Re: Puzzle: Add One Without Using + OR -

From: Alf P. Steinbach (alfps_at_start.no)
Date: 09/20/04 

Date: Mon, 20 Sep 2004 21:47:54 GMT

* Method Man:
> Q:
> Write a C/C++ program that takes an unsigned integer and adds 1 to it
> without using the plus or minus signs. Don't worry about overflow issues. It
> is cheating to use assembly commands, or to write a preprocessor that uses
> ASCII codes to insert the plus or minus sign. There are many solutions, some
> more elegant than others.
>
> Here's the solution I came up with. I'm wondering if there's a solution that
> is more elegant or efficient.
>
>
> void addOne(unsigned int &i) {
> int j = 1;
>
> while (i & 1) {
> i >>= 1;
> j <<= 1;
> }
> i |= 1;
>
> while !(j & 1) {
> i <<= 1;
> j >>= 1;
> }
> }

Good attempt, but regarding style, preferentially use a function rather than
an in/out argument, and regarding C++, the above shouldn't compile: always
post code that compiles (except when the point is that it doesn't compile).

At the risk of doing your HOMEWORK for you:

    unsigned next( unsigned x )
    {
        if( x == ~0 ) { return 0; }
        unsigned flipmask = ~0;
        while( (x | flipmask) == ~0 )
        {
            flipmask <<= 1;
        }
        return x ^ ~flipmask;
    }

State the assumptions that this solution relies on. 

-- 
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

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