C语言中一些非常酷的技巧（cool tricks）

来自Quora，认为不错，就实践了一下。

1. #if 0 ...... #endif 块中的内容不会被编译，由于凝视不同意嵌套，我们能够把临时不用的代码块放在

这里面。

2. 数组初始化的时候能够指定索引，并且能够给特定范围的数组赋值。

比方 int array[] = { [0 ... 9] = 1, [10 ... 20] = 2, [30 ... 40] = 3};

等价于 int array[] = { 1, 1, 1, 1, ..... 2, 2, 2 ...... 3, 3, 3};

演示样例：

#if 0

here will not enter into compile
we can move the code we not need now here

#endif

#include<stdio.h>

int  main(){
int i = 0;
int arr[] = {[1]=5, [5]=10, [2]=20};
int arr2[] = {[0 ... 9] = 10, [10 ... 19] = 20, [20 ... 29] = 30};
for(i = 0; i < 6; i++)
printf("arr[%d] = %d ,", i, arr[i]);
printf("\n");
for(i = 0; i < 30; i++)
printf("arr2[%d] = %d ,", i, arr2[i]);
return 0;
}

3. 要巧妙使用scanf，支持正则式，并且能够超越每次读到空白就终止的限制。

演示样例：

#include<stdio.h>
#define MAX 100

int  main(){
int i = 0;
char s1[MAX],s2[MAX];

//scanf("%[^\n]\n",s1);
//read till meet '\n',and then trash the \n

//scanf("%[^,]",s1); //?? also will  trash the coma
//scanf("%[^,],",s1); // this does not trash the coma

//this * can make us skip some input
//for example,we just care the last-name
scanf("%*s %s",s1);

printf("%s\n",s1);

return 0;
}

4. 理解offset宏定义，求一个成员在结构体中的偏移量。

演示样例：

#include <stdio.h>
#include <stdlib.h>

#define offset(TYPE, MEMBER) ((size_t)(&((TYPE *)0)->MEMBER))

int main(){
struct test{
int a;
int b[4];
int c;
};

printf("offset a : %lu\n", offset(struct test, a));
printf("offset b : %lu\n", offset(struct test, b));
printf("offset c : %lu\n", offset(struct test, c));

struct test *t = (struct test *)0;
printf("%d\n", &(t->c));  // right
printf("%d\n", t->c);  //?but cannot do this, core dump

return 0;
}

5. s[i] 是 *(s + i) 的语法糖，所以等价于 i[s]。

演示样例：

#include <stdio.h>

int main(){
char s[] = "vonzhou";
printf("%c\n", 2[s]);

return 0;
}

6. 再次是printf 和 scanf 的技巧。

#include <stdio.h>

int main(){

int n = 6;
int val = 1000;
char s[100];
printf("%*d", n, val);// with a minimum width of n,default right aligned
printf("hello\n");

//scanf("%*d");//read an integer and ignore it

//scanf has regex built into it
//read only selected chars into a string
//scanf("%[abcd]s", s);

//read everything excepted heading with abcd
//scanf("%[^abcd]s" ,s);

/* some complain that scanf reads upto a whitespace,
so this trick can be used to consume everything upto a newline */
scanf("%[^\n]s", s);
// reads all chars (including whitespaces) till newline is encountered.

printf("s = %s\n", s);
/* Another trick with scanf is to consume what is required.
For example - If  a user enters his date of birth in YYYY-MM-DD
then you can directly read the year month and date into integer variables */
//scanf("%d-%d-%d", &yy, &mm, &dd);
/* where yy, mm, dd are variables.
Note that this will work only with that format of input.
Anything else like YYYY/MM/DD,and the program will mostly crash.
You specify a hyphen, it expects only a hyphen! */

return 0;
}

7.利用scanf的返回值来识别是否到达文件尾EOF；当errno=0时%m格式会输出“Success”;brk(0)能够作为 return 0 的替代；
演示样例：

#include <stdio.h>

int main(){
int n;
//util reach EOF(ctrl + D)
while(~scanf("%d",&n)){
//logic process
printf("%d^2 = %d\n", n, n*n);
}

// %m print success only if errno = 0
printf("%m\n");

brk(0);
}

7.变量会被隐式初始化为0和1;
演示样例：

<pre name="code" class="cpp">#include <stdio.h>

// ??
// implicit initiliazation of variables with 0 and 1
g;main(i){

printf("%d,%d\n", g, i);

return 0;
}