c++ bitwise AND, expressing number literal

**rodrigorules** · 05-10-2010

lets say I have a string of characters, (I am sure they are all letters)

I want to make an is_uppercase function, thus my goal in this exercise is to check if the 5 bit is a 0 or not... (adding 32 to any uppercase gives me a lowercase)

Code:

bool is_uppercase(std::string word)
{
	BOOST_FOREACH(char c, word)
	{
		if(c & 32)
		{
			return false;
		}
	}
	return true;
}

The above code seems to work, though in this case I do not have to do any operations on the number.

but If i needed to use the ~ operator on the number 32 it would have messed me up by making the number negative...

Code:

 
if ( ~c | ~32)
// yields incorrect result

ive tried

Code:

 ~(32u)

but it also leads me to -33 .. (which is wrong)

how would I express a number as 'unsigned' ?

**tabstop** · 05-10-2010

I'm not sure what your question is, exactly. ~32 is bitwise flip, so instead of
00010000
you have
11101111. Or'ing that with ~c yields a number with 1's where either c or 32 had a zero, or to put it another way it gives ~(c & 32).

**iMalc** · 05-11-2010

Originally Posted by rodrigorules

but If i needed to use the ~ operator on the number 32 it would have messed me up by making the number negative...

Code:

if ( ~c | ~32)
// yields incorrect result

ive tried

Code:

 ~(32u)

but it also leads me to -33 .. (which is wrong)

No it doesn't. ~32u is the correct way to represent an unsigned number with the desired bits set.

if ( ~c | ~32) is the same as writing if (true) because or-ing a number with another number can only set additional bits, so you're guaranteed to get a non-zero result from the or-expression and hence a true condition overall.
What were you actually wanting that to do?

**prog-bman** · 05-11-2010

Is the goal to learn how to check if a character is upper case or just to write a function to see if a string is all upper case.

If its the latter you can just use this:
isupper - C++ Reference

**iMalc** · 05-11-2010

If you were trying to check for the other case then all you want to do is to negate the expression you had. Thus you could do:

Code:

if (!(c & 32))

Bitwise negating c would also work:

Code:

if (~c & 32)

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c++ bitwise AND, expressing number literal