Home > Articles > Programming > Ruby

Containers in Ruby

  • Print
  • + Share This
This chapter is from the book

This chapter is from the book


A string holds characters and nothing else. It is Ruby's only single-purpose container.

As the name suggests, a string has a beginning and an end; it is an ordered container. It might just as accurately have been called a "chain." If the organizing principle were a "bag" instead, then the characters could be scrambled; we would be able to distinguish list from still, but not fare from fear.

Two strings are considered equal only if they have the exact same characters in the exact same sequence.

 "fare" == "fear"  #–> false
 "Night" == "night" #–> false
 "nine " == " nine " #–> false
 "rabbit" == "rabbit" #–> true

Specifying Substrings by Position

Each position in a string has a numerical index. In good computer science tradition, Ruby's indices are counted starting with zero instead of one. Segments of a string, or substrings, are referred to by their starting positions and lengths. Putting this pair of numbers in square brackets, we can examine or modify individual characters or longer substrings.

 foo = "wishbone"
 foo[0,1] #–> "w"
 foo[2,5] #–> "shbon"
 foo[0,1] = "f"  # foo == "fishbone"
 foo[5,1] = "a"  # foo == "fishbane"
 foo[0,4] = "wolf" # foo == "wolfbane"

A replacement string does not need to be the same length as the segment it is replacing. The affected string expands or contracts as needed.

 foo[1,5] = "i" # foo == "wine"
 foo[1,2] = "edg" # foo == "wedge"

It's sometimes more convenient to count from the end of the string than from the beginning. The last character is considered to be in position -1, the next-to-last in position -2, and so on.

 bar = foo.upcase + foo.reverse # bar == "WEDGEegdew"
 bar[–1,1]      #–> "w"
 bar[–7,4]      #–> "GEeg"
 bar[–7,4] = ""     # bar == "WEDdew"

Let's not get lost in notation. If you take a look at bar.methods and scan through the long list it gives you, you'll see "[]" and "[]=" in there. This means that using square brackets to feed position information to a string is just another way of applying a method to an object; the notation may be new, but the concept is no different from what we were talking about on Day 2. "[]" could have been called "substring" or "slice", and "[]=" could have been called "replace_substring."


In fact, Ruby offers slice as a synonym for [].

 	 "Radio".slice(2,3) #–> "dio"

If we wanted to be absolutely strictly consistent with method notation, we could do this:

 qaz = "Mona Lisa"
 qaz.[] (0,5)   #–> "Mona " (refer to substring)
 qaz.[]= (3,6,"day") # qaz == "Monday" (replace substring)

It looks strange, but it works, and it follows the standard dot notation for applying methods: object, dot, method name, and arguments, in that order. Always remember: In Ruby, all you are doing is applying methods (that is, passing messages) to objects. Sometimes it isn't obvious, because some specialized notation is provided to let you say things in another form, but under the hood, it's always the same story.

Providing an alternate way to write something is sometimes known as sugaring the syntax; it doesn't make the language any more nutritious, so to speak, but does make it a little more pleasant to work with. This particular syntax sugar is borrowed from Perl, and it helps Perl programmers feel at home using Ruby's strings.

Individual Characters

The length parameter can be left off when using "[]" or "[]=", in which case the length defaults to one. The results are what you would expect when replacing substrings:

 s = "012345678"
 s[3] = "waffle" # s == "012waffle45678"

But then something surprising happens when you look at an individual character:

 s = "AaBbCc 012"
 s[0] #–> 65 
 s[1] #–> 97
 s[6] #–> 32
 s[7] #–> 48

What's that all about? Depending upon your experience, you might or might not recognize the above as ASCII codes, a common way of representing characters as numbers in the range 0 to 255. Ruby doesn't have a separate Character type, so when we talk about characters, we really mean these numbers.


Don't expect to understand how this works just yet, but if you want to see the ASCII codes of all characters of a string at once, you can try this:

		 "Book".split(//).collect{|c| c[0]}
		   #–> [66, 111, 111, 107] 

Being able to get a visible indication of which ASCII codes are associated with which characters is handy when you're trying to figure out lexicographic (or alphabetical) order. To get characters in this form, you can omit the length parameter to [] as shown above, but to see characters as tiny strings, you can either specify the substring length as 1 or use the chr method to do the necessary conversion. chr is a method of the Integer class.

 s[0,1] #–> "A" 
 s[2].chr #–> "B"
 70.chr #–> "F"
 10.chr #–> "\n" (linefeed)

You may not often have occasion to use it, but Ruby provides a simple way of expressing characters in ASCII form. A character literal is a question mark followed by a single character.

