Thread: help with setbits function

Hi, I've been reading The C Programming Language book and one of the excersises is to write a setbits(x, p, n, y) function that returns x with the n bits that begin at position p set to the rightmost n bits of y and am really stuck on how to do it.
Here's my code so far, all i need is to finish the result variable:

Code:

/* setbits.c
 * program to return x with the n bits at position p to the rightmost n bits of y
 */
 
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

unsigned setbits(unsigned *x, int *p, int *n, unsigned *y);

void binary_out(unsigned *num);

int main(int argc, char **argv)
{
	if (argc < 5)
	{
		fprintf(stderr, "Usage: %s <num1> <num2> <num3> <num4>\n", argv[0]);
		return -1;
	}
	
	unsigned a = atoi(argv[1]);
	int b = atoi(argv[2]);
	int c = atoi(argv[3]);
	unsigned d = atoi(argv[4]);
	
	setbits(&a, &b, &c, &d);
	return 0;
}

void binary_out(unsigned *num)
{
	int i;
	for (i = 256; i > 0; i>>=1)
	{
		if ((*num & i) == i)
			printf("%d", 1);
		else
			printf("%d", 0);
	}
	printf("\n");
}

// return x with the n bits at position p to the rightmost n bits of y
unsigned setbits(unsigned *x, int *p, int *n, unsigned *y)
{
	printf("x is: ");
	binary_out(x);
	printf("y is: ");
	binary_out(y);
	unsigned result = ((*x 
	printf("and the result is: ");
	binary_out(&result);
}

could anyone guide me through it with examples or anything.
Thanks.

The line below is missing part of the code; it will not compile as written.
Edit: And, since your comment is not enough for me to guess what it should do; I can not help you.
I suggest you think on what input should give what output and how you can do that.

Code:

unsigned result = ((*x

In spots, looks like poorly written code.
Edit: Poorly written include this function "binary_out"; why does an output only function needed to be passed an pointer.

Other parts look good.

Tim S.

Originally Posted by cprog12

yeah, i know the result variable is missing code that's what i need help with.

And, it does what?
Example input and output?

Since you passed the parameters as pointers; are you planning to change all the input parameters value in the function?


Several year old thread on same topic
Bitwise setbits function (knr 2-6)