用好printf和scanf

在C中，printf系列函数(fprintf, sprintf...)和scanf(fscanf, sscanf...)函数功能很强大，用好了的话，很多字符操作都可以很简单，甚至一些简单的正则匹配的工作也可以用scanf来完成，下面是几个例子。

1. printf：

a) 前导“0”：printf("%05d", n); // n=10, print: "00010"

b) 左对齐：printf("%-5d|%5d", n, n); // print: "10   |   10"

c) 自动类型前缀：printf("%#x", n); // print: "0xa"

d) 指定宽度：printf("%*d", w, n); // w=5, print: "   10"
                  printf("%*.*lf", 5, 1, f); // f=123.45, print: "123.5"

e) 取得输出长度： printf("%d%n", n, &len); // print: "10" and len=2, %n must be at the end of the formatting string

2. scanf：

// s="Used with 12,4.5|0x123 specifiers_the\n value.";

a) 基本用法：sscanf(s, "%s %s %d,%f|%x", s1, s2, &a, &f, &b); // s1="Used"; s2="with"; a=12; f=4.5; b=0x123

b) 跳过一些结果：sscanf(s, "%*s %s", s1); // %*s reads "Used" and ignores it. s1="with"

c) 限定长度：sscanf(s, "%10s", buf); // read at most 10 chars into buf.

d) 简单的正则匹配：sscanf(s, "%[a-z]", buf); // buf="with" not "Used"!
                    sscanf(s, "%3c", buf); // buf="Use"

e) 读入一整行： sscanf(s, "%[^\n]", buf); // buf="Used with 12,4.5|0x123 specifiers_the"