help with setbits function

This is a discussion on help with setbits function within the C Programming forums, part of the General Programming Boards category; Hi, I've been reading The C Programming Language book and one of the excersises is to write a setbits(x, p, ...

  1. #1
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    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.
    Last edited by cprog12; 05-23-2011 at 12:03 PM.

  2. #2
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    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.
    Last edited by stahta01; 05-23-2011 at 11:33 AM.

  3. #3
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    yeah, i know the result variable is missing code that's what i need help with.

  4. #4
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    Quote Originally Posted by cprog12 View Post
    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)
    Last edited by stahta01; 05-23-2011 at 11:48 AM.

  5. #5
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    i need to use bitwise operators to return x with the n bits at position p to the rightmost n bits of y, but i'm stuck on how to write it.

  6. #6
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    Thanks, the thread really helps

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