#include <stdio.h>
void binary_to_ascii( unsigned int value)
{
unsigned int quotient;
quotient=value/10;
if(quotient!=0)
binary_to_ascii(quotient);//recursion
putchar(value%10+'0');
}
value=4267 quotient=?
value=4267 quotient=426
value=426 quotient=42
value=42 quotient=4
value=4 quotient=0
value=42 quotient=4
value=426 quotient=42
value=4267 quotient=426
13.variable parameter
#include <stdarg.h>
float average(int n_values,...)
{
va_list var_arg;
int count;
float sum=0;
var_start(var_arg,n_values);
for(count=0; count<n_values;count+=1)
{
sum+=va_arg(var_arg, int);
}
va_end(var_arg);
return sum/n_values;
}
14. array
//(1)
int array[10];
2[array] == *(2+(array)) ==array[2];
//(2)
int array[10], a;
for(a=0;a<10;a=a+1)
array[a]=0;
int array[10], *ap;
for(ap=array; ap<array+10;ap++) // more efficient here than array
*ap=0;
// when accessing the array elements by a fixed increment, using pointer would more efficient than that of array index;
//(3)
#include <stdio.h>
#include <stdlib.h>
void my_parameter1(char string[]);
void my_parameter2(char *string);
int main()
{
char arr[]="zhiguang";
char *arr1="zhiguang";
printf("%d\n",sizeof(arr));//9--array: 'z' 'h' ... 'g','\0'
printf("%d\n",strlen(arr));//8--array: 'z' 'h' ... 'g'
printf("%d\n",sizeof(arr1));//4--pointer size
printf("%d\n",strlen(arr1));//8-- the content length the pointer points to
my_parameter1(arr);//4--pointer size, it is not 9, because it only pass the array as pointer in fact
my_parameter1(arr1);//4--pointer size
my_parameter2(arr);//4--pointer size
my_parameter2(arr1);//4--pointer size
return 0;
}
void my_parameter1(char string[])
{
printf("%d\n",sizeof(string));
}
void my_parameter2(char *string)
{
printf("%d\n",sizeof(string));
}
//(4)
char arr[]="zhiguang";
char *arr1="zhiguang";//constant
arr[2]='w';// ok
arr1[2]='w';//error, because it is constant
*(arr1+2)='w';//error, because it is constant
//(5)
int matrix[3][10];
matrix;//pointer, points to the first whole row, 10 elements
matrix+1;//pointer, points to the second whole row, 10 elements
*(matrix+1);// it is an array, also the points to the first element in second row;
*(matrix+1)+5;//pointer, points to matrix[1][5]
*(*(matrix+1)+5);//value, content of matrix[1][5]
*(matrix[1]+5);//value, content of matrix[1][5]
matrix[4,3]==matrix[3];//because of ','
//(6)
int vector[10], *vp=vector;//ok
int matrix[3][10], *mp=matrix;//error;
int (*p)[10]=matrix;//ok, p is a pointer first, points to an array with 10 elements;
int *pi=&matrix[0][0];//ok
int *pi=matrix[0];//ok
//(7)
// if the parameter mat is 2-D array, you can declare the function as:
void fun(int (*mat)[10]);//ok
void fun(mat[][10]);//ok
//however, the following is wrong
void func2(int **mat);//error
//(8)
char *keyword[5];// an array, each element is a pointer;
char const *keyword[]={"do","for","while"};
// the best way to determine the length of this array is:
sizeof(keyword)/sizeof(keyword[0]);
//(9)
void function0(int);
void function1(int);
void function2(int);
int main(void)
{
void (*f[3])(int) = {function0,function1,function2};
//f is first an array, each element is a pointer, pointing to a func with an int parameter
return 0;
}
15. string
//strlen returns an unsigned in, so if(strlen(x)>strlen(y)) //always right if(strlen(x)-strlen(y)>=0) //sometimes, error if(strlen(x)>=10)//always right if(strlen(x)-10>=0) //sometimes, error
16. void *
//any type pointer could be converted to void*, and that is why void *memcpy(void *dst, void const *src, size_t length); void *memmove(void *dst, void const *src, size_t length); void *memcmp(void *a, void const *b, size_t length); //...
17. structure
//(1)
struct{
int a;
int b;
}x;
struct SIMPLE{
int a;
int b;
};
struct SIMPLE x;
typedef struct {
int a;
int b;
} Simple;
Simple x;
//(2)
typedef struct{
int a;
struct SELF_REF3 *b;//error, there is no SELF_REF3 yet;
int c;
} SELF_REF3;
typedef struct SELF_REF3_TAG{
int a;
struct SELF_REF3_TAG *b;//OK, there is SELF_REF3_TAG already;
int c;
} SELF_REF3_TAG;
//(3)
// the following is error
struct A{
struct B *partner;
};
struct B{
struct A *partner;
}
//the following is correct
struct B;// declare first, even if it is not complete
struct A{
struct B *partner;
};
struct B{
struct A *partner;
18. memory
//(1)
void *malloc(size_t size);
void free(void *pointer);
// if malloc failed, it would return an NULL
//(2)
int *pi;
pi=malloc(25*sizeof(int));
if(pi==NULL)
{
printf("out of memory!\n");
exit(1);
}
//(3)
// the pointer passed to free must be generated by malloc, calloc, or realloc, not others;
//(4)
// simplest way of memory leak
for(int i=0; i<10;i++)
int *p=malloc(25*sizeof(int));
