STL Algorithms

algorithms independent of containers

STL Algorithms Description• independent of container types: operate on iterators

– operate on half-open range of elements of a container specified by iterators• often behavior can be modified through callbacks – references to code• callbacks

– function pointers (C-style)– function objects (functors)– lambda expressions (C++11)

• most algorithms are declared in <algorithm>, some are in <numeric>

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Algorithm Example: find• looks for specific element in range• needs <algorithm>• find(beginRange, endRange, toFind)• returns iterator to first matching element in range of the container

– if associative – not necessarily first, use lower_bound• if not found – returns endRange (past last element)• linear complexity

find() method in map and set is faster (logarithmic), use it instead

find() method in list should be used instead

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find_if with Function Pointer Callback• find_if(beginRange, endRange, condition)• returns iterator to first element in range satisfying condition• third parameter is a callback• may be the name of a function (function pointer)

– need to accept element type– need to be a predicate (return boolean)

• example:

vector<int> vect;

…

auto it=find_if(vect.begin(), vect.end(), moreThan5);

…

bool moreThan5(int elem){

return elem>5;

}

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Lambda Expressions• anonymous function

• defined in C++11

• syntax: [capture] -> returnType (parameters){body}• capture – passing discipline and (optionally) name of variables taken from

outside of scope of the lambda expressions– [] no variables defined. Attempting to use any external variables in the

lambda is an error

• clang/gcc apparently still capture external variables, it is not standard– [x, &y] x is captured by value, y is captured by reference – [&] any external variable is implicitly captured by reference – [=] any external variable is implicitly captured by value

• compiler may be able to deduce return type from return statement: returnType is optional

• if no parameters, parentheses are optional

• example: []{cout << ”Hello, World!” << endl;} • can be assigned to function pointer variables

– watch out for implicit captures

• can be used as parameters for other functions, have to conform to signature5

count_if, generate, for_each• count_if – counts number of elements that satisfy callback condition

int num=55;

int cnt = count_if(vect.begin(), vect.end(), [num](int i){return i==num;});

• generate – fills elements with value returned by callback

generate(vect.begin(), vect.end(), []{return rand()%10;});

• for_each – executes callback for each element

for_each(vect.begin(), vect.end(),

[](int i){cout << i << " ";});

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accumulate with various callbacks• accumulates data about container• two forms

accumulate(beginRange, endRange, initialValue) – sums elements, sum initialized to initialValue, returns accumulated value

accumulate(beginRange, endRange, initialValue, callback) invokes callback with two arguments, first is accumulator

• callback can be function or lambda

int product(int num1, int num2){

return num1 * num2;

}

double mult = accumulate(vect.begin(), vect.end(), 1, product);

or

double multLambda = accumulate(vect.begin(), vect.end(), 1,

[](int num1, int num2){return num1 * num2;});

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Function Objects (Functors)• can overload function call operator: operator() in a class

– such class is functor– may have any number of arguments and return any value

• then invoke like a standalone function

class MyFunctor{

public:

MyFunctor(int x) : x_(x) {}

int operator() (int y) {return x_+y;}

private:

int x_;

};

…

MyFunctor addOne(1); // creating a functor object

cout << addOne(2) << endl; // call it like a regular function

• may keep state between calls. Do not use this feature for algorithms!• for simple tasks lambda functions are preferred

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Predefined Functors, Arithmetic

• STL provides a number of predefined functors• defined in <functional>• in std namespace (need to be imported or scope resolved)• arithmetic: plus, minus, multiplies, divides, modulus• have to be instantiated with type

plus<int> myPlus;

int result = myPlus(3,4);

cut << result << endl;• may be used in algorithms as callbacks

int sum = accumulate(vect.begin(), vect.end(), 0, plus<int>());• regular operators cannot be used as callbacks, functors are adapters that wrap regular

arithmetic operators

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Comparison and Logical Functors

• comparison functors: equal_to not_equal_to less greater less_equal greater_equal– less is used as default comparison in priority_queue container adapter– may be changed, have to specify container, usually vector– example: reversing sorting order in priority_queue

priority_queue<string, std::vector<string>,std::greater<string>> workWeekR;

• logical functors: logical_and logical_or logical_not – example: logical_and in accumulate to determine if all boolean elements are true

vector<bool> flags;

…

bool allTrue=accumulate(flags.begin(), flags.end(), true,

std::logical_and<bool>());

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Functor Adapters (Binders)• binder (function adapter) – a specialized function that creates a function by assigning

(binding) a value of parameter of another function• bind() a C++11 feature – most flexible binder

newFunctior bind(oldFunctor, arguments)

where– newFunctor – pointer to new functor with bound parameters– oldFunctor – old functor– arguments – arguments to old functor

• free specified as _1 _2, etc defined in std::placeholders namespace• bound

• auto is useful as return type or it gets complicated• examples

auto f1 = bind(myFunc, _1, str); // binds second parameter to string str

auto f2 = bind(myFunc, _2, _1); // swaps parameters

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Using Binders to Form Callbacks• binders useful in forming callbacks for algorithims inline

using namespace std::placeholders;

bool passingScore(int s, int threshold){

return s>=threshold;

}

...

