C++ Tutorial - Hash Tables

RE: C++ Tutorial - Hash Tables

pjcar2000 — Thu, 10 May 2018 15:03:50 GMT

Very helpful article. FYI - the last example with the MakeHash class is not a complete program on its own. Here is the code that I put together using portions of the earlier code samples to complete it:

#include <iostream>

#include <string.h>

#include <unordered_map>

using namespace std;

class Make

{

public:

string Name;

Make(string name)

{

Name = name;

}

bool operator==(const Make &make) const

{

return Name == make.Name;

}

};

class Model

{

public:

string Name;

int Year;

Model(string name, int year)

{

Name = name;

Year = year;

}

bool operator==(const Model &model) const

{

return (Name == model.Name && Year == model.Year);

}

};

class MakeHash

{

public:

size_t operator()(const Make &make) const

{

return hash<string>()(make.Name);

}

};

class ModelHash

{

public:

size_t operator()(const Model &model) const

{

return hash<string>()(model.Name) ^ hash<int>()(model.Year);

}

};

int main()

{

unordered_map<Make, vector<Model>, MakeHash> make2model;

Model camry2005("Camry", 2005);

Model tercel1993("Tercel", 1993);

Make toyota("Toyota");

vector<Model> toyotaModels;

toyotaModels.push_back(camry2005);

make2model.emplace(toyota, toyotaModels);

make2model[toyota].push_back(tercel1993);

for (auto &make : make2model)

{

cout << make.first.Name << endl;

vector<Model> models = make.second;

for(auto &model : models)

{

cout << " " << model.Name << " " << model.Year << endl;

}

return 0;

}

RE: C++ Tutorial - Hash Tables

Former Member — Mon, 09 Nov 2015 11:47:25 GMT

thanks for solved this problem

RE: C++ Tutorial - Hash Tables

Former Member — Sat, 23 May 2015 00:45:05 GMT

Thanks for that. I found this blog on the topic of books for learning C++: http://stackoverflow.com/questions/388242/

Also, I'm discovering the difference between bringing Armadillo into Raspbian (Raspberry Pi) vs. into VS 2013 on Windows, 10 minutes (which was mostly just install time) vs. several hours and counting.

Any recommendations for learning that sort of stuff? Eg. how to correctly set up dependencies, libraries, .dlls, that sort of thing? Or is that wishful thinking? I encountered the same nightmare working Java in Netbeans or Eclipse a few years back. In other words, it's one thing to learn the language, it's another to learn how to put everyone else's parts together so you don't have to build everything yourself and refine it forever until you optimize your run time. In other words, I ain't recreating Armadillo. Any points on how to learn that stuff? Or is that just the way it goes? I know a chief problem there is that unless you know just how to search, the answers you find apply only partly to your problem--and frequently partial solutions don't work--you need the whole solution, which is almost always unique for everyone's' uniquely composited problem.

That will be my last question here, I promise. It is just that it can be discouraging to learn to program only to find out you don't get far with it in the real world of releasing software or building something outside of the confines of prototyping environments. I can do a lot in some nice environment like Octave, but if I have to compile and build anything using the wonderful work of others' libraries, flip a coin as to whether I am going to get anywhere in a given environment--at least not without extensive delays. If I have a method to attack that sort of problem, however, I should think it would be well worth the effort. Again, thanks.

RE: C++ Tutorial - Hash Tables

Former Member — Wed, 13 May 2015 05:25:10 GMT

Also, any chance you could explain a few things here:

1) Why the == after the return variable declaration and before the () parameter, and why the second mention of const after the () parameter in the operator functions, eg.

bool operator==(const Make &make) const

2) and in this one why the initial set of () before the parameter in the parentheses?

size_t operator()(const Model &model) const

Thanks for the tutorial. Edit: here's a link for more info about point 1 (above), and of course, about C++ in general:

http://en.cppreference.com/w/cpp/language/operator_comparison

RE: C++ Tutorial - Hash Tables

Former Member — Wed, 13 May 2015 04:02:52 GMT

A few errors:

include <string> //needed to get code to work

Make toyota("Toyota", "Japan");//doesn't compile. Two arguments given, only one accepted in constructor. Need to change to:

Make toyota("Toyota");//