Let's say I have some int x. I only want to print the first 5 bits of x. How can I do this? I've heard that I can do something like
printf("%d", x & 0x05);
to print the first five bits of x but this just outputs 0 in my function.
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1To get the five lowest bits, write `x & ((1 << 5) - 1)`. For the five highest bit, also need to use the right-shift operator (`>>`). – NPE Sep 10 '17 at 19:31
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10x05 is just the first and third bits. The low 5 are 0x1f. – Kevin Sep 10 '17 at 19:32
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4The mask `0x05` will only isolate bits `0` and `2`. To isolate bits `0` thru `4` use the mask `0x1F` – Weather Vane Sep 10 '17 at 19:32
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1You need 1f if you do the maths – Ed Heal Sep 10 '17 at 19:32
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possible duplicate of https://stackoverflow.com/questions/349931/how-do-i-print-one-bit – MKR Sep 10 '17 at 19:33
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When using this for 32 bits it only outputs 28 bits – Leonardo the Vinchi Sep 10 '17 at 19:56
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nth bit printf("%u", !!(x & (1 << nbit))); nbits bits from the nbit position printf("%u", ((x & (((1 << nbits) - 1)) << nbit) >> nbit)); nbits <1, > nbit <0,> – 0___________ Sep 10 '17 at 19:56
1 Answers
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If you are compiling with GCC, Visual Studio 2015 (or later), or Clang, then you can use the 0b (binary) prefix, so you can see the bitmask that you want, instead of the 0x (hexadecimal) prefix, which is a little less intuitive.
int x = 127;
printf("%d\n", x & 0b11111000); // Prints 120
printf("%d\n", x & 0b00011111); // Prints 31
These examples will mask out the 3 least significant bits (1, 2 and 4), or the 3 most significant bits (64, 32 and 16), leaving only the other 5.
If you are compiling with a compiler not listed above, then you can always import the BOOST_BINARY macro.
If you want to view the actual bits, then you might want to see this question.
Addison
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0b is a non-standard extension supported by recent versions of gcc and clang, but not generally by other compilers – Chris Dodd Sep 11 '17 at 01:48
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Huh. Touche @ChrisDodd. I always assumed this was part of the C standard, but you are indeed correct. I'll update to reflect that. – Addison Sep 11 '17 at 01:55