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unsigned long aval = 0xaf;
unsigned long mask = 0xfc;
unsigned long ans  = aval & mask;
printf("0x%x\n", ans);

¼Â¹Ô·ë²Ì:
0xac

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a += a << 2;

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#include <stdlib.h>
#include <stdio.h>
#define USAGE "ÃÍ¡§0¡¡Éä¹æÊÑ´¹¡Ê¡Ü¡¢¡Ý¤òµÕ¤Ë¡Ë\n"\
    "ÃÍ¡§1¡¡Ãͤò5Çܤˤ¹¤ë\n"\
    "ÃÍ¡§2¡¡Ãͤò1/4¤Ë¤¹¤ë\n"\
    "ÃÍ¡§¤½¤Î¾¡¡Ãͤò0¤Ë¤¹¤ë"
void PrintBinary(int n) {
    div_t d = div(n, 2);
    if (d.quot) PrintBinary(d.quot);
    putchar(d.rem ? '1' : '0');
}
void Print(int n) {
    printf("½½¿Ê:%4d ½½Ï»¿Ê:%x Æó¿Ê:", n, n);
    PrintBinary(n); putchar('\n'); putchar('\n');
}
void MaskLowOrder2Bits(int * n) {
    *n &= ~0 ^ 3;
}
void Negative(int * n) {
    *n = ~*n;
    ++*n;
}
void MultiplyBy5(int * n) {
    int tmp = *n << 2;
    tmp += *n;
    *n = tmp;
}
void DivideBy4(int * n) {
    *n >>= 2;
}
void Move0(int * n) {
    *n ^= *n;
}
int main(void) {
    int n, pattern;
    printf("ÃÍ(½½¿Ê):"); if (scanf("%d", &n) != 1) return 1;
    puts("¤Î¡¼¤Þ¤ë"); Print(n);
    MaskLowOrder2Bits(&n);
    puts("¤Þ¤¹¤¯"); Print(n);
    printf(USAGE "\n¥Ñ¥¿¡¼¥ó:"); if (scanf("%d", &pattern) != 1) return 2;
    switch (pattern) {
    case 0: puts("Éä¹æÊÑ´¹"); Negative(&n); break;
    case 1: puts("* 5"); MultiplyBy5(&n); break;
    case 2: puts("/ 4"); DivideBy4(&n); break;
    default: puts("= 0"); Move0(&n); break;
    }
    Print(n);
    return 0;
}

#include<stdio.h>

main(){
    int value,aftermask,mask(int);

    printf("value = ");
    scanf("%d",&value);//ÃͤòÆþÎÏ
    aftermask = value & 3;//­¡¡¡²¼°Ì2¥Ó¥Ã¥È¤ò¥Þ¥¹¥¯
    printf("%d\n",aftermask);

    mask(value);
    printf("%d\n",value);//­¤
}

int mask(int value){
    int i,n;
    int answer[32],rest[32];

    answer[0] = value;¡¡¡¡¡¡¡¡¡¡¡¡

    for(i=0;;i++){
        answer[i+1] = answer[i]/2;¡¡
        rest[i] = answer[i] - answer[i+1]*2;
        if(answer[i+1]==0){
            n = i;           //²¿ÈÖÌܤÎÇÛÎó¤Þ¤Ç;¤ê¤¬Æþ¤Ã¤¿¤«¤òn¤ËÆþ¤ì¤Æ¤ª¤¯
            break;
        }
    }

    for(i=n;i>=0;i--){
        printf("%d",rest[i]);//­¢¡¡nÈÖÌܤ«¤éµÕ¤ËÇÛÎó¤òʤ٤ë
    }
    printf("\n");
    return(rest[i]);//­£¡¡value¤ËÊÖ¤¹
}

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aval1 = ÆþÎÏÃÍ;
mask = 0x03;
sw = aval1 & (mask);
aval2 = aval1 & (~mask);

switch(sw) {
case ¤Ê¤ó¤È¤«:
    ans = Áàºî(aval1);  // ¤³¤ì¤«¤Ê?
    ans = Áàºî(aval2);  // ¤³¤ì¤«¤Ê?
    ans = Áàºî(sw);  // ¤³¤ì¤«¤â?
    break;
// Ãæά
}