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 = ; 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) == )
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 = ; i < n; i++)
{
void * elemAddr = (uchar*)base + i*elemSize;
if( memCmpa( elemAddr,key) == )
return elemAddr;
}
return NULL;
}
applied in array search
uchar *array[] = {"one","two","three","four","five"};
uchar *aim="two";
uchar **k;
k = lsearch(&aim,array,,sizeof(uchar*), myStrCmp);
// the first parameter add & to get the same depth with the second
if(k == NULL)
return -;
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 = ;
right = n - ;
while(left <= right)
{
mid = (left + right)/; elemAddr = (char *)base + mid * elemSize;
compare = memCmpa(key,elemAddr); if( == compare)
{
return elemAddr;
}
else if(compare < )
{
right = mid - ;
}
else if(compare > )
{
left = mid + ;
}
}
return NULL;
}
//__I1__
int compareInt(const void *d1,const void *d2)
{
int t1,t2;
t1 = *(int *)d1;
t2 = *(int *)d2;
if(t1 > t2)
return ;
else if(t1 < t2)
return -;
else
return ;
}
//__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[]);
result = (char **)bsearchMy(&key,a,n,sizeof(a[]),compareStr);
if(result == NULL)
printf("key not found");
else
printf("%s",*result);
}