C: Access individual bits in uint16_t number?

Question

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C printing bits

Lets say I have a uint16_t number something like:

uint16_t myVar = 0x3A44;

and I want to print the binary value if 0x3A44 from the myVar, how could I iteratively access and print each bit?

Treat it as a bit array?

score 6 · Accepted Answer · edited Mar 31 '12 at 02:45

6

Integers effectively are bit arrays. You can just loop through the value of the number, shifting the value to the right 16 times.

#include <stdio.h>
#include <stdint.h>

int main() {
    uint16_t x = 0xFF00;
    for (int i = 0; i < 16; i++) {
        printf("%d", (x & 0x8000) >> 15);
        x <<= 1;
    }
    printf("\n");
}

Results in:

1111111100000000

edited Mar 31 '12 at 02:45

R.. GitHub STOP HELPING ICE

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answered Mar 30 '12 at 20:35

Dave

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3

That works but it prints back to front – Paul R Mar 30 '12 at 20:36
`#include ` is C++, not C. – Kaz Mar 30 '12 at 21:21
2

@Kaz: Normally I don't edit people's answers, but this seems like a really clear-cut case of wrong-language that's easily fixed, so I fixed it. – R.. GitHub STOP HELPING ICE Mar 31 '12 at 02:45

Adam Liss · Answer 2 · 2012-03-31T02:48:42.390

2

while (myVar != 0) {
  printf("%c", (myVar & 0x8000) == 0 ? '0' : '1');
  myVar <<= 1;
}
printf("\n");

edited Mar 31 '12 at 02:48

answered Mar 30 '12 at 20:36

Adam Liss

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Liked your answer better than mine! – David Pointer Mar 30 '12 at 20:37
`(myVar & 0x8000)` can never be equal to 1... – R.. GitHub STOP HELPING ICE Mar 31 '12 at 02:47
Wow, how embarrassing! Fixed; thanks for the catch. – Adam Liss Mar 31 '12 at 02:49

score 2 · Answer 3 · edited Mar 31 '12 at 12:20

2

Use something like the following:

for (i = 15; i >= 0; --i) {
    printf("%d", (bool) (myVar & (1U << i)));
}

edited Mar 31 '12 at 12:20

Abel

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answered Mar 30 '12 at 20:49

ouah

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+1; this is one of the cleanest and most correct answers so far. Happy to compensate for the downvote... Perhaps `!!` would be clearer (and more widely supported) than `(bool)`. – R.. GitHub STOP HELPING ICE Mar 31 '12 at 02:48

score 1 · Answer 4 · answered Mar 30 '12 at 20:44

1

Well, recursion for a change! (a lazy way to print bits in order)

print(int x) {
if(!x) return;
print(x>>1);
printf("%d", x & 1);
}

answered Mar 30 '12 at 20:44

P.P

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If `x` is signed and negative, shifting right is undefined. Where is the sign bit, and what happens when you shift it? – Bo Persson Mar 31 '12 at 08:49

score 1 · Answer 5 · answered Mar 31 '12 at 02:51

1

Simplest I can think of:

for (int i=15; i>=0; i--) putchar('0'+((x>>i)&1);

answered Mar 31 '12 at 02:51

R.. GitHub STOP HELPING ICE

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score 0 · Answer 6 · answered Mar 30 '12 at 20:36

To get the lowest-order bit one would write:

int is_set = (myVar >> 0) & 1;

The first part of the expression shifts the integer's bits towards the lower bits. A shift of zero does nothing, but it is easy to see how nonzero values would be used there.

The second part of the expression masks out all the bits save for the lowest order bit.

When these two are combined, you are able to detect whether or not a single bit is set or not.

cHao · Answer 7 · 2012-03-30T21:04:20.757

void print_binary(uint16_t n) {
    char buffer[17];                  /* 16 bits, plus room for a \0 */
    buffer[16] = '\0';                
    for (int i = 15; i >= 0; --i) {   /* convert bits from the end */
        buffer[i] = '0' + (n & 1);    /* '0' + bit => '0' or '1' */
        n >>= 1;                      /* make the next bit the 'low bit' */
    }

    /* At this point, `buffer` is `n` turned to a string of binary digits. */

    printf("%s\n", buffer);
}

score 0 · Answer 8 · answered Mar 30 '12 at 21:33

const char *nyb[5] = {
  "0000", "0001", "0010", "0011",
  "0100", "0101", "0110", "0111",
  "1000", "1001", "1010", "1011",
  "1100", "1101", "1110", "1111",
};

#define NYB(NUM, NY) (nyb[((NUM) >> (4*(NY))) & 0xf])

/* ... */

void print_u32(unsigned long u)
{
  printf("%s%s%s%s%s%s%s%s",
         NYB(u, 7), NYB(u, 6), NYB(u, 5), NYB(u, 4),
         NYB(u, 3), NYB(u, 2), NUB(u, 1), NYB(u, 0));
}

:) :) :)

C: Access individual bits in uint16_t number?

8 Answers8