So how can I still be able to write beautiful code such as:
'im a string meing!'.pop
Note: str.chop isn't sufficient answer
It is not what an enumerable string atually enumerates. Is a string a sequence of ...
lines,
characters,
codepoints or
bytes?
The answer is: all of those, any of those, either of those or neither of those, depending on the context. Therefore, you have to tell Ruby which of those you actually want.
There are several methods in the String class which return enumerators for any of the above. If you want the pre-1.9 behavior, your code sample would be
'im a string meing!'.bytes.to_a.pop
This looks kind of ugly, but there is a reason for it: a string is a sequence. You are treating it as a stack. A stack is not a sequence, in fact it pretty much is the opposite of a sequence.
That's not beautiful :)
Also #pop is not part of Enumerable, it's part of Array.
The reason why String is not enumerable is because there are no 'natural' units to enumerate, should it be on a character basis or a line basis? Because of this String does not have an #each
String instead provides the #each_char and #each_byte and #each_line methods for iteration in the way that you choose.
Since you don't like str[str.length], how about
'im a string meing!'[-1] # returns last character as a character value
or
'im a string meing!'[-1,1] # returns last character as a string
or, if you need it modified in place as well, while keeping it an easy one-liner:
class String
def pop
last = self[-1,1]
self.chop!
last
end
end
#!/usr/bin/ruby1.8
s = "I'm a string meing!"
s, last_char = s.rpartition(/./)
p [s, last_char] # => ["I'm a string meing", "!"]
String.rpartition is new for 1.9 but it's been back-ported to 1.8.7. It searches a string for a regular expression, starting at the end and working backwards. It returns the part of the string before the match, the match, and the part of the string after the match (which we discard here).
String#slice! and String#insert is going to get you much closer to what you want without converting your strings to arrays.
For example, to simulate Array#pop you can do:
text = '¡Exclamation!'
mark = text.slice! -1
mark == '!' #=> true
text #=> "¡Exclamation"
Likewise, for Array#shift:
text = "¡Exclamation!"
inverted_mark = text.slice! 0
inverted_mark == '¡' #=> true
text #=> "Exclamation!"
Naturally, to do an Array#push you just use one of the concatenation methods:
text = 'Hello'
text << '!' #=> "Hello!"
text.concat '!' #=> "Hello!!"
To simulate Array#unshift you use String#insert instead, it's a lot like the inverse of slice really:
text = 'World!'
text.insert 0, 'Hello, ' #=> "Hello, World!"
You can also grab chunks from the middle of a string in multiple ways with slice.
First you can pass a start position and length:
text = 'Something!'
thing = text.slice 4, 5
And you can also pass a Range object to grab absolute positions:
text = 'This is only a test.'
only = text.slice (8..11)
In Ruby 1.9 using String#slice like this is identical to String#[], but if you use the bang method String#slice! it will actually remove the substring you specify.
text = 'This is only a test.'
only = text.slice! (8..12)
text == 'This is a test.' #=> true
Here's a slightly more complex example where we reimplement a simple version of String#gsub! to do a search and replace:
text = 'This is only a test.'
search = 'only'
replace = 'not'
index = text =~ /#{search}/
text.slice! index, search.length
text.insert index, replace
text == 'This is not a test.' #=> true
Of course 99.999% of the time, you're going to want to use the aforementioned String.gsub! which will do the exact same thing:
text = 'This is only a test.'
text.gsub! 'only', 'not'
text == 'This is not a test.' #=> true
references:
Ruby String Documentation
Related
I was recently asked this in an interview and was figuring out a way to do this without using regex in Ruby as I was told it would be a bonus if you can solve it without using regex.
Question: Assume that the hash has 1 million key, value pairs and we have to be able to sub the variables in the string that are between % % this pattern. How would I be able to do this without regex.
We have a string str = "%greet%! Hi there, %var_1% that can be any other %var_2% injected to the %var_3%. Nice!, goodbye)"
we have a hash called dict = { greet: 'Hi there', var_1: 'FIRST VARIABLE', var_2: 'values', var_3: 'string', }
This was my solution:
def template(str, dict)
vars = value.scan(/%(.*?)%/).flatten
vars.each do |var|
value = value.gsub("%#{var}%", dict[var.to_sym])
end
value
end
There are many ways to solve this, but you will probably need some kind of parsing and / or lexical analysis if you don't want to use built-in pattern matching.
Let's keep it very simple and say that your string's content falls into two categories: text and variable which are separated by %, e.g. (you could also think of the variables being enclosed by %, but that's harder to implement)
str = "Hello %name%, hope to see you %when%!"
