C语言中一些很酷的技巧（cool tricks）

来自Quora，觉得不错，就实践了一下。

1.  #if 0 ...... #endif 块中的内容不会被编译，因为注释不允许嵌套，我们可以把暂时不用的代码块放在
这里面。
2. 数组初始化的时候可以指定索引，而且可以给特定范围的数组赋值。
比如  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};
示例：

#if 0here will not enter into compilewe 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，而且可以超越每次读到空白就终止的限制。
示例：

#include<stdio.h>#define MAX 100int  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。
示例：

#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]。
 示例：

#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)可以作为 return 0 的替代；

示例：

#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;

示例：

<pre name="code" class="cpp">#include <stdio.h>// ??// implicit initiliazation of variables with 0 and 1g;main(i){    printf("%d,%d\n", g, i);         return 0;}