為什么我不能在下面的程序中初始化指向變量的指針值

Question

#include<Stdio.h>

int main()
{
    int a ,b;
    int *p;
    int *q;
    printf("enter the value of a and b");
    scanf("%d%d",&a,&b);
    p = &a;
    q = &b;
    printf("value of a and b is %d and %d",a,b);
    a = *q;
    b = *p;
    printf("value of a and b is %d and %d",a,b);
}

我無法更改 b 的值，即使將其重新定義為指針 p。

輸出

Answer 1

實際上，您正在為b分配一個值，但是由於您更改了a的值，因此您不會注意到分配，假設您為a輸入4 ，為b輸入5 ：

a = *q;  // q points at b, which is 5, so a = 5
b = *p;  // p points at a, which is now 5, so, b = 5

要交換值，您可以改為：

int tmp = a; // store the value of a, 4
a = b;       // assign, a = 5
b = tmp;     // assign, b = 4 (the stored value)

或者用它做一個函數：

void swap(int *lhs, int *rhs) {
    int tmp = *lhs;
    *lhs = *rhs;
    *rhs = tmp;
}

並稱之為：

swap(&a, &b);