Home > Articles > Programming > C/C++

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

Iterators

Containers such as lists and maps do not behave like arrays, so you can't use a for loop to go through the elements in them. Likewise, because these containers are not accessible randomly, you cannot use a simple integer index. You can use iterators to refer to elements of a container.

Iterating through a Container

Each container type has a distinct iterator associated with it. For instance, this is how you declare the iterator for list<int>:

;> list<int>::iterator ili;

You have previously seen the operator :: in two forms: as the global scope operator (where it has one operand) and in the constant ios::app. Each container type has a scope that contains a type name (iterator), and the scope operator (::) allows you to access that type name. Again, using typedef can make for easier typing and understanding, as in the following example; remember that ILI is a completely different name from ili in C++:

;> typedef list<int> LI;
;> typedef LI::iterator ILI;
;> LI ls;  ls.push_back(1);  ls.push(2);  
;> ILI ili = ls.begin();
;> *ili;
(int) 1
;> ++ili;
;> *ili;
(int) 2
;> for(ili = ls.begin(); ili != ls.end(); ++ili)
;1}  cout << *ili << endl;
1
2

In this example, the iterator ili is used for accessing the contents of the list ls. First, a list ls is created, and the numbers 1 and 2 are added to it. Then the iterator ili is declared and set to ls.begin(). The expression *ili gives you the first value in ls, and ++ili moves the iterator to the next value. You use the dereference operator (*) to extract the value and the increment operator (++) to move to the next list item. (Note that * is used for both dereferencing and multiplication, in the same way that - is used for both -2.3 and 2-3. The unary and binary forms of the operator are quite different from one another.) The method begin() returns an iterator that points to the beginning of the list, but the method end() returns an iterator that is just beyond the end of the list. Once ++ili has moved the iterator past the end, then ili becomes equal to ls.end(). Therefore, the for loop in the example visits each item in the list. This technique works for any list type, and it is a very common way of iterating over all items in a list. The vector—and in fact, any standard container—can also be traversed by using an iterator, as in the following example:

;> vector<string>::iterator vsi;
;> string tot;
;> for(vsi = vs.begin(); vsi != vs.end(); ++vsi) tot += *vsi;

In a case like this, you would use a plain for loop and use the vector as if it were an array, but being able to iterate over all containers like this allows you to write very general code that can be used with both vectors and lists.

Finding Items

Reinventing the wheel wastes your time and confuses those that follow you. The standard library provides a number of ready-to-use algorithms that do common tasks like searching and sorting. For instance, find() does a simple linear search for a value and returns an iterator that points to that value if it is successful. You specify the data to be searched by a pair of iterators. Assume that you have a list of strings, ls, which contains "john", "mary", and "alice":

;> list<string> iterator ii;
;> ii = find(ls.begin(), ls.end(), "mary");
;> *ii
(string&) `mary'
;> *ii = "jane";
(string&) `jane'

Note that *ii can be assigned a new value, too: It is a valid lvalue (short for left-hand value) and so can appear on the left-hand side of assignments. Because *ii is a reference to the second item in the list, modifying *ii actually changes the list. If find() does not succeed, it returns ls.end().

You might wonder why you can't just pass the list directly to find(). The standard algorithms could do that but they prefer to work with sequences. Consider the following example of finding the second occurrence of 42 in a list of integers li. You use the result of the first find() as the start of the sequence for the second find(). We have to increment the iterator because it will be pointing to 42 after the first find().

;> list<int>::iterator ili;
;> ili = find(li.begin(),li.end(),42); // first position
;> ++ili;                              // move past the `42'
;> ili = find(ili,li.end(),42);        // second position
;> *ili;
(int&) 42

find() accepts a sequence and not a container argument for another good reason: find() can work with ordinary arrays, which are not proper containers and have no size information. The following example illustrates this, with the standard algorithm copy(), which copies a sequence to a specified destination:

;> int tbl[] = {6,2,5,1};
;> int cpy[4];
;> copy(tbl,tbl+4,cpy);
;> show_arr(cpy,4);
6 2 5 1
;> int array[20];
;> copy(li.begin(), li.end(), array); 
;> *find(tbl,tbl+4,5) = 42;
;> show_arr(tbl,4);
6 2 42 1

Note in this example how you specify the end of an array. copy() is very useful for moving elements from one type of container to another. For example, in this example, you move a list of integers into an array. Again, it is important that the array be big enough for the whole list; otherwise, you could unintentionally damage program memory. Likewise, you can use find(). The call returns a reference to the third element of tbl, which is then changed to 42.

If you have a sorted array-like sequence, then using binary_search() is much faster than find(), as discussed previously. A binary search requires a random access sequence, so it does not work for lists. Maps already have their own find() method, and the generic find() won't work on them.

Erasing and Inserting

Both vectors and lists allow you to erase and insert items, although these operations are faster with lists than with vectors. You specify a position using an iterator, such as the iterator returned by find(). Insertion occurs just before the specified position, as in the following example:

;> list<string> ls;
;> list<string>::iterator ils;
;> ls.push_back("one");
;> ls.insert(ls.end(),"two");       // definition of push_back()!
;> ls.insert(ls.begin(),"zero");    // definition of push_front()!
;> ils = find(ls.begin(),ls.end(),"two");
;> ls.insert(ils, "one-and-a-half");
;> while (ls.size() > 0) { 
;1}   cout << ls.back() << ` `;
;1}   ls.pop_back(); // emptying the list in reverse
;1} }
two one-and-a-half one zero

vectors have methods for inserting and erasing elements, and they all involve moving elements. To erase the second element in a vector (remember that we are counting from zero and begin() refers to the first element), you would use code like this:

;> vi.erase(vi.begin() + 1);
;> vi.erase(vi.end() -  1);   // same as pop_back() !
  • + 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.

Overview


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.

Surveys

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.

Newsletters

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.

Security


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

Children


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

Marketing


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.

Choice/Opt-out


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.

Links


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