Re: any suggestion to improve the following code?

"Daniel T." <>
Thu, 03 Jan 2008 00:04:39 -0500
Fei Liu <> wrote:

Hello, the following code is an implementation computing the 3n+1
problem, specifically maximum number of steps to complete for any
integer range. The algorithm aims to be as efficient/fast as possible.

It would probably be faster if it wasn't recursive.

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.


#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)
// 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]
        return steps[n];

The above forces an insert even if the value doesn't currently exist in
the map. Instead use find.

    if(n == 1) return 1;
        n = 3*n+1;
        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];
        ifstream inf("record_h.txt", ios::binary);
        while( *)two, 2*sizeof(unsigned int)))
            steps[two[0]] = two[1];

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

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