【书中示例】--《C程序设计语言》第二章：类型、运算符、表达式

/*  string(),返回字符串s的长度  */
int string(char s[])
{
	int i;

	for (i = 0;s[i] != '\0';i++)
		;
	return i;
}



/*  判断是否为闰年  */
if ((0 == year % 4 && 0 != year % 100) || 0 == year % 400)
printf("%d is a leap year", year);//是闰年



/*  atoi()，将字符串s转换为对应的int型  */
//使用此函数有个前提：字符串s中全是数字，无其他符号；否则转化的整形会有截断
int atoi(char s[])
{
	int i, n;

	n = 0;
	for (i = 0;s[i] >= '0'&&s[i] <= '9';++i)
		n = n * 10 + (s[i] - '0');

	return n;
}
/*++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
1、s[i]-'0' 可以计算出该字符所对应的数字值
2、此函数功能的缺陷：
   1.数字是无符号数
   2.字符串中无法有空白符
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++*/


/*  lower()，把大写字母转化成小写字母 (只限与ASCII字符集)  */
//如果该字符不是大写字母，则返回该字符本身
char lower(char c)
{
	if (c >= 'A'&&c <= 'Z')
		return c - 'A' + 'a';//将大写字母变成小写字母
	else
		return c;
}
/*+++++++++++++++++++++++++++++++++++++++++++++++++++++++
1.if (c >= '0'&&c <= '9') 可以用函数isdigit(c)
2.大写字母变小写字母 可以用函数tolower(c)
3.以上两个函数都在头文件<ctype.h>中
++++++++++++++++++++++++++++++++++++++++++++++++++++++++*/


/*  rand(),生成伪随机数  */
//next全局变量为种子
unsigned long int next = 1;
int rand(void)
{
	next = next * 1103515245 + 12345;
	return (unsigned int)(next / 65536) % 32768;
}
/*  srand(),为rand()设置随机种子  */
void srand(unsigned int seed)
{
	next = seed;
}



/*  squeeze(s,c),删除字符串s中出现的所有字符c  */
void squeeze(char s[], char c)//传入了字符串头指针，在函数内的操作，会对字符串造成改变（引用传递）
{
	int i, j;

	for (i = j = 0;s[i] != '\0';i++)
		if (s[i] != c)
			s[j++] = c;
	s[j] = 0;
}
/*++++++++++++++++++++++++++++++++++++++++++++++++++++
1.s[j++]=c;相当于：
  s[j]=c;
  j++;
2.适当运用前置、后置运算符++ -- ，可以使代码更加简洁明了
+++++++++++++++++++++++++++++++++++++++++++++++++++++++*/



/*  srecat(),将字符串t接到字符串s的尾部；假设s的空间足够大  */
void srecat(char s[], char t[])
{
	int i, j;

	i = j = 0;
	while (s[i] != '\0')//找到字符串s的末尾
		++i;
	while ((s[i++] = t[j++]) != '\0')//拷贝t
		;
}
/*+++++++++++++++++++++++++++++++++++++++++++++++++++++++
1.注意拷贝操作：
  s[i++]=t[j++];
2.赋值运算符返回它左边的值.
++++++++++++++++++++++++++++++++++++++++++++++++++++++++*/