Troubleshooting an 0x80 Mask for Char Bit Display

[Hypercase]

mask =  0x80;
for(i = 0; i < 8; i++)
{
	if((testvalue & mask)!= 0)
            {			
                cout<<"1";
            }
	else
            {
	cout<<"0";
            }
	mask = mask >> 1;

}

The code displayed was for displaying the bits in a char,however it doesn't work. Please help me with the code.
Thanx.

 

[chroot]

Is this homework?

This code appears by inspection to be correct. Is mask the same data type as testvalue?

- Warren

 

[Hypercase]

char testvalue,mask ;

No it isn't HW.

 

[Hypercase]

for(i = 0; i < 8; i++)
{
      if((test & 0x80)!= 0)
      {
       cout<<"1";
      }
      else
      {
       cout<<"0";
      }
    test = test << 1;

 }

This code works well however the testvalue is altered. I didnt want that to happen ,and I therefore wrote the code displayed in post1, where the mask is shifted.

 

[rgrig]

it doesn't work.

Please be more specific: what exactly doesn't work? As chroot said, the code looks OK.

 

[rgrig]

You might find this code interesting (this is how I'd probably write it, I'd just add some more checks in real code).

#include <iostream>
#include <cassert>

using namespace std;

template <typename T>
void print_bits(const T& value, int to_go = sizeof(T) * 8)
{
        if (to_go == 0) return;
        assert (to_go > 0);
        bool last = value & T(1);
        print_bits(value >> 1, to_go - 1);
        cout << last;
}

int main()
{
        print_bits('a'); cout << endl;
        print_bits(-17); cout << endl;
        print_bits(unsigned(-17)); cout << endl;
        print_bits(short(-17)); cout << endl;
}

Run it and see what it prints. BTW, I think the "*8" part is not guaranteed by the standard to be portable but I don't know any platform where this would be a problem.

 

[rgrig]

Ah, I know why it doesn't work! if you declared "mask" as "char" then 0x80 is actually a negative value and the sign bit is preserved. you should have written those declarations in the first piece of code :) I've barely nticed you mantioned it latter.