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Const-Correctness .... with example.

Some of my friend have trouble with the terrible "const" keyword this morning

They have a class 'test', an instance of that class and they want to pass it through a function as 'const test*' parameter.

The question was: what 'const classname*' means ? and by the way what is the difference with 'const classname* const' ?

To answer, let create a 'test' class defined with 'const' and not const function:


class test
{
public:
test(){}
~test(){}

int DoConstOp() const { return 0;}
int DoNotConstOp() { return 0;}

};


And now we create different version of a method call 'process_blahblah' in which we will call the 2 test's function. Those method will take a object of type 'test' but with 4 different signature:

• const test&
• const test*
• const test* const
• test* const

And call the 4 method from a main:

process_as_const_ref(t);
process_as_const_ptr(&t);
process_as_const_ptr_const(&t);
process_as_ptr_const(&t);


And now have a look at their implementation. I put under comment the lines that doesn't compile (under VS'12 at least... but result looks correct regarding the C++ standard !) + error message.



void process_as_const_ref(const test& t)
{
t.DoConstOp();
//t.DoNotConstOp();//Cannot build: cannot convert 'this' pointer from 'const test' to 'test &'
}

void process_as_const_ptr(const test* t)
{
t->DoConstOp();
//t->DoNotConstOp(); //Cannot build: cannot convert 'this' pointer from 'const test' to 'test &'
(const_cast<test*>(t))->DoNotConstOp(); //trick
}

void process_as_const_ptr_const(const test* const t)
{
t->DoConstOp();
//t->DoNotConstOp();//Cannot build: cannot convert 'this' pointer from 'const test' to 'test &'
(const_cast<test* const>(t))->DoNotConstOp(); //trick remove the const on the class not on pointer
//t++;//but this you can't do : 't' : you cannot assign to a variable that is const
}

void process_as_ptr_const(test* const t)
{
t->DoConstOp();
t->DoNotConstOp();
//t++;//but this you can't do : 't' : you cannot assign to a variable that is const
}


So the answer is:

• const test& or const test* declare the the test instance you use as const, thats why you cannot non-const method.
• and with const test* const or test* const, the 2nd const keyword declare the pointer as const, it means that you cannot do any modification of the pointer value (ptr++ ptr = another ptr ...etc ...).

Note that using the "test* const" syntax may be confusing and useless as by default the pointer you get is a "copy" of the pointer you have in the caller !

HTML/CSS for fixed background image and a nice scrolling effect

As I was looking some news about the next XBox on http://www.wired.com/gadgetlab/2013/05/xbox-one/ I found the web page design and scrolling effect really nice. And I'm wondering if that kind of effect required JS or if it only use CSS. Note that I'm not a web developer, but just as usual curious of "how it works ?" ...

In fact after inspecting the page (thank you to the Firefox Inspector ! a nice functionality) I found that a small piece of CSS is enough and than the main trick is in the size of the image !!!!

Look at the following image they use http://www.wired.com/images_blogs/gadgetlab/2013/05/0521-tall-controller-v1.jpg. It's a 1050x2000 pixels image.

Below I tested the insertion on their image (Thank you Microsoft ...) to do the same in the middle of a fake text ....


#container div.fixedbackgroundimg {
position: relative;
overflow: visible;
}
.fixedbackgroundimg.example {
background: url("http://www.wired.com/images_blogs/gadgetlab/2013/05/0521-tall-controller-v1.jpg") repeat-y fixed 50% center transparent !important;
}

<div class="fixedbackgroundimg example" data-type="background" data-speed="5">
<h2 class=""> test </h2>
<img src="http://www.wired.com/images_blogs/gadgetlab/2013/05/0521-tall-controller-v1.jpg" style="visibility: hidden; height: 500px" data-lazy-loaded="true"></img>
</div>


The trick is to create a div containing an hidden image, or with a fixed width and height, and in the style of that div, you specify a background image bigger than the div and centered inside !

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Mastermind : build the k-combination of n colors(with repetitions)

In my 'Mastermind serie', I wrote a small C++ code that i would template later to create the list of the 4-combination of 6 peg (with repetitions) as we already know that for the classic mastermind game:

• we have 6 colors,
• the code is 4 peg long,
• so the total number of combination is 6^4 = 1296

To build that list, I apply the recursion/induction method and obtain finally a short and cool code :)
 Recall that our colors are describe by letters from 'A' to 'F'.


const color_code_map_t color_code = map_list_of (color_e::white, 'A')(color_e::blue, 'B')(color_e::green, 'C')(color_e::yellow, 'D')(color_e::orange, 'E')(color_e::red, 'F') ;


