Teuntje - 1 year ago 56

C Question

I'm trying to get my program to work where a certain bit is being flipped. I have this function called

`flipbit(int *p, int m)`

`8`

`00001000`

`2`

`00001010`

EDIT: I made a stupid mistake, I want to count starting from 0, so the 2nd bit in 8 flipped is actually

`00001100`

`00001010`

`#include <stdio.h>`

#include <stdlib.h>

void flipbit(int *p, int m) {

int digit;

digit = *p;

int bit;

bit = &m;

int result;

//printf("The numbers are %d %d", digit, bit);

printf("%d", result);

}

int main() {

int number1;

int number2;

printf("Give number and bit: ");

scanf("%d, %d",&number1, &number2);

flipbit(&number1, &number2);

return 0;

}

Answer Source

When checking your source code, there are mixture between pointer and value. To comply the call of your flipbit() function with the declaration, the call should be:

```
// first parameter is a pointer and second parameter is a value
flipbit(&number1, number2); // void flipbit(int *p, int m);
```

Inside the flipbit() function, the mixture continues because digit is a value and p is a pointer. The code should be:

```
int digit;
// 'digit' is a value and 'p' is a pointer
digit = p[0]; // 'digit' is the first value pointed by 'p'
```

Same error kind of error with the 'bit' parameter

```
int bit;
// 'bit' is a value and 'm' is a value
bit = m;
```

And the result to flip a bit is the XOR operation.

the bit number 2, so the 2nd

Due to your specification, you have to shift only of (bit - 1). So, in your case: 0x0001(or 0000.0000.0000.0001b) << (2 - 1) = 0x0002(or 0000.0000.0000.0010b)

result is 0x0010(or 0000.0000.0000.1000b) XOR 0x0002(or 0000.0000.0000.0010b) = 0x0012(or 0000.0000.0001.0010b).

```
int result;
result = digit ^ (0x0001 << (bit - 1));
```

Did you enter '9, 1' to comply with the scanf("%d, %d",..) ?