Thread: Got a question about odd numbers

Code:

while (counter1 !=0 ,&(counter1&1===>counter1))

am I doing this wrong? I am trying to compare the least significant bit to determine if counter1 is odd.
Thanks

What do you think ",&" and "===>" are supposed to mean?

I'm not sure, though I was told it was a conditional test to see if a number is odd
,& means AND

it's something someone told me a while back as a simple test to determine if counter1 is odd instead of using a modulo calculation.

Originally Posted by Once-ler

I'm not sure, though I was told it was a conditional test to see if a number is odd
,& means AND

If you mean AND, that's "&&", not ",&". Someone may very well have told you that "counter1&1" is a test to see if a number is odd, because it is. The rest of it ....

,&??? you should use correct syntax.

Originally Posted by grumpy

that doing it with a bitwise operation is either unnecessary obfuscation or premature optimisation.

Are you serious?

I've written a non-optimized version

Code:

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <math.h>

enum {
    PARITY_ODD,
    PARITY_EVEN
};


int *primefactors(unsigned long long n);
int parityeven(long long n);
void fatalerr(int err);
static int primefactors_r(unsigned n, int **mem, size_t *sz);


int main(void)
{
    int n;
    
    const char *paritystr[2] = {"odd", "even"};
    
    for (n = -10; n <= 11; n++)
        printf("%d is %s\n\n", n, parityeven(n)[paritystr]);
    
    return 0;
}

int parityeven(long long n)
{
    int *pfacts;
    int i, r = PARITY_ODD;
    
    if (n < 0)
        n = -n;
    
    if (n == 0 || n == 2) 
        return PARITY_EVEN;
    if (n == 1 || n == 3)
        return PARITY_ODD;
    
    pfacts = primefactors(n);
    
    if (!pfacts)
        fatalerr(errno);
    
    if (pfacts[0] == 0)
        printf("Is prime");
    else
        printf("Prime Factors:");

    for (i = 0; pfacts[i] != 0; i++) {
        printf(" %d", pfacts[i]);
        if (pfacts[i] == 2) {
            r = PARITY_EVEN;
        }
    }       
    printf(" => ");
    return r;
}

void fatalerr(int err)
{
    fprintf(stdout, "errr... %s\n", strerror(err));
    exit(1);
}

int *primefactors(unsigned long long n)
{
    int *mem;
    size_t sz = 1;
    
    if ((mem = malloc(sizeof *mem * sz)) == NULL)
        return NULL;
    
    if (n < 4) {
        mem[0] = 0;
        return mem;
    }
    
    primefactors_r(n, &mem, &sz);
    mem[sz - 1] = 0;
    
    return mem;
}

static int primefactors_r(unsigned n, int **mem, size_t *sz)
{
    int i, maxn;
    int *newmem; 
    
    if (n < 2)
        return 0;
    
    maxn = n / 2;
    for (i = 2; i <= maxn; i++) {
        if (n % i == 0) {
            if (primefactors_r(i, mem, sz)) {
                (*mem)[*sz - 1] = i;
                newmem = realloc(*mem, ++(*sz) * sizeof(**mem));
                if (!newmem) {
                    int err = errno;
                    free(mem);
                    fatalerr(err);
                }
                *mem = newmem;
            }
            if (primefactors_r(n / i, mem, sz)) {
                (*mem)[*sz - 1] = n / i;
                newmem = realloc(*mem, ++(*sz) * sizeof(**mem));
                if (!newmem) {
                    int err = errno;
                    free(mem);
                    fatalerr(err);
                }
            }
            break;
        }
    }
    /* return 0 (false) if no 'lower' primefactors found;
     * i.e. the above loop didn't terminate early */
    return i > maxn;
}