Updating fields of bits incorrectly

Question

I'm trying to solve a question. It says,

Initialize a new variable to the value 17512807u.

Assume we number the bits as usual from 0 as least significant (on the right) to 31 (most significant, on the left). Update bits 18 through 21 with the integer value 8 and bits 10 through 14 with value 17 (decimal). Print the resulting value as an eight digit hexadecimal number to show all of the digits.

Here's the code I came up with:

#include <stdio.h>

int main(){
    int value = 17512807u;
    int L = 21; // starting left position
    int R = 18; // starting right position

    int mask = (1 << (L - R + 1) - 1) << R; 
    int newField = (8 << R) & mask; // integer value 8, shifting to right
    int newValue = value & (~mask); // remove range of bits
    value = newField | newValue; // update range of bits

    L = 14;
    R = 10;

    mask = (1 << (L - R + 1) - 1) << R;
    newField = (17 << R) & mask;
    newValue = value & (~mask);
    value = newField | newValue;

    printf("%08x\n", value);
}

The answer I get is 012b7d67

However, I am told this is the incorrect answer. I do not know what the correct answer is.

Answer 1

int mask = ((1 << (L - R + 1)) - 1) << R;

Answer 2

Your expressions to compute mask are incorrect. You changed the parentheses in the second expression but both are incorrect, and do not even compile:

int mask = ((1 << (L - R + 1) - 1) << R;
...
mask = ((1 << (L - R + 1) - 1 << R);

should be written:

mask = ((1UL << (L - R + 1)) - 1) << R;

Answer 3

Looks like what you are asked to do is use the bit fields. For example, given the type

struct bits{
    int a:5;
    unsigned short b:3;
    unsigned char c:2;
    bool d:1;
};

The above struct will have 4 members, each of specific bit length.

If you union that struct with an int you get a "dual-view" of the bits. As a list of fields or as a single integer:

union U{
    struct bits fields;
    int i;
};

Now the code like

U u;
u.i = 0;
u.fields.b = true;

becomes valid and gives you access to either the whole number or individual bit fields.