# TTTTTT VVVV TTTTTTTTTTTTTTTTTT VVVV T
As you can see, the categories are alternating. We can utilize this and write a little helper method that turns a string into a list of [type, value] pairs, something like this:
def each_part(str)
return enum_for(__method__, str) unless block_given?
type = [:text, :var].cycle
buf = ''
str.each_char do |char|
if char != '%'
buf << char
else
yield type.next, buf
buf = ''
end
end
yield type.next, buf
end
It starts by defining an enumerator that will cycle between the two types and an empty buffer. It will then read each_char from the string. If the char is not %, it will just append it to the buffer and keep reading. Once it encounters a %, it will yield the current buffer along with the type and start a new buffer (next will also switch the type). After the loop ends, it will yield once more to output the remaining characters.
It outputs this kind of data:
each_part(str).to_a
#=> [[:text, "Hello "],
# [:var, "name"],
# [:text, ", hope to see you "],
# [:var, "when"],
# [:text, "!"]]
We can use this to convert the string:
dict = { name: 'Tom', when: 'soon' }
output = ''
each_part(str) do |type, value|
case type
when :text
output << value
when :var
output << dict[value.to_sym]
end
end
p output
#=> "Hello Tom, hope to see you soon!"
You could of course combine parsing and evaluation, but I like the separation. An full-fledged parser might involve even more steps.
A very simple approach:
First, split the string on '%':
str = "%greet%! Hi there, %var_1% that can be any other %var_2% injected to the %var_3%. Nice!, goodbye)"
chunks = str.split('%')
Now we can assume given the way the problem has been specified, that every other "chunk" will be a key to replace. Iterating with the index will make that easier to figure out.
chunks.each_with_index { |c, i| chunks[i] = (i.even? ? c : dict[c.to_sym]) }.join
Result:
"Hi there! Hi there, FIRST VARIABLE that can be any other values injected to the string. Nice!, goodbye)"
Note: this does not handle malformed input well at all.
print str[i].upcase is not working and i have to capitalize specific letters determined using an index. Can someone help me with this?
def mumble_letters
str = nil
print "Please write a string : "
str = gets.to_str
# puts str.length
while str.length == 1
print "Please write a string : "
str = gets.to_str
end
for i in 0..str.length
print str[i].upcase!
i.times{ print str[i].capitalize}
if i != str.length - 1
print"-"
end
end
end
mumble_letters
the error I get is : undefined method `upcase' for nil:NilClass (NoMethodError)
Did you mean? case
Problem
str[i].upcase! mutates the single character in the Array value into an uppercase character. However, at least on Ruby 2.7.1, it won't actually change the contents of your original String object until you reassign the element back to the String index you want modified. For example:
str[i] = str[i].upcase
However, the approach above won't work with frozen strings, which are fairly common in certain core methods, libraries, and frameworks. As a result, you may encounter the FrozenError exception with the index-assignment approach.
Solution
There's more than one way to solve this, but one way is to:
split your String object into an Array of characters,
modify the letter at the desired indexes,
rejoin the characters into a single String, and then
re-assign the modified String to your original variable.
For example, showing some intermediate steps:
# convert String to Array of characters
str = "foobar"
chars = str.chars
# contents of your chars Array
chars
#=> ["f", "o", "o", "b", "a", "r"]
# - convert char in place at given index in Array
# - don't rely on the return value of the bang method
# to be a letter
# - safe navigation handles various nil-related errors
chars[3]&.upcase!
#=> "B"
# re-join Array of chars into String
chars.join
#=> "fooBar"
# re-assign to original variable
str = chars.join
str
#=> "fooBar"
If you want, you can perform the same operation on multiple indexes of your chars Array before re-joining the elements. That should yield the results you're looking for.
More concisely:
str = "foobar"
chars = str.chars
chars[3]&.upcase!
p str = chars.join
#=> "fooBar"
Personally, I find operating on an Array of characters more intuitive and easier to troubleshoot than making in-place changes through repeated assignments to indexes within the original String. Furthermore, it avoids exceptions raised when trying to modify indexes within a frozen String. However, your design choices may vary.
str[i].upcase returns the upcased letter, but does not modify it in place. Assign it back to the string for it to work.
str = 'abcd'
str[2] = str[2].upcase #=> "C"
str #=> "abCd"
I can see two problems with your code...
First, an empty string has a length of 0 so what you wanted to write is
while str.length == 0
Secondly, when you do...
for i in 0..str.length
You are iterating up to the string length INCLUDING the string length. If the string has five characters, it actually only has valid indexes 0 through 4 but you are iterating 0 through 5. And str[5] doesn't exist so returns nil and you cannot do upcase! on a nil.
