Please, tell me why this code does not work

[Ian Rust]

Please help. I am new to C. I am trying to display the character 'W' as it is written in binary using a bitmask. I am aware there are alternate ways to do this, but I want to know why this did not work. If you read the comments, I know the problem is somewhere in the 'if' statement, because the code works when I delete the if statement. The compilers error message says: ...17 (line of the if statement): "invalid operands to binary & (having 'int' and 'char *)". Yet letter & array are both char variables. So I do not understand:

#include <stdio.h>




int main(void){


int i = 0;
char letter = 'W';
char array[] = "0b00000000";
char *pointer = array;


// so first, i want the pointer + 2 to change the 0 (in array) to a 1. But I want this to be an increment, not a pointer switch.


    for (i = 2; i < 10; i++){
    *(pointer + i) = '1';


    // Now I have the bitmask. So I want to apply the bitmask to W using a boolean operator, and print either a 1 or 0 according
    // to which bit value is contained in 'W'. I also want to undo the '1' in the bitmask at the end of the loop, setting it to 0, then moving
    // on to the next bit in the bitmask; changing that to a 1, and repeating the process until all 8 bits have been checked.


    if (letter & array){
        printf("1");
    }
    // So at this point array = 0b10000000. The printf statement confirms that. 
    //
    // I am aware this code is not complete. I'd planned to add the statement:
    // if (~(letter & array)){
    // printf("0");
    // }


    printf("%s", array);

    // If I do not include the 'if' statement, the printf function runs and displays my bitmasks, incremented from 0b10000000 to 0b01000000
    //and so on. But, when I add the if statement, it no longer works. 

    *(pointer + i) = '0';
    }


return 0;
}

[laserlight]

The problem is that in the expression (letter & array), array is converted to a pointer to its first element, then bitwise and is computed. This does not result in the bitmask that you were expecting.

Rather, what you should do is to have something like:

unsigned int mask = 1 << (CHAR_BIT - 1);

Then:

if (letter & mask)

This way, you can then write:

mask >>= 1;

At the end of the loop, looping for CHAR_BIT number of iterations.

[Aslaville]

^_^

char array[] = "0b00000000"; //wrong

uint8_t/unsigned char array = 0b00000000; //correct

This link pretty much sums up everything about bitmasks How do you set, clear and toggle a single bit in C/C++? - Stack Overflow .

[Ian Rust]

Ok, thank you.