I build the list through the following code:

vector<char> elements;
std::for_each(color_code.begin(), color_code.end(), [&elements](const color_code_map_t::value_type& p) { elements.push_back(p.second); });
auto combi_found = combinations_with_repetitions(elements, TheHiddenCode.size());
std::cout << "total combination you can try = " << combi_found.size() << std::endl;



and in the 2 following function, I use the recursion/induction approach:


void combinations_with_repetitions_recursion(deque< vector<char> >& combi, const vector<char> &elems,
unsigned long req_len,
vector<unsigned long> &pos,
unsigned long depth)
{
// Have we selected the number of required elements?
if (depth >= req_len) {
vector<char> depth_r_combi;
for (unsigned long ii = 0; ii < pos.size(); ++ii){
depth_r_combi += elems[pos[ii]];
}
//copy(depth_r_combi.begin(), depth_r_combi.end(), ostream_iterator<char>(cout, " ")), cout << endl;
combi.push_back(depth_r_combi);
return;
}

// Try to select new elements to the right of the last selected one.
for (unsigned long ii = 0; ii < elems.size(); ++ii) {
pos[depth] = ii;
combinations_with_repetitions_recursion(combi, elems, req_len, pos, depth + 1);
}
return;
}

deque<vector<char>> combinations_with_repetitions(const vector<char> &elems, unsigned long r)
{
if(r > elems.size()){
throw std::logic_error("combination length (r) cannot be greater than the elements list (n)");
}

vector<unsigned long> positions(r, 0);
deque<vector<char>> combi;
combinations_with_repetitions_recursion(combi, elems, r, positions, 0);

return combi;
}


the 'depth' parameters is the key to stop the recursion and drill-up all the results. Based on those function we can easily create function to build permutations, combination(without repetitions), and ....

Mastermind, a small evolution to map with the true rules!

In my previous post about the Mastermind game, I didn't implement the real rules and my daughter said that I should use 'W'hite & 'B'lack peg as in the real game to say if a color is correctly placed, misplaced and empty if there is nothing to say !

So I added a short description of the rules, color code, number of allowed try, etc ....


cout << "Code of " << TheHiddenCode.size() << " peg" << endl;
cout << "Number of try to guess " << nb_line << endl;
cout << "Color are ", for_each(color_code.begin(), color_code.end(), [](const color_t& c){cout << c.second << " " ;}), cout << endl;


In the main loop, the 'answer' is now a vector of char to store our 'W' and 'B' peg!...


while( exact < TheHiddenCode.size() && nb_try < nb_line) {
cout << "\n\nguess--> ", cin >> guess;
if(guess.size() == 4) {
exact = color = 0;
transform( begin(TheHiddenCode), end(TheHiddenCode),
begin(guess), begin(answer),
Count( TheHiddenCode, color, exact ) );
cout << "Answer => " << "Incorrect place " << color << " Correct place " << exact << " : " ,
copy(answer.begin(), answer.end(), ostream_iterator<char>(cout, " ")),
cout << endl;
nb_try++;
} else {
cout << "incorrect number peg on the line" << endl;
}
}


And finally I updated my 'Count' class to check if a peg in the guess, if well or misplaced ....


struct Count {
Count( hidden_code_t code, int& incorrect_place, int& exact )
: incorrect_place_(incorrect_place), exact_(exact=0), code_(code) { }
char operator()( color_t c, char g ){
bool correct_place = (c.second == toupper(g));
if(correct_place) { //Argh, conditionnal are ugly !
exact_++;
return 'W';
} else {
//does that color be somewhere else in the code...
auto f = std::find_if(begin(code_), end(code_), [&g](const color_t& tobecheck_) { return (tobecheck_.second == toupper(g));} );
auto misplaced = (f != code_.end());
incorrect_place_ += (int) (misplaced);
if(misplaced)
return 'B';
else
return ' ';
}

}
~Count(){
}

int &incorrect_place_, &exact_;
hidden_code_t code_;
};


Based on that work, I will present later an API and Async Engine.

Mastermind: Search the code by yourself....

Hey good news after only 1hour more, i got a playable version with around 100 line of code....
I have more work to display the color code: A,B,C,D,E,F, to configure the program for a given number of try and to compute, but it's a correct improvement !


