Binary to unsigned int using bitwise operations and pointer arithmetic in C

Question

I can only use bitwise operations and pointer arithmetic to solve this problem. I am converting from binary to unsigned int.

The function I am writing is:

unsigned int atob(const char* nptr);

atob("101") should return 5, atob("11000") should return 24, atob("11$") should return 3, and atop("") should return 0.

I'm pretty new to bitwise operations, so I really need some help specifically in that area.

edit:

nptr can only be incremented, not other inc/dec's are allowed.

What have you tried? And is this homework? Are you not allowed to use comparisons? — unwind
– unwind, Commented Jul 3, 2012 at 12:21
How would you solve it if you were allowed to use * and +? (Hint: *2 === <<1, + can here be done with |.) — Daniel Fischer
– Daniel Fischer, Commented Jul 3, 2012 at 12:25
This is not homework. Just studying material. I have no problem at all converting from binary to decimal, but I am not coherent on how to do it with bitwise operations because I rarely use them for anything. — Jordan
– Jordan, Commented Jul 3, 2012 at 12:27

Richard J. Ross III · Accepted Answer · 2012-07-03 15:37:11Z

2

unsigned bits2val(char *bits)
{
    unsigned val;

    for (val = 0; *bits; bits++) {
        if (*bits == '1') 
            val = (val << 1) | 1;
        else if (*bits == '0' ) 
            val <<= 1;
        else 
            break;
    }

    return val;
}

edited Jul 3, 2012 at 15:37

Richard J. Ross III

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answered Jul 3, 2012 at 15:27

wildplasser

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7 Comments

Richard J. Ross III Over a year ago

Very difficult to read, do you mind cleaning up the post a bit?

Richard J. Ross III Over a year ago

Your code formatting is a bit off of the norm, I took the liberty of normalizing it.

wildplasser Over a year ago

Ah, you are discussing whitespace. I never use extra lines for single statements. (I still find mine easier to read. The extra lines and braces only distract, IMHO) BTW: by correcting my style you are takng the chance from people to be confronted with different styles.

Richard J. Ross III Over a year ago

Wildplasser, that is true, but here on SO, the style that I have there is the most widely used one. Feel free to use whichever you want, just make sure that your indentation is proper. (Also, I did give you a +1).

wildplasser Over a year ago

I hate corporate styles. Even more in non-corporate environments. Please learn how to cope with diversity, instead of imposing your views on others. Thank you.

|

Richard J. Ross III · Accepted Answer · 2012-07-03 12:42:27Z

1

Here's my example implementation, using just shifts and ors (assuming you can use ++ for string-manipulation):

unsigned atob(const char *input)
{
    unsigned result = 0;
    unsigned currentBit = 0;

    // we need to go right to left;
    const char *end = input;
    // make sure we only read '0's and '1's
    while ((*end == '0') || (*end == '1'))
    {
        end++;
    }

    while (--end >= input) {
        // check for overflow
        if ((currentBit >> 3) > sizeof(result))
            break;

        char isCurrentBitSet = *end == '1';
        unsigned setValue = (isCurrentBitSet << currentBit);
        result |= setValue;

        currentBit++;
    }

    return result;
}

edited Jul 3, 2012 at 12:42

answered Jul 3, 2012 at 12:36

Richard J. Ross III

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4 Comments

Jordan Over a year ago

Looks good except I cannot use division! That is why I am struggling.

Richard J. Ross III Over a year ago

@JordanCarney that's only to check for overflow. If you know the size of unsigned at compile time, it's a non-issue.

Jordan Over a year ago

OK, I understand. I actually did not need to check for overflow in this exercise, so that is okay then!

Jordan Over a year ago

One more question: Is it possible to do this by only incrementing const char* input and no other inc/dec?

Marco Manelli · Accepted Answer · 2012-07-03 12:23:07Z

0

Start with the basics http://www.wikihow.com/Convert-from-Decimal-to-Binary

answered Jul 3, 2012 at 12:23

Marco Manelli

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