编程范式 episode3 and 4，5

episode 3 

--storage structure. ampersand operate with asterisk

--library function

episode 4

--generic function 泛型函数

swap(void* pa,void*pb,int size);

-----ampersand  and asterisk

the original types does not matter much, the pointer does.

when a pointer is operated with a intergral, just remember to transit the intergral into the same type length with the pointer.

--big endian, little endian(most linux)

-----array

int array[10];

array = &arry[0];

array+k = &array[k];   //k 进行指针运算，要扩展成array同类型长度

*array = array[0];

*(array+k)=array[k];

*(array-4)=8;

array[-4] = 8;//there is no bond check in C, so it will put 8 at the corresponding address.This is not good                            //code, the result is unsure, it may crush.

-------liabrary function

..........strdup("adm"); //as it shows itself, only valid to string type.

memcpy function,return an adress pointed at heap which store"adm"(with string end sign).

.........strcpy(pAim, "4021");
"4021" is copied into the memory started at address pAim.

...........memcpy(void*to,void*from,int size)
copy designated size  to the memory.

..........memcmy(void *a, void *b, int size)

copare two storage, if equal,return to 0,otherwise, get a positive or negative value.

 

------void swap(void* pa,void*pb,int size);

void swap(void* pa,void*pb,int size)
{
    char temp[size];
    memcpy(temp,pa,size）；
    memcpy(pa,pb,size);
    memcpy(pb,temp,size);
}

 

-----void * lsearch(void *key, void *base, int n, int elemSize)

//a generic function of search, but not available for pointer type, as memcmp is used.

void * lsearch(void *key, void *base, int n, int elemSize)
{
      int i;
      for(i = 0; i < n; i++)
      {
          void * elemAddr = (char*)base + i*elemSize;  //void * type not allowed to do operation.
　　　　　　　　　　　　　　　　　　　　　　　//intergral operated with pointer here manually adjusted
          if( memcmp( elemAddr,key,elemSize) == 0 )   
              return elemAddr;
       }
       return NULL;
}

 -----void * lsearch(void *key, void *base, int n, int elemSize,int(*memCmpa)(void *, void *) )

 //*memCmpa , the *  is not necessary,explictly show function pointer  

void * lsearch(const void *key,const void *base, int n, int elemSize,int(*memCmpa)(void *,void *) )
{
      int i;
      for(i = 0; i < n; i++)
      {
          void * elemAddr = (uchar*)base + i*elemSize;
          if( memCmpa( elemAddr,key) == 0 )
              return elemAddr;
       }
       return NULL;
}

 applied in array search

    uchar *array[] = {"one","two","three","four","five"};
    uchar *aim="two";
    uchar **k;
   k = lsearch(&aim,array,5,sizeof(uchar*), myStrCmp);
   // the first parameter add & to get the same depth with the second
   if(k == NULL)
   return -1;
    printf("%s,",*k);

int strCmp(const void *a,const void *b)
{
    uchar *a1 = *(uchar**)a;
    uchar *b1 = *(uchar**)b;
    return strcmp(a1,b1);
}

 bSearch  针对已经排列好的有序数列

#define  NULL (void *)0
//generic binary search function
void *bsearchMy(const void *key, const void *base,int n, int elemSize, int (*memCmpa)(const void *,const void *) )
{
    int left,right;
    int mid;
    void *elemAddr;
    int compare;
    left = 0;
    right = n - 1;
    while(left <= right)
    {
        mid = (left + right)/2;

        elemAddr = (char *)base + mid * elemSize;
        compare = memCmpa(key,elemAddr);

        if(0 == compare)
        {
            return elemAddr;
        }
        else if(compare < 0)
        {
            right = mid - 1;
        }
        else if(compare > 0)
        {
            left = mid + 1;
        }
    }
    return NULL;
}
//__I1__
int compareInt(const void *d1,const void *d2)
{
    int t1,t2;
    t1 = *(int *)d1;
    t2 = *(int *)d2;
    if(t1 > t2)
        return 1;
    else if(t1 < t2)
        return -1;
    else
        return 0;
}
//__I2__
int compareStr(const void *d1,const void *d2)
{
    char *t1, *t2;
    t1 = *(char **)d1;
    t2 = *(char **)d2;
    return strcmp(t1,t2);
}

__I1__compareint, not return v1-v2, c程序实践中写到，if v2 is large and positive and v1 is large and negative or vice versa, the resulting overflow would produce an incorrct answer. Direct comparison is longer but safer.

__I2__the address of each entry in the array,&str[i](of type char**),not str[i] is passed to the function parameter. two steps to access the value.

int main()
{
    char *a[] = {"hello","miss","rabbit","you"};
    char *key = "rabbit";  //key[] wrong
    char **result;
    int n;
    n = sizeof(a)/sizeof(a[0]);
    result = (char **)bsearchMy(&key,a,n,sizeof(a[0]),compareStr);
    if(result == NULL)
        printf("key not found");
    else
        printf("%s",*result);
}