Re: any suggestion to improve the following code?

James Kanze <>
Thu, 3 Jan 2008 02:56:24 -0800 (PST)
On Jan 2, 9:03 pm, 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.

One thing I am unsatisfied of is the use of hash_map (deprecated but
supported by most C++ vendors).

Not deprecated; there was never a "standard" hash_map. And
while most C++ compilers do support some sort of hash table,
there are subtle differences between them.

The next version of the standard will contain an
std::unsorded_map, but until then, you're on your own, at least
if you want to be portable.

Is there a portable hash map implementation?

It's not hard to write. (There's one at my site, for example.)

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

unordered_map is presumably an implementation of the draft
standard, and definitly to be preferred if it is available.

I didn't use std::map because std::map lookup/insertion is
O(lgN) while hash map is O(1). What's your opinion?

How many elements? I've found that for up to a thousand or so,
it really doesn't matter.

Note too that the complexity of a hash map depends on the
quality of the hashing function. Use a bad hashing function,
and it rapidly becomes O(n).

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;

Personally, I don't like the "using namespace". I'd generally
write std:: wherever needed (and alias __gnu_cxx to something
like gcc). And I definitly wouldn't use a "using namespace"
until after all of the include files have been processed.

#include <boost/filesystem/convenience.hpp>

I'm not familiar with the algorithm, so I'll skip that part.

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];

The above seems a bit heavy. What's wrong with just:

    std::ifstream inf( "record_h.txt" ) ;
    if ( inf ) {
        // read file...

Note that the way you read the file in your loop is a recepe for
problems---recompile the code with different options or a
different version of the compiler, and you may read different
values than you wrote. The simplest solution would be to define
a text format for the data, and read and write it as text. This
will also simplify debugging enormously. If you do want to use
a raw memory image (which is probably acceptable since you
haven't lost anything critical if you can't read it), then
prefix it with some sort of signature, which is regenerated
(differently) every time you relink your code.

And of course, you really want this part in a separate function.
Or even a separate class, which encapsulates all of your file
cache management. (If you read and write in the same class,
then it's easy to modify the file format.)

    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 safel=


    while(!(cin >> i >> j)) {
        cout << i << ' ' << j << endl;
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
    assert(i != 0 && j >= i);

I'm not sure what your trying to do here. If you only need two
integers, and you always need two integers, wouldn't it be more
appropriate to pick them up from the command line. Something

    asInt( char const* arg )
        int result ;
        std::istringstream tmp( arg ) ;
        tmp >> result >> std::ws ;
        return tmp && tmp.get() == EOF
            ? result
            : 0 ;

    main( int argc, char** argv )
        if ( argc != 3 ) {
            std::cerr << argv[ 0 ] << ": syntax is: "
                      << argv[ 0 ] << " i j" << std::endl ;
            return EXIT_FAILURE ;
        int i = asInt( argv[ 1 ] ) ;
        int j = asInt( argv[ 2 ] ) ;
        if ( i == 0 || j < i ) {
            std::cerr << argv[ 0 ] << ": illegal values" <<
std::endl ;
            return EXIT_FAILURE ;
        // ...

Note that it's probably best to do this before trying to read
the file with the cached values. No point in reading the file
if all you're going to do is terminate with an error.

Also, of course, if you're doing any amount of programming,
you'll quickly write some generic code to handle the command
line arguments, and use it here.

If you really have to read the two values from std::cin, then
I'd define a somewhat stricter format, say both in one line,
use std::getline() to read the data, and std::istringstream to
parse it. This avoids all problems with handling an error state
in the input or resynchronizing. Something like:

    int i = 0 ;
    int j = 0 ;
    while ( i == 0 ) {
        std::string s ;
        std::getline( std::cin, s ) ;
        if ( ! std::cin ) {
            std::cerr << argv[ 0 ]
                      << ": fatal error on standard in"
                      << std::endl ;
            return EXIT_FAILURE ;
        std::istringstream tmp( s ) ;
        tmp >> i >> j >> std::ws ;
        if ( ! tmp || tmp.get() != EOF || i == 0 || j < i ) {
            std::cerr << argv[ 0 ]
                      << ": illegal input, try again"
                      << std::endl ;
            i = j = 0 ;


    ofstream ofs("record_h.txt", ios::binary);
    hash_map<unsigned int, unsigned int, hash<unsigned int>>::const_iterat=

or it = steps.begin();

Are you sure you didn't want to use a typedef for the type of
the map?

    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));

Again, this more or less guarantees that you won't be able to
reliably read the data later, if you recompile the program for
any reasons.


    if ( ! ofs ) {
        std::cerr << argv[ 0 ]
                  << ": fatal error writing cache file"
                  << std::endl ;
        remove( "record_h.txt" ) ;
        return EXIT_FAILURE ;

I'm not sure about the return here---since it's only a cache
file, you might want to just note the error, without aborting
because of it.

The remove, on the other hand, is important; we've only written
part of the file, so it may be corrupt.

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

More of a style issue, but it's a lot cleaner if you add the
explicit return here:

    return EXIT_SUCCESS ;


You still have way too much in the function main. I'd probably
end up with something like:

    main( int argc, char** argv )
        int i = 0 ;
        int j = 0 ;
        DataMap map ;
        if ( parseArgs( argc, argv, i, j )
                || inputArgs( std::cin, i, j ) ) {
            CacheFileManager cache( filename, map ) ;
            process( dataStructure, i, j ) ;
            outputResults( i, j ) ;
        return exitStatus ;

Note the use of the destructor of CacheFileManager to output the

James Kanze (GABI Software)
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