replacing lines in ruby string

Question

i'm trying to loop through a Ruby string containing many lines using the each_line method, but I also want to change them. I'm using the following code, but it doesn't seem to work:

string.each_line{|line| line=change_line(line)}

I suppose, that Ruby is sending a copy of my line and not the line itself, but unfortunatelly there is no method each_line!. I also tried with the gsub! method, using /^.*$/ to detect each line, but it seems that it calls the change_line method only ones and replaces all lines with it. Any ideas how to do that? Thanks in advance :)

score 5 · Answer 1 · edited May 23 '17 at 11:46

@azlisum: You are not storing the result of your concatenation. Use:

output = string.lines.map{|line|change_line(line)}.join

Comparing four ways to process by line in a string:

# Inject method (proposed by @steenslang)
output = string.each_line.inject(""){|s, line| s << change_line(line)}

# Join method (proposed by @Lars Haugseth)
output = string.lines.map{|line|change_line(line)}.join

# REGEX method (proposed by @olistik)
output = string.gsub!(/^(.*)$/) {|line| change_line(line)}

# String concatenation += method (proposed by @Erik Hinton)
output = "" 
string.each_line{|line| output += change_line(line)}

The timing with Benchmark:

                   user     system      total        real
Inject Time:   7.920000   0.010000   7.930000 (  7.920128)
Join Time:     7.150000   0.010000   7.160000 (  7.155957)
REGEX Time:   11.660000   0.010000  11.670000 ( 11.661059)
+= Time:       7.080000   0.010000   7.090000 (  7.076423)

As @steenslag pointed out, 's += a' will generate a new string for each concatenation and is therefor not usually the best choice.

So given that, and given the times, your best bet is:

output = string.lines.map{|line|change_line(line)}.join

Also, this is the cleaner looking choice IMHO.

Notes:
Using Benchmark
Ruby-Doc: Benchmark

score 3 · Accepted Answer · answered Nov 23 '11 at 12:30

3

You should try starting out with a blank string too, each_lining through the string and then pushing the results onto the blank string.

output = "" 
string.each_line{|line| output += change_line(line)}

In your original example, you are correct. Your changes are occuring but they are not being ssved anywhere. Each in Ruby does not alter anything by default.

answered Nov 23 '11 at 12:30

Erik Hinton

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Can also be written as `output = string.lines.map{|line|change_line(line)}.join`. – Lars Haugseth Nov 23 '11 at 14:12
`str += "something"` will create a new string everytime. http://stackoverflow.com/questions/5686621/ruby-string-concatenation-and – steenslag Nov 23 '11 at 16:22

olistik · Answer 3 · 2011-11-23T13:34:40.793

2

You could use gsub! passing a block to it:

string.gsub!(/^(.*)$/) {|line| change_line(line)}

source: String#gsub!

edited Nov 23 '11 at 13:34

answered Nov 23 '11 at 12:31

olistik

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score 1 · Answer 4 · answered Nov 23 '11 at 12:32

1

String#each_line is meant for reading lines in a string, not writing them. You can use this to get the result you want like so:

changed_string = ""
string.each_line{ |line| changed_string += change_line(line) }

answered Nov 23 '11 at 12:32

Carl Zulauf

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score 0 · Answer 5 · answered Nov 23 '11 at 16:19

0

If you don't give each_line a block, you'll get an enumerator, which has the inject method.

str = <<HERE
smestring dsfg
line 2
HERE

res = str.each_line.inject(""){|m,line|m << line.upcase}

answered Nov 23 '11 at 16:19

steenslag

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replacing lines in ruby string

5 Answers5