Reading characters on a bit level

Question

I would like to be able to enter a character from the keyboard and display the binary code for said key in the format 00000001 for example.

Furthermore i would also like to read the bits in a way that allows me to output if they are true or false.

e.g.

01010101 = false,true,false,true,false,true,false,true

I would post an idea of how i have tried to do it myself but I have absolutely no idea, i'm still experimenting with C and this is my first taste of programming at such a low level scale.

Thankyou

Answer 1

For bit tweaking, it is often safer to use unsigned types, because shifts of signed negative values have an implementation-dependent effect. The plain char can be either signed or unsigned (traditionally, it is unsigned on MacIntosh platforms, but signed on PC). Hence, first cast you character into the unsigned char type.

Then, your friends are the bitwise boolean operators (&, |, ^ and ~) and the shift operators (<< and >>). For instance, if your character is in variable x, then to get the 5th bit you simply use: ((x >> 5) & 1). The shift operators moves the value towards the right, dropping the five lower bits and moving the bit your are interested in the "lowest position" (aka "rightmost"). The bitwise AND with 1 simply sets all other bits to 0, so the resulting value is either 0 or 1, which is your bit. Note here that I number bits from left significant (rightmost) to most significant (leftmost) and I begin with zero, not one.

If you assume that your characters are 8-bits, you could write your code as:

unsigned char x = (unsigned char)your_character;
int i;

for (i = 7; i >= 0; i --) {
    if (i != 7)
        printf(",");
    printf("%s", ((x >> i) & 1) ? "true" : "false");
}

You may note that since I number bits from right to left, but you want output from left to right, the loop index must be decreasing.

Note that according to the C standard, unsigned char has at least eight bits but may have more (nowadays, only a handful of embedded DSP have characters which are not 8-bit). To be extra safe, add this near the beginning of your code (as a top-level declaration):

#include <limits.h>
#if CHAR_BIT != 8
#error I need 8-bit bytes!
#endif

This will prevent successful compilation if the target system happens to be one of those special embedded DSP. As a note on the note, the term "byte" in the C standard means "the elementary memory unit which correspond to an unsigned char", so that, in C-speak, a byte may have more than eight bits (a byte is not always an octet). This is a traditional source of confusion.