Question

I made an object that actually represents an array of 8 booleans stored in a char. I made it to learn something more about bitwise operators and about creating your own objects in C. So I’ve got two questions:

1. Can I be certain if the below code
   always works?
2. Is this a good implementation to
   make an object that can’t get lost
   in C, unless you release it
   yourself.

The Code:

/*
 *  IEFBooleanArray.h
 *  IEFBooleanArray
 *
 *  Created by ief2 on 8/08/10.
 *  Copyright 2010 ief2. All rights reserved.
 *
 */

#ifndef IEFBOOLEANARRAY_H
#define IEFBOOLEANARRAY_H

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

typedef char * IEFBooleanArrayRef;

void IEFBooleanArrayCreate(IEFBooleanArrayRef *ref);
void IEFBooleanArrayRelease(IEFBooleanArrayRef ref);
int IEFBooleanArraySetBitAtIndex(IEFBooleanArrayRef ref, 
                                 unsigned index, 
                                 int flag);
int IEFBooleanArrayGetBitAtIndex(IEFBooleanArrayRef ref, 
                                 unsigned index);

#endif

---

/*
 *  IEFBooleanArray.c
 *  IEFBooleanArray
 *
 *  Created by ief2 on 8/08/10.
 *  Copyright 2010 ief2. All rights reserved.
 *
 */

#include "IEFBooleanArray.h"

void IEFBooleanArrayCreate(IEFBooleanArrayRef *ref) {
    IEFBooleanArrayRef newReference;

    newReference = malloc(sizeof(char));
    memset(newReference, 0, sizeof(char));
    *ref = newReference;
}

void IEFBooleanArrayRelease(IEFBooleanArrayRef ref) {
    free(ref);
}

int IEFBooleanArraySetBitAtIndex(IEFBooleanArrayRef ref, unsigned index, int flag) {
    int orignalStatus;

    if(index < 0 || index > 7)
        return -1;

    if(flag == 0)
        flag = 0;
    else
        flag = 1;

    orignalStatus = IEFBooleanArrayGetBitAtIndex(ref, index);
    if(orignalStatus == 0 && flag == 1)
        *ref = *ref + (int)pow(2, index);
    else if(orignalStatus == 1 && flag == 0)
        *ref = *ref - (int)pow(2, index);

    return 0;
}

int IEFBooleanArrayGetBitAtIndex(IEFBooleanArrayRef ref, unsigned index) {
    int result;
    int value;

    value = (int)pow(2, index);
    result = value & *ref;

    if(result == 0)
        return 0;
    else
        return 1;
}

I’m more of an Objective-C guy, but I really want to learn C more. Can anyone request some more “homework” which I can improve myself with?

Thank you,
ief2

Answer 1

---

pow(2,index) is among the more inefficient ways to produce a bit mask. I can imagine that using the Ackermann function could be worse, but pow() is pretty much on the slow side. You should use (1<<index) instead. Also, the C’ish way to set/clear a bit in a value looks different. Here’s a recent question about this:

• Simple way to set/unset an individual bit

---

If you want to munge bits in C in an efficient and portable way, then you really should have a look at the bit twiddling page, that everyone here will suggest to you if you mention “bits” somehow:

• http://graphics.stanford.edu/~seander/bithacks.html

---

The following code sequence:

if(result == 0)
        return 0;
    else
        return 1;

can be written as return (result != 0);, return resultor return !!result (if result should be forced to 0 or 1) . Though it’s always a good idea to make an intent clear, most C programmer will prefer ‘result result;’ because in C this the way to make your intent clear. The if looks iffy, like a warning sticker saying “Original developer is a Java guy and knows not much about bits” or something.

---

newReference = malloc(sizeof(char));
memset(newReference, 0, sizeof(char));

malloc + memset(x,0,z) == calloc();

---

You have a way to report an error (invalid index) for IEFBooleanArraySetBitAtIndex but not for IEFBooleanArrayGetBitAtIndex. This is inconsistent. Make error reporting uniform, or the users of your library will botch error checking.