#include <string>
#include <map>
#include <algorithm>
#include <time.h>
#include <iostream>
#include <sstream>
using namespace std;

#include "boost/assign.hpp"
using namespace boost::assign;

enum color_e { white = 0, blue, green, yellow, orange, red };
typedef map<color_e, char> color_code_map_t;
typedef pair<color_e, char> color_t;
typedef vector<color_t> guess_code_t;
typedef vector<color_t> hidden_code_t;

const color_code_map_t color_code = map_list_of (color_e::white, 'A')(color_e::blue, 'B')(color_e::green, 'C')(color_e::yellow, 'D')(color_e::orange, 'E')(color_e::red, 'F') ;
const auto comb_length = 4;
const auto nb_line = 8;
hidden_code_t TheHiddenCode(comb_length);
vector<bool> answer(comb_length);
//how many of 4(k) peg can make from 6(N) ==> N^k
string guess;

struct acc_char {
void operator () (color_t c) {
acc_ += c.second;
}
string get() {return acc_;}
string acc_;
};

template<typename T> string colorCodeString(const T& m) {
bool is_vector_of_peg = std::is_same<T, guess_code_t>::value || std::is_same<T, hidden_code_t>::value ;
if(!is_vector_of_peg)
return ""; //or throw ?
acc_char acc;
return std::for_each(begin(m), end(m), acc).get();
}
color_t ChooseRandomPeg() { return *color_code.find((color_e)(rand() % color_code.size())); }

struct Count {
Count( int& incorrect_place, int& exact )
: incorrect_place_(incorrect_place), exact_(exact=0) { }
bool operator()( color_t c, char g ){
exact_ += c.second == toupper(g);
return (c.second == toupper(g));
}
~Count(){
}

int &incorrect_place_, &exact_;
};

class WinnerOrLoser {
public:
WinnerOrLoser(int exact, int code_length, int nb_try, int nb_line) : state_(false), msg_("not found") {
state_ = (exact == code_length);
stringstream builder;
if(state_) {
builder << "found in " << nb_try << "/" << nb_line;
msg_ = builder.str();
}
}
~WinnerOrLoser(){}
bool getState() {return state_;}
string getMsg() {return msg_;}
private:
bool state_;
string msg_;
};

int main() {

int color, exact = color = 0;

srand( time(0) ),
generate( TheHiddenCode.begin(), TheHiddenCode.end(), ChooseRandomPeg );

int nb_try = 0;
while( exact < TheHiddenCode.size() && nb_try < nb_line) {
cout << "\n\nguess--> ", cin >> guess;
if(guess.size() == 4) {
transform( begin(TheHiddenCode), end(TheHiddenCode),
begin(guess), begin(answer),
Count( color, exact ) );
cout << color << ' ' << exact << " : " ,
for_each(answer.begin(), answer.end(), [](bool pegExact){ cout << ((pegExact) ? "T" : "F") ;}),
cout << endl;
nb_try++;
} else {
cout << "incorrect number peg on the line" << endl;
}
}
cout << "Hidden code " << colorCodeString(TheHiddenCode) << " " << WinnerOrLoser(exact, TheHiddenCode.size(), nb_try, nb_line).getMsg() << "!\n";
}




Mastermind: a starting point...

As I start to build an algorithm to compete in an on-line Mastermind tournament, I build the following code to generate random hidden code of 4 peg from a list of 6 color (6^4 possibilities...).

The next step will be the interactive guess from a user (stdin), check to put the white and black mark, and finally the state win or lose.... (let see in future post)...


#include <string>
#include <map>
#include <algorithm>
#include <time.h>
#include <iostream>
using namespace std;

#include "boost/assign.hpp"
using namespace boost::assign;

enum color_e { white = 0, blue, green, yellow, orange, red };
typedef map<color_e, char> color_code_map_t;
typedef pair<color_e, char> color_t;
typedef vector<color_t> gess_code_t;
typedef vector<color_t> hidden_code_t;

const color_code_map_t color_code = map_list_of (color_e::white, 'A')(color_e::blue, 'B')(color_e::green, 'C')(color_e::yellow, 'D')(color_e::orange, 'E')(color_e::red, 'F') ;
const auto comb_length = 4;
const auto nb_line = 8;
hidden_code_t TheHiddenCode(comb_length);

struct acc_char {
void operator () (color_t c) {
acc_ += c.second;
}
string get() {return acc_;}
string acc_;
};

template<typename T> string colorCodeString(const T& m) {
bool is_vector_of_peg = std::is_same<T, gess_code_t>::value || std::is_same<T, hidden_code_t>::value ;
if(!is_vector_of_peg)
return ""; //or throw ?
acc_char acc;
return std::for_each(begin(m), end(m), acc).get();
}
color_t ChooseRandomPeg() { return *color_code.find((color_e)(rand() % color_code.size())); }



And a small main to check the random 'hidden code' generation:


int main() {
do
{
srand( time(0) ),
generate( TheHiddenCode.begin(), TheHiddenCode.end(), ChooseRandomPeg );
std::cout << colorCodeString(TheHiddenCode);
} while (getchar() != EOF);
return 0;
}