How do I print one bit?

Question

Please tell me how do I print a bit, like printf("%d",bit);.

This is a good question that needs rephrasing, but I don't have edit privileges yet.... — Jarred McCaffrey, Dec 18 '08 at 15:35

score 20 · Answer 1 · answered Dec 08 '08 at 15:55

20

If bit is just an int that contains the value you want in the least significant bit, then:

printf("%d", bit & 0x1);

should do it. The & is doing a binary-AND with a number with only the first significant bit set, so you're removing all the rest of the bits in the integer.

answered Dec 08 '08 at 15:55

Herms

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Keep in mind that extracting one bit from a multi-byte number makes you run into endianess issues. – gnud Dec 08 '08 at 15:57
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@gnud: That only applies when done against memory, using pointers. bit & 0x1 will always work, regardless of the way the value is stored in memory. – unwind Dec 08 '08 at 16:04

plinth · Answer 2 · 2008-12-18T15:31:55.517

10

If you need to generalize more than Herms, you could do this:

#define IsBitSet(val, bit) ((val) & (1 << (bit)))

/* ... your code ... */

printf ("%c", IsBitSet(bit, 0) ? '1' : '0');

The printf is equivalent to Herms answer as is.

If you're talking about bitfield in C, you can do this:

struct foo { int b:1; } myFoo;

printf("%c", myFoo.b ? '1' : '0');

edited Dec 18 '08 at 15:31

answered Dec 08 '08 at 16:01

plinth

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There is a missing closing parenthesis at the end of the macro definition, no? – bortzmeyer Dec 18 '08 at 10:35
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This seems a bit roundabout - why not `((val) >> (bit)) & 0x1` so you can print it as an integer instead of the ternary? – Aaron Dufour Oct 06 '12 at 04:33

score 6 · Answer 3 · edited May 23 '17 at 12:18

6

Related question: How do you set, clear, and toggle a single bit? is an extended discussion of single-bit access in c and c++.

edited May 23 '17 at 12:18

Community

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answered Dec 08 '08 at 16:17

dmckee --- ex-moderator kitten

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score 3 · Answer 4 · edited Mar 11 '17 at 16:51

3

To print the m-th bit (m from 1..16 or 32) of n:

void print_bit(n, m)
{
    printf("%d", n & (1 << (m - 1)));
}

Remove the - 1 bit if your bit counter starts at 0.

edited Mar 11 '17 at 16:51

gsamaras

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answered Dec 08 '08 at 16:00

Keltia

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That won't quite work. You're masking the bit you want, but you're leaving that bit in place, so you won't get 1 or 0. To always get 1 or 0 for that bit you'd need to shift the variable right, not shift the mask left. – Herms Dec 08 '08 at 20:07
1

What is bit in your function? And "n" is not used. Did you mean printf("%d", n & (1 << (m - 1)) ? – Koshmaar May 07 '16 at 03:58

score 1 · Answer 5 · answered Dec 09 '08 at 10:10

1

The C++ answer is easier than the C89 one, with the native bool type:

bool b = true;
std::cout << b;

C99 is quite similar:

_Bool b = 1;
printf("%d", b);

answered Dec 09 '08 at 10:10

MSalters

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Bools are generally not one bit in size. – Jasper Bekkers Dec 18 '08 at 16:34

score 1 · Answer 6 · edited Mar 02 '14 at 01:38

1

You can use "union":

union bitshow {
    unsigned bit1:1;
    int i;
};

int main() {
    union bitshow bit;
    cin >> bit.i;
    cout << bit.bit1;
    return 0;
}

edited Mar 02 '14 at 01:38

Qantas 94 Heavy

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answered Jul 11 '11 at 11:38

Sajjad Mohammadadeh

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Would that print the most significant bit, least significant bit, or something else? You can't know, because it isn't portable. – interjay Jul 11 '11 at 12:06

How do I print one bit?

6 Answers6

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