# [AIdrifter CS 浮生筆錄](https://hackmd.io/s/rypeUnYSb) <br> Pointer,Address, and String

## Array and Address

// assume a's address = 100
// assume b's address = 200
// assume c's address = 300
// assume d's address = 400

int a[4], *b, *c;
char s[10];

for (i=0; i<4; i++)
      a[i] = i;

b = a;
c = &a[2];
strcpy(s,“123”);

- Question

printf(“%c”, s);     // ?? (S 的 Addr 轉成 char)
printf(“%d”, s[0]);  // 49 (‘1’ 的 ASCII code)
printf(“%s”, s[0]);  // Runtime error(crash)
printf(“%c”, s[0]);  // 1

## String Array

char a[100];
strncpy(a, "123");

char s1[] = "abcde";

char *s2  = "abcde";

char *s3;
s3 = malloc(6);
strcpy(s3, "abcde");


- Question

s1 = a;            // compile error
s2 = a;            // OK
s3 = a;            // OK, but memory leak
strcpy(s1, a);     // OK, stack can rw
strcpy(s2, a);     // [FIXME] Wrong => code seegment (read only)
strcpy(s3, a);     // OK, heap can rw

- String Coding Style

#define LEN 5
char s1[LEN],s2[LEN];


strncpy(s1,”abcdefgh”,LEN-1);
s1[LEN-1] = ‘\0’;

//        |  |  |  |  |   |
// s1[5]  |a |b |c |d |\0 |
//         0  1  2  3   4

strncpy(s2,”ab”,LEN-1);
s2[LEN-1] = ‘\0’;

//        |  |  |   |  |   |
// s2[5]  |a |b |\0 |? |\0 |
//         0  1   2  3   4

## Pointer To Pointer
- ~~Double Pointer~~ 是錯誤的說法 Jserv大神會生氣喔
### 2D Array
- dynmically allocate array size

int i;                                     
int N = 3,M = 5;                           
int **a = NULL;                            
                                               
a = (int **)malloc(sizeof(int *)*N);       
if (a)                                     
{                                          
    for (i=0; i<N; i++) 
    {                    
        a[i] = (int *)malloc(sizeof(int)*M);
        if (!a[i]) 
            break;                  
    }                                      
} // (i != N) or (a == NULL) =>  malloc() failed

- How to free it ?

if (a) 
{
	for (i=0; i<N; i++)
	    free(a[i]);
	free(a);
	a = NULL;
}

### Pointer as Parameter
- `*b` <=> `a`

int foo (int **b) 
{
    int i=0, n=10;
	
    if ((*b = (int *)malloc(n * sizeof(int))) == NULL)
        return 0;
         
    for (i=0;i<n;i++)
        (*b)[i] = i; // important!!! (*b) <=> a

    return n;
}

int main() 
{
    int *a = NULL; 
    int n = 0, i;

    if ( n=foo(&a) ) 
    {
        for (i=0;i<n;i++)
            printf("%d\n", a[i]);
	
        if(a)
        {
            free(a);
            a = NULL;
        }
    }
}

- **`*b`(pointer to pointer) <=> `a`(pointer)**