 87.chr #–> "W"
 ?W  #–> 87
 ?\t  #–> 9 (tab character)
 "W"[0] #–> 87
 "W"[0,1] #–> "W"
 ?W < ?X #–> true (because 87 < 88)
 "W" < "X" #–> true (correct lexicographic order)
 ?W < "X" # error


Here is an example of Ruby refusing to try to read your mind. If comparisons between integers and strings were allowed, you would run into situations like this:

		 "4" < ?3

This is ambiguous because the ASCII code for "3" is 51, which is of course larger than 4. So the comparison might be either true or false depending on which conversion you had in mind.

		 "4" < ?3.chr #–> false (comparing 1-character strings)
		 "4".to_i -< ?3 #–> true (interpreting "4" as the number it  represents)

Specifying Substrings by Matching

Often it's useful to deal with substrings based on content rather than position. We've been supplying a position and length to [], but if we supply a string instead, Ruby will search the target string for it and figure out the position and length for itself.

 footwear = "blue suede shoes"
 footwear["suede"]    #–> "suede" 
 footwear["leather"]   #–> nil
 footwear["blue"] = "red"  # footwear == "red suede shoes"
 footwear["socks"] = "sandals" # footwear == "red suede shoes"

Notice that if the search fails, no replacement happens, but there is also no error; the target string is simply unaffected.


Looking ahead: You can search not only for an exact substring but also for an abstract pattern. Here we replace the first vowel in a word with an asterisk:

		 s = "strongbox"
		 s[/[aeiou]/i] = "*" # s == "str*ngbox"

We'll learn all about string matching patterns on Day 8.

A Few Useful String Instance Methods


Append either another string or a character:

 x = "one "
 x << 49  #–> "one 1"  (49 is ASCII for "1")
 x << " two " #–> "one 1 two "
 x << ?2  #–> "one 1 two 2" (same as x << 50)

ljust(length), center(length), rjust(length)

Pad with leading and trailing spaces as necessary to grow to the given length.

 "abc".ljust(7) #–> "abc "
 "abc".rjust(7) #–> " abc"
 "abc".center(7) #–> " abc "
 "abc".center(6) #–> " abc " (odd spaces go to the right)
 "abc".center(2) #–> "abc"  (no change if length is too small) 


Return the number of characters that match those in the description string. A range of characters can be specified with a dash, as in "a-c".

 s = "abcde abcde"
 s.count("c")  #–> 2
 s.count("b–e") #–> 8


Like count, but return a copy with all matching characters removed.

 s = "abcde abcde"
 s.delete("ac–e") #–> "b b"

downcase, upcase, swapcase, capitalize

Return a copy with capitalization changed.

 "aBc".downcase #–> "abc"
 "aBc".upcase  #–> "ABC"
 "aBc".swapcase #–> "AbC"
 "aBc".capitalize #–> "Abc"


Return true or false depending on whether the string contains spec, which can be a string or a character.

 "Haystack".include?("needle") #–> false
 "Haystack".include?("sta") #–> true
 "Haystack".include?(72)  #–> true (because ?H is 72)

index(spec, [offset])

Find the index where spec is found, starting either from the beginning or from offset. Again, spec can be either a string or a character.

 "Mississippi".index("ssi") #–> 2
 "Mississippi".index("ssi",3) #–> 5
 "Mississippi".index("sp") #–> nil (not found)

rindex(spec, [limit])

Like index, but find the last match instead of the first. The limit stops the search.

 "Mississippi".rindex("i") #–> 10
 "Mississippi".rindex("i",6) #–> 4


Remove whitespace (invisible characters such as spaces, tabs, linefeeds, and so forth) from the beginning and end.

 " Erie Canal \n".strip #–> "Erie Canal"

tr(spec, repl)

Short for "translate." Return a copy with characters from spec replaced by the corresponding characters from repl. The first example here simulates the downcase method.

 "DOS_FILE.EXT".tr("A–Z","a–z") #–> "dos_file.ext"
 "Monkey".tr("ym–q","O*:^)?") #–> "M^:keO"
  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020