To handle that common situation, Ruby has the tripe dot operator
for i in 0...str.length
...which will stop at the integer before the length, which is what you want.
It's also more ruby-eque to do
(0...str.length).each do |i|
So here is the string I want to convert to an array, where then I want to reverse each word without reversing the entire sentence, and then join them back and provide the output.
For instance, I want to change "Hello there, and how are you?" to "olleH ,ereht dna woh era ?uoy"
This is the string:
sentence1="Hello there, and how are you?"
and, this is my code in which I have to incorporate .each(which i know is wrong, but don't know how)
def reverse_each_word(sentence1)
split_array = sentence1.split
reversed_array = split_array.reverse
reversed_array.each do |joined_array|
joined_array.join(' ')
end
end
and as mentioned, the desired result has to be:
"olleH ,ereht dna woh era ?uoy"
You're calling join in a string, since you're iterating over each element in reversed_array, and all those ones are string objects:
p sentence1.split.first.join(' ')
# undefined method `join' for "Hello":String (NoMethodError)
It might work if you use something to store the value in each iteration within the block, it can be a variable declared outside the iteration, or better map, after that, you can just reverse each string and then join everything:
def reverse_each_word(sentence1)
sentence1.split.map do |joined_array|
joined_array.reverse
end.join(' ')
end
p reverse_each_word(sentence1) # "olleH ,ereht dna woh era ?uoy"
Notice this can be written as sentence1.split.map(&:reverse).join(' ') too.
In case you're looking for each to solve this problem, you'll need a variable where to store each "modified" string as long as you're iterating over each of those elements:
memo = ''
sentence1.split.each { |joined_array| memo << "#{joined_array.reverse} " }
p memo.rstrip # "olleH ,ereht dna woh era ?uoy"
There you have a memo variable which is an empty string, just for the reason to be filled with each reversed string, you reverse the string and add a white space to the right. The last string is going to have an additional whitespace, so rstrip helps you to "remove" it.
For collect you can use the map approach, because they're aliases.
I would be inclined to use String#gsub with a regular expression.
str = "Hello there, and how are you?"
str.gsub(/\S+/) { |s| s.reverse }
#=> "olleH ,ereht dna woh era ?uoy"
The regular expression reads, "match one or more characters other than whitespace characters".
I want to convert all the words(alphabetic) in the string to their abbreviations like i18n does. In other words I want to change "extraordinary" into "e11y" because there are 11 characters between the first and the last letter in "extraordinary". It works with a single word in the string. But how can I do the same for a multi-word string? And of course if a word is <= 4 there is no point to make an abbreviation from it.
class Abbreviator
def self.abbreviate(x)
x.gsub(/\w+/, "#{x[0]}#{(x.length-2)}#{x[-1]}")
end
end
Test.assert_equals( Abbreviator.abbreviate("banana"), "b4a", Abbreviator.abbreviate("banana") )
Test.assert_equals( Abbreviator.abbreviate("double-barrel"), "d4e-b4l", Abbreviator.abbreviate("double-barrel") )
Test.assert_equals( Abbreviator.abbreviate("You, and I, should speak."), "You, and I, s4d s3k.", Abbreviator.abbreviate("You, and I, should speak.") )
Your mistake is that your second parameter is a substitution string operating on x (the original entire string) as a whole.
Instead of using the form of gsub where the second parameter is a substitution string, use the form of gsub where the second parameter is a block (listed, for example, third on this page). Now you are receiving each substring into your block and can operate on that substring individually.
def short_form(str)
str.gsub(/[[:alpha:]]{4,}/) { |s| "%s%d%s" % [s[0], s.size-2, s[-1]] }
end
The regex reads, "match four or more alphabetic characters".
short_form "abc" # => "abc"
short_form "a-b-c" #=> "a-b-c"
short_form "cats" #=> "c2s"
short_form "two-ponies-c" #=> "two-p4s-c"
short_form "Humpty-Dumpty, who sat on a wall, fell over"
#=> "H4y-D4y, who sat on a w2l, f2l o2r"
I would recommend something along the lines of this:
class Abbreviator
def self.abbreviate(x)
x.gsub(/\w+/) do |word|
# Skip the word unless it's long enough
next word unless word.length > 4
# Do the same I18n conversion you do before
"#{word[0]}#{(word.length-2)}#{word[-1]}"
end
end
end
The accepted answer isn't bad, but it can be made a lot simpler by not matching words that are too short in the first place:
def abbreviate(str)
str.gsub(/([[:alpha:]])([[:alpha:]]{3,})([[:alpha:]])/i) { "#{$1}#{$2.size}#{$3}" }
end
abbreviate("You, and I, should speak.")
# => "You, and I, s4d s3k."
Alternatively, we can use lookbehind and lookahead, which makes the Regexp more complex but the substitution simpler:
def abbreviate(str)
str.gsub(/(?<=[[:alpha:]])[[:alpha:]]{3,}(?=[[:alpha:]])/i, &:size)
end
I want to replace the last occurrence of a substring in Ruby. What's the easiest way?
For example, in abc123abc123, I want to replace the last abc to ABC. How do I do that?
How about
new_str = old_str.reverse.sub(pattern.reverse, replacement.reverse).reverse
For instance:
irb(main):001:0> old_str = "abc123abc123"
=> "abc123abc123"
irb(main):002:0> pattern="abc"
=> "abc"
irb(main):003:0> replacement="ABC"
=> "ABC"
irb(main):004:0> new_str = old_str.reverse.sub(pattern.reverse, replacement.reverse).reverse
=> "abc123ABC123"
"abc123abc123".gsub(/(.*(abc.*)*)(abc)(.*)/, '\1ABC\4')
#=> "abc123ABC123"
But probably there is a better way...
Edit:
...which Chris kindly provided in the comment below.
So, as * is a greedy operator, the following is enough:
"abc123abc123".gsub(/(.*)(abc)(.*)/, '\1ABC\3')
#=> "abc123ABC123"
Edit2:
There is also a solution which neatly illustrates parallel array assignment in Ruby:
*a, b = "abc123abc123".split('abc', -1)
a.join('abc')+'ABC'+b
#=> "abc123ABC123"
Since Ruby 2.0 we can use \K which removes any text matched before it from the returned match. Combine with a greedy operator and you get this:
'abc123abc123'.sub(/.*\Kabc/, 'ABC')
#=> "abc123ABC123"
This is about 1.4 times faster than using capturing groups as Hirurg103 suggested, but that speed comes at the cost of lowering readability by using a lesser-known pattern.
more info on \K: https://www.regular-expressions.info/keep.html
Here's another possible solution:
>> s = "abc123abc123"
=> "abc123abc123"
>> s[s.rindex('abc')...(s.rindex('abc') + 'abc'.length)] = "ABC"
=> "ABC"
>> s
=> "abc123ABC123"
When searching in huge streams of data, using reverse will definitively* lead to performance issues. I use string.rpartition*:
sub_or_pattern = "!"
replacement = "?"
string = "hello!hello!hello"
array_of_pieces = string.rpartition sub_or_pattern
( array_of_pieces[(array_of_pieces.find_index sub_or_pattern)] = replacement ) rescue nil
p array_of_pieces.join
# "hello!hello?hello"
The same code must work with a string with no occurrences of sub_or_pattern:
string = "hello_hello_hello"
# ...
# "hello_hello_hello"
*rpartition uses rb_str_subseq() internally. I didn't check if that function returns a copy of the string, but I think it preserves the chunk of memory used by that part of the string. reverse uses rb_enc_cr_str_copy_for_substr(), which suggests that copies are done all the time -- although maybe in the future a smarter String class may be implemented (having a flag reversed set to true, and having all of its functions operating backwards when that is set), as of now, it is inefficient.
Moreover, Regex patterns can't be simply reversed. The question only asks for replacing the last occurrence of a sub-string, so, that's OK, but readers in the need of something more robust won't benefit from the most voted answer (as of this writing)
You can achieve this with String#sub and greedy regexp .* like this:
'abc123abc123'.sub(/(.*)abc/, '\1ABC')
simple and efficient:
s = "abc123abc123abc"
p = "123"
s.slice!(s.rindex(p), p.size)
s == "abc123abcabc"
string = "abc123abc123"
pattern = /abc/
replacement = "ABC"
matches = string.scan(pattern).length
index = 0
string.gsub(pattern) do |match|
index += 1
index == matches ? replacement : match
end
#=> abc123ABC123
I've used this handy helper method quite a bit:
def gsub_last(str, source, target)
return str unless str.include?(source)
top, middle, bottom = str.rpartition(source)
"#{top}#{target}#{bottom}"
end
If you want to make it more Rails-y, extend it on the String class itself:
class String
def gsub_last(source, target)
return self unless self.include?(source)
top, middle, bottom = self.rpartition(source)
"#{top}#{target}#{bottom}"
end
end
Then you can just call it directly on any String instance, eg "fooBAR123BAR".gsub_last("BAR", "FOO") == "fooBAR123FOO"
.gsub /abc(?=[^abc]*$)/, 'ABC'
Matches a "abc" and then asserts ((?=) is positive lookahead) that no other characters up to the end of the string are "abc".