Lecture 21 --- Hash Tables, part II

Today’s Lecture

Function Objects
Continuing with Hash Tables
STL Queue and STL Stack

21.1 Function Objects, a.k.a. Functors

In addition to the basic mathematical operators + - * / < > , another operator we can overload for our C++ classes is the function call operator. Why do we want to do this? This allows instances or objects of our class, to be used like functions. It’s weird but powerful.
Here’s the basic syntax. Any specific number of arguments can be used.

class my_class_name {
public:
	// ... normal class stuff ...
	my_return_type operator() ( /* my list of args */ );
};

See and run this simple functor example.

21.2 Why are Functors Useful?

One example is the default 3rd argument for std::sort. We know that by default STL’s sort routines will use the less than comparison function for the type stored inside the container. How exactly do they do that?
First let’s define another tiny helper function:

bool float_less(float x, float y) {
return x < y;
}

Remember how we can sort the my_data vector defined above using our own homemade comparison function for sorting:

std::sort(my_data.begin(),my_data.end(),float_less);

If we don’t specify a 3rd argument:

std::sort(my_data.begin(),my_data.end());

This is what STL does by default:

std::sort(my_data.begin(),my_data.end(),std::less<float>());

What is std::less? It’s a templated class. Above we have called the default constructor to make an instance of that class. Then, that instance/object can be used like it’s a function. Weird!
How does it do that? std::less is a teeny tiny class that just contains the overloaded function call operator.

template <class T>
class less {
public:
bool operator() (const T& x, const T& y) const { return x < y; }
};

You can use this instance/object/functor as a function that expects exactly two arguments of type T (in this example float) that returns a bool. That’s exactly what we need for std::sort! This ultimately does the same thing as our tiny helper homemade compare function!

21.3 Another more Complicated Functor Example

Constructors of function objects can be used to specify internal data for the functor that can then be used during computation of the function call operator! For example:

class between_values {
private:
float low, high;
public:
between_values(float l, float h) : low(l), high(h) {}
bool operator() (float val) { return low <= val && val <= high; }
};

The range between low & high is specified when a functor/an instance of this class is created. We might have multiple different instances of the between_values functor, each with their own range. Later, when the functor is used, the query value will be passed in as an argument. The function call operator accepts that single argument val and compares against the internal data low & high.
This can be used in combination with STL’s find_if construct. For example:

between_values two_and_four(2,4);
if (std::find_if(my_data.begin(), my_data.end(), two_and_four) != my_data.end()) {
	std::cout << "Found a value greater than 2 & less than 4!" << std::endl;
}

Alternatively, we could create the functor without giving it a variable name. And in the use below we also capture the return value to print out the first item in the vector inside this range. Note that it does not print all values in the range.

std::vector<float>::iterator itr;
itr = std::find_if(my_data.begin(), my_data.end(), between_values(2,4));
if (itr != my_data.end()) {
	std::cout << "my_data contains " << *itr
	<< ", a value greater than 2 & less than 4!" << std::endl;
}

21.5 Using STL’s Associative Hash Table (Map)

Using the default std::string hash function.
- With no specified initial table size. (map a std::string type key to a class Foo type value)
```
std::unordered_map<std::string,Foo> m;
```
- Optionally specifying initial (minimum) table size.
```
std::unordered_map<std::string,Foo> m(1000);
```
Using a home-made std::string hash function. Note: We are required to specify the initial table size.
- Manually specifying the hash function type.
```
std::unordered_map<std::string,Foo,std::function<unsigned int(std::string)> > m(1000, MyHashFunction);
```
- Using the decltype specifier to get the “declared type of an entity”.
```
std::unordered_map<std::string,Foo,decltype(&MyHashFunction)> m(1000, MyHashFunction);
```
Using a home-made std::string hash functor or function object.
- With no specified initial table size.
```
std::unordered_map<std::string,Foo,MyHashFunctor> m;
```
- Optionally specifying initial (minimum) table size.
```
std::unordered_map<std::string,Foo,MyHashFunctor> m(1000);
```

21.6 Additional STL Container Classes: Stacks

Stacks allow access, insertion and deletion from only one end called the top.
- There is no access to values in the middle of a stack.
- Stacks may be implemented efficiently in terms of vectors and lists, although vectors are preferable.
- All stack operations are O(1).

21.7 Additional STL Container Classes: Queues

Queues allow insertion at one end, called the back and removal from the other end, called the front.
- There is no access to values in the middle of a queue.
- Queues may be implemented efficiently in terms of a list. Using vectors for queues is also possible, but requires more work to get right.
- All queue operations are O(1).

21.8 Leetcode Exercises

Leetcode problem 1451: Rearrange Words in a Sentence. Solution: p1451_rearrange_words_in_a_sentence.cpp.
Leetcode problem 508: Most Frequent Subtree Sum. Solution: p508_most_frequent_subtree_sum.cpp.
Leetcode problem 225: Implement Stack using Queues. Solution: p225_stack_using_queues.cpp.
Leetcode problem 232: Implement Queue using Stacks. Solution: p232_queue_using_stacks.cpp.
Leetcode problem 102: Binary Tree Level Order Traversal. Solution: p102_level_order_traversal.cpp.
- Play this animation to see how the level order traversal works.
Leetcode problem 20: Valid Parentheses. Solution: p20_valid_parentheses.cpp

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