Pn * Pn * exp(\$lambda * \$mismatch) * 12;
# refine guess at lambda
if (\$sum > 1) {
\$high = \$lambda;
\$lambda = (\$lambda + \$low)/2;
}
else {
\$low = \$lambda;
\$lambda = (\$lambda + \$high)/2;
}
}

trying to rewrite this in ruby
while (high - low > 0.001)
#calculate the sum of all normalized scores
sum = Pn * Pn **(lambda * match) * 4 + Pn * Pn **(lambda * mismatch) *
12

#refine guess at lambda
if sum > 1
high = lambda
lambda = (lambda + low)/2
else
low = lambda
lambda = (lambda+high)/2
end
end

Is this the right way?

i seem to get different results when i execute the ruby version.
any ideas?

M

#### Matthias Reitinger

George said:
i have some perl code

while (\$high - \$low > 0.001) { # precision
# calculate the sum of all normalized scores
my \$sum = Pn * Pn * exp(\$lambda * \$match) * 4
+ Pn * Pn * exp(\$lambda * \$mismatch) * 12;
[...]
trying to rewrite this in ruby
while (high - low > 0.001)
#calculate the sum of all normalized scores
sum = Pn * Pn **(lambda * match) * 4 + Pn * Pn **(lambda * mismatch) *
12

It seems like you're using the ** operator wrong. You should use Ruby's
Math.exp instead (which is the direct equivalent to Perl's exp).

-Matthias

G

#### George George

It seems like you're using the ** operator wrong. You should use Ruby's
Math.exp instead (which is the direct equivalent to Perl's exp).

-Matthias

Thank you Mat
i have changed that. here is the full script in ruby:
Pn = 0.25 #probabilty of any nuclotide

match = 10
mismatch = -10

expected_score = match * 0.25 + mismatch * 0.75
if match <=0 or expected_score >= 0
puts "illegal scores"
# quit
end

#calculate lambda
#initial estimates
lambda = 1
high = 2
low = 0

while (high - low > 0.001) do
#calculate the sum of all normalized scores
sum = Pn * Pn * Math.exp(lambda * match) * 4 + Pn * Pn *
Math.exp(lambda * mismatch) * 12

#refine guess at lambda
if sum > 1
high = lambda
lambda = (lambda + low)/2
else
low = lambda
lambda = (lambda+high)/2
end
end

#compute target frequencies and H
target_id = Pn * Pn * Math.exp(lambda * match) * 4
h = lambda * match * target_id + lambda * mismatch * (1 - target_id)

puts "expected_score #{expected_score}\n"
puts "lambda: #{lambda} nats, #{lambda/Math.log(2)} bits"
puts "H: #{h} nats , #{h/Math.log(2)} bits"
puts "%ID: #{target_id * 100}"

and the original perl script is here:
#!/usr/bin/perl -w use strict;
use constant Pn => 0.25; # probability of any nucleotide

die "usage: \$0 <match> <mismatch>\n" unless @ARGV == 2;

my (\$match, \$mismatch) = @ARGV;

my \$expected_score = \$match * 0.25 + \$mismatch * 0.75;
die "illegal scores\n" if \$match <= 0 or \$expected_score >= 0;

# calculate lambda
my (\$lambda, \$high, \$low) = (1, 2, 0); # initial estimates

while (\$high - \$low > 0.001) { # precision
# calculate the sum of all normalized scores
my \$sum = Pn * Pn * exp(\$lambda * \$match) * 4
+ Pn * Pn * exp(\$lambda * \$mismatch) * 12;
# refine guess at lambda
if (\$sum > 1) {
\$high = \$lambda;
\$lambda = (\$lambda + \$low)/2;
}
else {
\$low = \$lambda;
\$lambda = (\$lambda + \$high)/2;
}
}

# compute target frequency and H
my \$targetID = Pn * Pn * exp(\$lambda * \$match) * 4;
my \$H = \$lambda * \$match * \$targetID
+ \$lambda * \$mismatch * (1 -\$targetID);

# output
print "expscore: \$expected_score\n";
print "lambda: \$lambda nats (", \$lambda/log(2), " bits)\n";
print "H: \$H nats (", \$H/log(2), " bits)\n";
print "%ID: ", \$targetID * 100, "\n";

The ruby version seems to get into an infinite loop. while the perl
version executes nicely and with the expected results.

F

#### Fritz Heinrichmeyer

in ruby lambda, high and low are integers and divide by two is also
integer arithmetic. Perl uses float for scalars by default.

Am Mittwoch, den 08.04.2009, 07:57 -0500 schrieb George George:

P

#### Peter Zotov

Quoting George George said:
i have some perl code

while (\$high - \$low > 0.001) { # precision
# calculate the sum of all normalized scores
my \$sum = Pn * Pn * exp(\$lambda * \$match) * 4
+ Pn * Pn * exp(\$lambda * \$mismatch) * 12;
# refine guess at lambda
if (\$sum > 1) {
\$high = \$lambda;
\$lambda = (\$lambda + \$low)/2;
}
else {
\$low = \$lambda;
\$lambda = (\$lambda + \$high)/2;
}
}

Pn * exp(\$lambda * \$match) is Pn * (Math::E ** (lambda + match)), not
Pn ** (lambda + match).

WBR, Peter Zotov

F

#### Fritz Heinrichmeyer

define lambda, high and low as floats, lambda = 1.0, high=2.0, low = 0.0

Am Mittwoch, den 08.04.2009, 07:57 -0500 schrieb George George: