Algorithms: I
- 1. Level up your coding skills with the C++ Standard Template Library (STL): Algorithms: I BY JOYJIT CHOUDHURY
- 2. Algorithms ï” In the C++ Standard Library, algorithms are components that perform algorithmic operations on containers and other sequences ï” The C++ standard provides some standard algorithms collected in the <algorithm> standard header. A handful of algorithms are also in the <numeric> header ï” All algorithms are in the std namespace
- 3. Sort ï” sort(), sorts the elements in the range [first, last) ï” The sort is performed by, either using the < operator (resulting in an ascending order), or a Boolean âcompare functionâ (aka. Binary Predicate) passed as an argument to the sort function. ï” Time Complexity : O(N*logN) ï” Works on containers which provide random access iterators: vector, deque, array ï” Defined in the header <algorithm> ï” Belongs to the namespace std
- 5. So, what is this âBinary Predicateâ? ï” Sorting is a result of comparisons. This âcompare - binary predicateâ decides where the elements are put after each comparison ï” Think of it as a function, which is passed any two elements from the sequence and this function returns a true or false, depending on which the relative ordering (which element in the pair comes first, and which comes second) of each of these pair of elements in the final sorted sequence, is defined ï” In binary predicate, binary stands for the fact that it accepts two elements as input, and predicate indicates that it produces a boolean output, i.e. true or false
- 6. ï” This binary predicate is passed two elements of the type contained in sequence. It returns true if the first argument goes before the second argument in the ordering it defines, false otherwise. ï” The binary predicate in the form of a function is given here
- 7. ï” Letâs take for example: ï” Now we create a vector containing objects of stuff class and insert a few values : a = 23 b = 34 a = 43 b = 2 a = 23 b = 98 a = 1 b = 2 a = 45 b = 54myV
- 8. ï” Sort the elements of myV in the descending order of itâs data member int b ï” The global function compare : ï” After calling sort() and passing compare as the third argument : a = 23 b = 98 a = 45 b = 54 a = 23 b = 34 a = 43 b = 2 a = 1 b = 2 myV
- 9. struct as a binary predicate ï” A function isnât the only way to define a binary predicate ï” It can also be done by creating a struct Defining the structâs ()operator (which is done similar to a function definition) And then passing an object of this struct as the third parameter to sort()
- 10. ï” From the previous example, the binary predicate could also be defined as a struct
- 12. Using templates ï” Suppose you want to sort a vector in descending order, it might contain data of int, float or char or some other type ï” Do you write separate functions/structs for the different types ? ï” No. You can use templates to create a generic function/struct and use it accordingly
- 14. ï” Achieving a descending order of elements in a container can be made even easier by the use of std::greater ï” Itâs a binary function object class whose call returns whether the its first argument compares greater than the second (as returned by operator >) ï” Which is exactly what our compare function/struct did in the previous example
- 15. ï” Thatâs not all. There are many other functions and techniques that could come in handy. Read about them on cplusplus.com or cppreference.com or somewhere else. Just google it!