cpp/CPrimerPlus6E/Ch15/invert4.c

/* invert4.c -- using bit operations to display binary */
#include <stdio.h>
#include <limits.h>
char * itobs(int, char *);
void show_bstr(const char *);
int invert_end(int num, int bits);

int main(void)
{
    char bin_str[CHAR_BIT * sizeof(int) + 1];
    
    int number;
    
    puts("Enter integers and see them in binary.");
    puts("Non-numeric input terminates program.");
    while (scanf("%d", &number) == 1)
    {
        itobs(number,bin_str);
        printf("%d is\n", number);
        show_bstr(bin_str);
        putchar('\n');
        number = invert_end(number, 4);
        printf("Inverting the last 4 bits gives\n");
        show_bstr(itobs(number,bin_str));
        putchar('\n');
    }
    puts("Bye!");
    
    return 0;
}

char * itobs(int n, char * ps)
{
    int i;
    const static int size = CHAR_BIT * sizeof(int);
    
    for (i = size - 1; i >= 0; i--, n >>= 1)
        ps[i] = (01 & n) + '0';
    ps[size] = '\0';
    
    return ps;
}

/* show binary string in blocks of 4 */
void show_bstr(const char * str)
{
    int i = 0;
	
    while (str[i])  /* not the null character */
    {
        putchar(str[i]);
        if(++i % 4 == 0 && str[i])
            putchar(' ');
    }
}

int invert_end(int num, int bits)
{
    int mask = 0;
    int bitval = 1;
    
    while (bits-- > 0)
    {
        mask |= bitval;
        bitval <<= 1;
    }
    
    return num ^ mask;
}