Beginner's q about eval and/or code blocks

This ruby program successfully prints all 5-letter words starting with
‘j’
(in a text file containing all current U.S. English Scrabble words).

fp = File.open(‘C:\Documents and Settings\User\twl06FromMac.txt’)
fp.each do |line|
puts line if line =~ /^j…$/
end

I would like to allow the user to enter the regex rather than have it
hard-coded as above. I’ve tried to get eval to work after building a
string to pass it. No luck so far.

I’m positive this is easy in Ruby but currently it’s driving me nuts.

Brian C. wrote:

Jeff Myers wrote:

This ruby program successfully prints all 5-letter words starting with
‘j’
(in a text file containing all current U.S. English Scrabble words).

fp = File.open(‘C:\Documents and Settings\User\twl06FromMac.txt’)
fp.each do |line|
puts line if line =~ /^j…$/
end

I would like to allow the user to enter the regex rather than have it
hard-coded as above. I’ve tried to get eval to work after building a
string to pass it. No luck so far.

Try something like:

str = $stdin.gets.chomp
re = Regexp.new(str)

puts line if line =~ re

Excellent! Thanks. Now I’ll spend some time digging into the Why of
it all. Apparently the match operator needs one side to be a
regular expression object.

Jeff Myers wrote:

This ruby program successfully prints all 5-letter words starting with
‘j’
(in a text file containing all current U.S. English Scrabble words).

fp = File.open(‘C:\Documents and Settings\User\twl06FromMac.txt’)
fp.each do |line|
puts line if line =~ /^j…$/
end

I would like to allow the user to enter the regex rather than have it
hard-coded as above. I’ve tried to get eval to work after building a
string to pass it. No luck so far.

Try something like:

str = $stdin.gets.chomp
re = Regexp.new(str)

puts line if line =~ re

Brian C. wrote:

Jeff Myers wrote:

This ruby program successfully prints all 5-letter words starting with
‘j’
(in a text file containing all current U.S. English Scrabble words).

fp = File.open(‘C:\Documents and Settings\User\twl06FromMac.txt’)
fp.each do |line|
puts line if line =~ /^j…$/
end

I would like to allow the user to enter the regex rather than have it
hard-coded as above. I’ve tried to get eval to work after building a
string to pass it. No luck so far.

Try something like:

str = $stdin.gets.chomp
re = Regexp.new(str)

puts line if line =~ re

If I put the user input into a simple loop, it has trouble:
"private method ‘chomp’ called for nil:NilClass (NoMethodError).
Is there a problem with redefining re?

Jeff Myers wrote:

If I put the user input into a simple loop, it has trouble:
"private method ‘chomp’ called for nil:NilClass (NoMethodError).
Is there a problem with redefining re?

No. It’s because you’re trying to do nil.chomp. Possibly, $stdin.gets is
returning nil for end-of-file. You need to post your actual code if you
want an accurate answer, otherwise I have to guess.

(The “private method” error is confusing; just pretend it said “no such
method”. It’s because there’s also a standalone ‘chomp’ method defined
in the Kernel module. This is a legacy Perlism, and serves little
purpose except to confuse newcomers :slight_smile:

Better to test for nil before you chomp. Example:

loop do
print "Enter a pattern: "
line = $stdin.gets
break if line.nil?
line.chomp!
re = Regexp.new(line)
… rest of logic goes here
end

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