// biding second argument of function passingScore to 70

auto it=find_if(vect.begin(), vect.end(),

bind(passingScore, _1, 70));

// binding second argument of standard functor greater_equal

auto it = find_if(vect.begin(), vect.end(), bind(std::greater_equal<int>(), _1, 70));

• this example is probably easier to read with lambda, how would you implement it?

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Containers of Objects• algorithms perform on non-basic types correctly provided that proper operators are

defined– e.g. operator< for sorting, operator== for searching

• common task: invoke a method on each object– suppose myclass declares myfunc() method and container cont holds

elements of myclass

may be accomplished as follows

for_each(cont.begin(), cont.end(), &myclass::myfunc);– if need to pass parameters, use bind()

for_each(cont.begin, cont.end(), bind(&myclass::myfunc, _1, value));

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Algorithm Categories

• utility – not operating on containers but useful• non-modifying – not updating the container

– search: min_element, max_element, find_fist_of, search, search_n

– comparison: equal, mismatch, liexicographical_compare– operational: for_each– numerical processing: count, count_if, accumulate

• modifying – updating the container• sorting – sorting or (dis)ordering container• set – set functions

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Utility Algorithms• min, max, minmax, swap• operate on a couple of elements• use operator<• use function templates• examples

int x=1,y=2;

cout << min(x, y); // prints1

cout << max(x, y); // prints 2

swap(x, y); cout<< x << y; // prints 21

auto pair = minmax(x,y);

cout << pair.first << pair.second; // prints 12

• In C++11, utility algorithms operate on initializer lists: max({1,2,3,4,5});

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Search Algorithms• return iterator to first element found• accept range• by default use opeator== or operator<• find, find_if, find_if_not – already covered• min_element, max_elment – locate element

auto it=min_element(vect.begin(), vect.end());• adjacent_find – finds the first pair of matching consecutive elements• find_first_of – finds first occurrence of elements in target range• search – finds target subsequence• search_n – finds consecutive elements• searches that work on ordered sequences (sorted vector, map, multimap,

set, multiset): binary_search, lower_bound, upper_bound, equal_range

• C++11 functions: find_if_not, minmax_element, all_of, any_of, none_of

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Comparison and Operational• comparison – compare entire ranges of elements

equal() – returns true if elements in both ranges are equal

mismatch() returns iterator to the first mismatched element

lexicographical_compare() – dictionary order comparison of elements

• operational

for_each() – executes callback on each element of the range: may print a copy of every element, accumulate info about all elements, etc.

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Modifying Algorithms• usually operate on two ranges: source range and destination (target) range,

ranges may be independent, overlapping or the same (in place operation)• transform() – similar to for_each() expects callback to return a value

to be stored in the target range– variant: has two source ranges, callback accepts two parameters – one

for each source range and stores value for the target range. Can be used to process two containers

• copy() – copies source to target range• copy_if() – copies if callback returns true C++11

– returns iterator past the last element copied – can be used to trim unused after copy elements

• replace() – replaces elements with particular value with a different one• replace_if() – replaces by new value if callback returns true• reverse() – reverses elements in container• move() – moving elements with C++11 move semantics, leaves source

elements in unspecified by valid state• unique() – eliminates consequent duplicates – useful with sorted

containers

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Remove (and Erase)• remove() – removes elements with specific value• remove_if() – removes if callback returns true• both modifying algorithms• do not erase elements from containers (do not know if whole or full range)

– instead move remaining elements forward– return iterator past last remaining elements

• remove-erase-idiom – get the returned iterator and then use the container’s erase() function to eliminate removed elements

– can be done in single line• removes are linear

– preferred to iterative erase() invocation – for random access containers memory reorganization to keep continuous, results in quadratic complexity

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Sorting• sort() – n log(n) sort of the range• merge() – linear merge of source ranges

– target range has to be large enough– does not return iterator; no elements are removed – number of elements

in target container is sum of source sizes: use resize() or erase() to trim target

• unique() – eliminates duplicates, returns iterator past the last element• binary_search() – log(n) search in sorted container for a value, returns

true if found– lower_bound() is same complexity but more useful

• random_shuffle() – reshuffles range in linear time, internally uses rand()

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Setoperate on sorted containers with unique elements, not necessarily sets; in fact,

sequential containers are recommended• includes() – returns true if first range includes second• set_union() – computes union (duplicates eliminated) from two source

ranges, puts it in destination range, returns pointer past last element• set_intersection() – computes intersection of two source ranges• set_difference() – difference (complement) of first range with second –

elements of first range that are not present in the second• set_symmetric_difference() – elements of first range that are not

present in second and v.v.

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