MhdBanat MhdBanat - 10 months ago 34
C Question

Masks bits of array members

Can you direct me where is my problem here? I am trying to test values of an array.

Every value presents 16 bits (flags) and I need to check if the flag is set or not.

My current output is shown below and also the expected output.

#include <stdio.h>

unsigned short value[1];
unsigned int bitCheck(unsigned int mask, int pin);
unsigned short mask;

int main(void){
value[0]=0;
value[1]=4095;


int pin0 = 0;
int pin1 = 1;

unsigned int bit0= bitCheck( mask, pin0);
unsigned int bit1= bitCheck( mask, pin1);


for (int i =0;i<=1; i++)
{
mask=value[i];
printf("Mask = %d ==>>\n", mask);

if ( bit0 == 1 ){
printf("Pin %d is Set\n", pin0);
}else{
printf("Pin %d is not Set\n", pin0);
}

if ( bit1 == 1 ){
printf("Pin %d is Set\n", pin1);
}else{
printf("Pin %d is not Set\n", pin1);
}

printf("\n");
}

}

unsigned int bitCheck(unsigned int mask, int bit){
if ( (mask >> bit ) & 1){
return 1;
}else{
return 0;
}
}


My output is:

Mask = 0 ==>>
Pin 0 is not Set
Pin 1 is not Set

Mask = 4095 ==>>
Pin 0 is not Set
Pin 1 is not Set


and it must be:

Mask = 0 ==>>
Pin 0 is not Set
Pin 1 is not Set

Mask = 4095 ==>>
Pin 0 is Set
Pin 1 is Set

Answer Source

This code is unnecessarily complex and through it's complexity, hides a blatant logical error. You're doing your calls to bitCheck(mask, ...) before you ever assign a value to mask.

Change this:

    unsigned int bit0= bitCheck( mask, pin0);
    unsigned int bit1= bitCheck( mask, pin1);


  for (int i =0;i<=1; i++)
{
    mask=value[i];
   printf("Mask = %d ==>>\n", mask);

to this:

  for (int i =0;i<=1; i++)
{
    mask=value[i];

    unsigned int bit0= bitCheck( mask, pin0);
    unsigned int bit1= bitCheck( mask, pin1);

   printf("Mask = %d ==>>\n", mask);

That said, here's a sane way to do all this:

#include <stdio.h>

#define BIT(n, v) (!!((v) & (1U << (n))))

int main(void)
{
    unsigned short values[] = {0, 4095};
    for (int i = 0; i < 2; ++i)
    {
        printf("value: %hu\n", values[i]);
        for (int bit = 0; bit < 16; ++bit)
        {
            printf( BIT(bit, values[i]) ?
                    "bit %d is set\n" :
                    "bit %d is not set\n", bit);
        }
    }
    return 0;
}

For one reason why your approach isn't considered sane, read about the DRY principle. Writing 16 times the same code is a simple example of violating this principle -> you need a loop instead.

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