##1. Principle
- Array is NOT point
- Avoid using unsafe api
- Deeply understand point
##2. math
2.1 mod
if n%2k=x, then x=n&(2k-1)
2.2 even odd check
if the lowest bit of binary mode is 0, even number, otherwise odd.
2.3 multi
a*5 = a<<2+a
##3.structure
3.1 size
int arr[] = {1, 5, 3, 18, 107, 21, 88};
size of arr is sizeof(arr)/sizeof(arr[0])
struct _test_ {
int a;
double b;
char c;
}
size of _test_ is sizeof(_test_)
##4. Puzzle
4.1 Print the numbers between 1 and 1000.
- Add a rule: Don’t use any loops.
- Add another rule: No recursions.
- We can’t do that? OK. Please look at this one.
#include "stdio.h"
#define A(x) x;x;x;x;x;x;x;x;x;x;
int main (void){
int n = 1;
A(A(A(printf ("%d ", n++))));
return 0;
}
4.2 swap two variables
a = a^b;
b = a^b;
a = a^b;
##5. What we need to avoid using?
- gets –> fgets
- scanf –> scanf_s sscanf
- sprintf –> snprintf sprintf_s
- strcpy –> strlcpy
- strcat –> strlcat
- strtok –> strtok_r
##6. v[index] and index[v] are the same
##7. use _Generic macro
#include <stdio.h>
#include <stdlib.h>
void printchar(char X) { printf("char: %c\n", X); }
void printint(int X) { printf("int: %d\n", X); }
void printfloat(float X) { printf("float: %f\n", X); }
void printdouble(double X) { printf("double: %lf\n", X); }
#define print(X) _Generic((X), \
char: printchar, \
int: printint, \
float: printfloat, \
double: printdouble)(X)
int main()
{
print(1);
print(1.2);
print((char)'a');
return 0;
}
GCC version don’t support _Generic macro currently, but clang work.
Compile this code with clang as clang test.c
##8. Reference