Operators & Expressions

#include <stdio.h>

int main(void) {
    int a = 7, b = 2;

    /* Integer division — truncates, does NOT round */
    printf("%d / %d = %d\n", a, b, a / b);     /* 3, not 3.5 */
    printf("%d %% %d = %d\n", a, b, a % b);    /* 1 (remainder) */

    /* To get a float result, cast one operand to double BEFORE dividing */
    printf("(double)%d / %d = %.1f\n", a, b, (double)a / b); /* 3.5 */

    /* Careful: (double)(a/b) is NOT the same — int division first! */
    printf("(double)(%d/%d) = %.1f\n", a, b, (double)(a / b)); /* 3.0 */

    /* Modulo: useful for even/odd check, cycling, etc. */
    for (int i = 0; i < 6; i++) {
        printf("i=%d  i%%3=%d\n", i, i % 3);   /* cycles 0,1,2,0,1,2 */
    }
    return 0;
}

#include <stdio.h>

int main(void) {
    unsigned int a = 0b1010;  /* 10 in decimal */
    unsigned int b = 0b1100;  /* 12 in decimal */

    printf("a   = %u  (binary: 1010)\n", a);
    printf("b   = %u  (binary: 1100)\n", b);
    printf("a&b = %u  (binary: 1000) — AND: both bits must be 1\n", a & b);
    printf("a|b = %u  (binary: 1110) — OR: at least one bit 1\n",  a | b);
    printf("a^b = %u  (binary: 0110) — XOR: bits differ\n",        a ^ b);
    printf("~a  = %u  — NOT: all bits flipped\n", ~a);
    printf("a<<1= %u  (binary: 10100) — left shift: multiply by 2\n", a << 1);
    printf("b>>1= %u  (binary: 0110)  — right shift: divide by 2\n",  b >> 1);

    /* Practical: test if a bit is set */
    int flags = 0b00001010;
    int bit1 = (flags >> 1) & 1;   /* extract bit 1 */
    printf("Bit 1 of flags: %d\n", bit1);  /* 1 */

    /* Set bit 2 */
    flags |= (1 << 2);
    printf("After setting bit 2: %d\n", flags);  /* 14 = 0b00001110 */

    return 0;
}

#include <stdio.h>

int main(void) {
    int x = 5, y;

    /* Postfix: y gets old value (5), then x becomes 6 */
    y = x++;
    printf("x=%d, y=%d\n", x, y);  /* x=6, y=5 */

    /* Prefix: x becomes 7 first, then y gets new value (7) */
    y = ++x;
    printf("x=%d, y=%d\n", x, y);  /* x=7, y=7 */

    /* Short-circuit: second operand not evaluated if unnecessary */
    int a = 0, b = 0;
    /* a is 0 (false), so the whole && is false — b++ never runs */
    if (a && b++) {
        printf("never reached\n");
    }
    printf("b after a&&b++: %d\n", b);  /* b is still 0 */

    /* a is 0, but || checks second: b++ runs */
    if (a || b++) {
        printf("reached because b++ made it true\n");
    }
    printf("b after a||b++: %d\n", b);  /* b is now 1 */

    /* Ternary operator */
    int m = (x > y) ? x : y;
    printf("max(%d,%d) = %d\n", x, y, m);

    return 0;
}

#include <stdio.h>
#include <stdint.h>

// Pack a 3-bit mode (bits 5-3) and 3-bit priority (bits 2-0) into one byte.
// Bit layout: [unused][unused][mode2][mode1][mode0][pri2][pri1][pri0]
#define MODE_MASK     0x38   // 0011 1000
#define PRIORITY_MASK 0x07   // 0000 0111
#define MODE_SHIFT    3

uint8_t pack(int mode, int priority) {
    return ((mode << MODE_SHIFT) & MODE_MASK) | (priority & PRIORITY_MASK);
}

int get_mode(uint8_t packed)     { return (packed & MODE_MASK) >> MODE_SHIFT; }
int get_priority(uint8_t packed) { return packed & PRIORITY_MASK; }

int main(void) {
    uint8_t p = pack(5, 3);           // mode=5 (101), priority=3 (011)
    printf("packed = 0x%02X\n", p);   // 0x2B = 0010 1011
    printf("mode = %d\n", get_mode(p));       // 5
    printf("priority = %d\n", get_priority(p)); // 3
    return 0;
}