Re: reverse a string with 0 terminator in one pass

James Kanze <>
Wed, 4 Mar 2009 05:22:22 -0800 (PST)
On Mar 4, 12:40 am, Juha Nieminen <nos...@thanks.invalid> wrote:

dbtouch wrote:

I have been thinking solution for this problem: to reverse a string
with 0 terminator in one pass without using auxillary memory. My
initial thought is to use recursion, but I have not succeeded finding
an answer.

I don't even think the problem is solvable strictly in one
single pass even if you are allowed to use additional memory

Sure it is:

    reverse( char* source )
        std::size_t cap = 32 ;
        char* result
            = static_cast< char* >( std::malloc( cap + 1 ) ) ;
        assert( result != NULL ) ;
        *result = 0 ;
        for ( std::size_t i = 0 ; source[ i ] != '\0' ; ++ i ) {
            if ( cap <= i ) {
                cap += cap / 2 ;
                result = static_cast< char* >(
                                realloc( result, cap + 1 ) ) ;
                assert( result != NULL ) ;
            std::memmove( result + 1, result, i + 1 ) ;
            result[ 0 ] = source[ i ] ;
        std::strcpy( result, source ) ;
        std::free( result ) ;

Obviously, it's not the most efficient solution in the world.
But artificial constraints (only one pass) can artificially
increase runtime. (Of course, you really have two passes
anyway. The first, generating the reversed string, and the
second, copying the results back into the original.)

Of course, if all that counts is what's in your own code:

    reverse( char* source )
        std::string tmp( source ) ;
        std::reverse( tmp.begin(), tmp.end() ) ;
        std::strcpy( source, tmp.c_str() ) ;

Which is probably more efficient than my code, above, but I'm
willing to bet that there's a call to strlen hidden somewhere in
the constructor of std::string that I use.

The whole question is, of course, silly. If you're doing any
string manipulation, you won't loose the size to begin with.
And if you know the size, you don't need strlen to recalculate
it, and std::reverse can be used immediately.

James Kanze (GABI Software)
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