Stl iterators 1 The iterator concept A generalization

Stl iterators 1

The iterator concept �A generalization of a pointer. �An objects that gives access to elements of another object. �The iterator concept is the set of all the operations it can perform on the referenced object (container, stream…). �The iterator required operations are derived from the nature of the referenced object. 2

Iterator concepts hierarchy 3

Iterator operations �Output: write only and can write only once �Input: read many times each item �Forward supports both read and write �Bi-directional support also decrement �Random supports random access (just like C pointer) 4

Iterators & Containers �Bidirectional iterators for: �list, map, set �Random access iterators for: �Vector, array �Input/output/forward iterators for: �iostreams (not a container) 5

Iterators and Containers • Each container defines an itertor type, and begin() and end() functions that return iterators. class Name. Of. Container {. . . typedef … iterator; // iterator type of Name. Of. Container iterator begin(); // first element of Name. Of. Container iterator end(); // element after last of Name. Of. Container 6

Iterator basic usage �With templates: Name. Of. Container<. . . > c. . . Name. Of. Container<. . . >: : iterator i; for(i= c. begin(); i!=c. end(); ++i) // do something that changes *i 7

const_iterators & Containers class Name. Of. Container {. . . typedef … const_iterator; const_iterator begin() const; const_iterator end() const; // iterator type of Name. Of. Container // first element // element after last Name. Of. Container<. . . > c Name. Of. Container<. . . >: : const_iterator it; for( it= c. begin(); it!=c. end(); ++it) // do something that does not changes *it 8

Const iterators. . . iterator begin(); iterator end(); . . . const_iterator begin() const; const_iterator end() const; �Note that the begin() and end() methods that return regular iterator are not const methods. i. e: if we get a container by const reference we can't use these methods, the methods that return const_iterator will be used. 9

Iterators & Sequence Containers Seq. Container. Name<. . . > c; Seq. Container. Name<. . . >: : iterator i, j; c. insert(i, x) // insert x before i c. insert(i, first, last) // insert elements from [first, last) before i c. erase(i) // erase element i points to c. erase(i, j) // erase elements in range [i, j) 10 first, last are any type of input iterator

Iterators & other Containers �insert and erase has the same ideas for different containers, except they keep the invariants of the specific container. �For example, a Sorted Associative Container will remain sorted after insertions and erases. 11

Iterators and Assoc. Containers �For example - what does c. insert(pos, x) does, when c is a Unique Sorted Associative Container? �Inserts x into the set, using pos as a hint to where it will be inserted: hint regarding the place to start searching for the correct point of insertion. �If the hint is good, will reduce the insertion time from O(log(N)) to O(1) 12

Algorithm that requires an input iterator template<class Input. Iterator, class T> Input. Iterator find ( Input. Iterator first, Input. Iterator last, const T& value ) { for ( ; first!=last; first++) // [first, last) { if ( *first==value ) break; } return first; 13 }

Iterators’ Validity �When working with iterators, we have to remember that their validity can be changed �What’s wrong with this code? Container c; Container: : iterator i; for(i=c. begin(); i!=c. end(); ++i) if(f(*i)) // f is some test { c. erase(i); } �If we enter the “if”: it nvalidates i, thus we cannot “++” it 14

Iterators’ Validity Two cases: �list, set, map �i is not a legal iterator �vector �i (might) point to the element after �In either case, this is not what we want… 15

Iterator validity – second try. . . Container c; Container: : iterator i= c. begin(); while(i!=c. end()) { Container: : iterator j= i; ++i; if(f(*j)) // f is some test { c. erase(j); . . . } �Works for set, map, list, not vector 16

How do you know? ! �Experience �Trial and error ; ) �http: //www. cplus. com/reference/stl/ �msdn 17

Stl algorithms

Inclusions �To use stl algorithmis, you must include: #include <algorithm> �Some of the stl algorithms are provided for numeric processing. To use them you must include: #include <numeric> �Sometimes you will need to use function adapters. They are defined in: #include <functional>

A brief introduction �Algorithms are global functions that work on iterators. �All algorithms process one or more iterartors range: �The first range is usually iterators for the first and last indexes of the container. �The caller must ensure that the range is valid. �The second range is the destination range. �Algorithms usually work in overwrite mode. It is the caller responsibility to ensure that the destination range has enough elements. �Several algorithms allow the user to pass user defined operations via function objects.

Suffixes �The _if suffix: �Use to specify a condition to an algorithm by a function object. �For example: find() Vs. find_if() �The _copy suffix: �Used to indicate that the elements are copied into the destination range. �For example: reverse() Vs. reverse_copy() �The suffix _n: �Perform an operation n times. �For example generate_n()

Algorithms classification �Non-modifying algorithms. �Modifying algorithms. �Sorting algorithms. �Numeric algorithms.

Non-modifying algorithms �Do not change the order or values of the objects they process. �Use input and forward iterators. �Can be used with all standard containers. �Partial list: �count – Returns the number of element (also count_if) �find, find_if – Search for the first element with a specified value, or condition. �search – Searches for the first occurrence of a sub-range. �for_each – Performs operation on each element (can also modify)

Algorithms Vs. containers member functions �Never use an algorithm, when there is a member function in your container that does the same. �For example, to find a key in a set: �find(my. Set. begin(), my. Set. end(), key) - linear complexity �my. Set. find(key) – logarithmic complexity �However, you can use specialized algoritms such as binary_search()

Modifying algorithms �Modify elements in the range, either directly or a copied version to the destination range. �Partial list: �transform – apply function to range. �copy – copy elements to range. �generate - generate values to range with a function �unique – remove consecutive duplicates.

Sorting and related algorithms �Sorting (partial list): �sort – sort elements in range. �stable_sort – preserve order of equivalents �partial_sort – partially sort elements in the range �Binary search: �binary_search – test for existence of a value �lower_bound – return iterator to lower bound �Merging: �merge – merge sorted ranges �Heap operations: �make_heap �sort_heap

Numeric algorithms �More specified algorithms for numeric operations �Partial list: �accumulate – compute sum of range + baseline �inner_product �adjacent_difference – compute the difference between neighbors in a range