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C Programming - Bitwise Operations

alexexcl (24) Hello, I am trying to write a C program that uses a function setbits(x,p,n,y) that returns x with the n bits that begin at position p set to the rightmost n bits of y, leaving the other bits unchanged. I am almost sure that I have the return function x in setbits() is correct. since I got the first output correct. I am confused on why my other three outputs are incorrect, can someone please tell me why or modify my code / setbits() function to get the Desired Output? Desired Output: setbits(x=0x1234, p=15, n=8, y=0xffff) => 0xff34 setbits(x=0x1234, p=14, n=3, y=0x2) => 0x2234 setbits(x=0x1234, p=20, n=6, y=0xf2) => 0x191234 setbits(x=0x1234, p=31, n=4, y=0x192f) => 0xf0001234 Actual Output: setbits(x=0x1234, p=15, n=8, y=0xffff) => 0xff34 setbits(x=0x1234, p=14, n=3, y=0x2) => 0x234 setbits(x=0x1234, p=20, n=6, y=0xf2) => 0x1234 setbits(x=0x1234, p=31, n=4, y=0x192f) => 0x1234 The C Code I have is below:

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#include <stdio.h> unsigned setbits(unsigned x,int p,int n,unsigned y); int main() { int x,p,n,y; printf("setbits(x=0x%x, p=%d, n=%d, y=0x%x) => 0x%x\n", 0x1234, 15, 8, 0xffff, setbits(0x1234, 15, 8, 0xffff)); printf("setbits(x=0x%x, p=%d, n=%d, y=0x%x) => 0x%x\n", 0x1234, 14, 3, 2, setbits(0x1234, 14, 3, 2)); printf("setbits(x=0x%x, p=%d, n=%d, y=0x%x) => 0x%x\n", 0x1234, 20, 6, 0xf2, setbits(0x1234, 20, 6, 0xf2)); printf("setbits(x=0x%x, p=%d, n=%d, y=0x%x) => 0x%x\n", 0x1234, 31, 4, 0x192f, setbits(0x1234, 31, 4, 0x192f)); } unsigned setbits(unsigned x,int p,int n,unsigned y) { return x & ~(~(~0 << n) << (p+1-n)) | ( y & (~(~0<<n)) << (p+1-n)); }

Duthomhas (13206) Extra tricky code makes extra tricky errors. Break it down. You need two masks, one for the N bits starting at xp, and one for the N LSB bits of y. It is also useful to put that shift expression as a variable. Also, I added a test case.

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#include <stdio.h> unsigned setbits(unsigned x,int p,int n,unsigned y); int main() { // int x,p,n,y; printf("setbits(x=0x%x, p=%d, n=%d, y=0x%x) => 0x%x\n", 0x1234, 15, 8, 0xffff, setbits(0x1234, 15, 8, 0xffff)); printf("setbits(x=0x%x, p=%d, n=%d, y=0x%x) => 0x%x\n", 0x1234, 14, 3, 2, setbits(0x1234, 14, 3, 2)); printf("setbits(x=0x%x, p=%d, n=%d, y=0x%x) => 0x%x\n", 0x1234, 20, 6, 0xf2, setbits(0x1234, 20, 6, 0xf2)); printf("setbits(x=0x%04x, p=%d, n=%d, y=0x%04x) => 0x%04x\n", 0x1234, 31, 4, 0x192f, setbits(0x1234, 31, 4, 0x192f)); printf("setbits(x=0x%X, p=%d, n=%d, y=0x%X) => 0x%04X\n", 0x1234, 8, 4, 0xE, setbits(0x1234, 8, 4, 0xE)); // 13D4 } unsigned setbits(unsigned x,int p,int n,unsigned y) { // return x & ~(~(~0 << n) << (p+1-n)) | ( y & (~(~0<<n)) << (p+1-n)); int shift = p - n + 1; unsigned mask = (1U << n) - 1; return (x & ~(mask << shift)) | ((y & mask) << shift); }

Which produces the correct output:

setbits(x=0x1234, p=15, n=8, y=0xffff) => 0xff34 setbits(x=0x1234, p=14, n=3, y=0x2) => 0x2234 setbits(x=0x1234, p=20, n=6, y=0xf2) => 0x191234 setbits(x=0x1234, p=31, n=4, y=0x192f) => 0xf0001234 setbits(x=0x1234, p=8, n=4, y=0xE) => 0x13D4

[edit] What happens if bad values of p and n are supplied? [/edit] Hope this helps. Last edited on Topic archived. No new replies allowed. Home page | Privacy policy© cplusplus.com, 2000-2024 - All rights reserved - v3.3.3Spotted an error? contact us