c程序设计语言_习题1-9_将输入流复制到输出流，并将多个空格过滤成一个空格

　　Write a program to copy its input to its output, replacing each string of one or more blanks by a single blank.

　　编写这样一个程序，实现将输入流复制到输出流，但是要将输入流中多个空格过滤成一个空格。 

1.旗帜变量方法

#include <stdio.h>
 
int main(void)
{
  int c;
  int inspace;
 
　//这里用了旗帜变量来过滤多余空格
  inspace = 0;
  while((c = getchar()) != EOF)
  {
    if(c == ' ')
    {
      if(inspace == 0)
      {
        inspace = 1;
        putchar(c);
      }
    }
 
    /* We haven't met 'else' yet, so we have to be a little clumsy */
    if(c != ' ')
    {
      inspace = 0;
      putchar(c);
    }
  }
 
  return 0;
}

2.保存上一个输入字符

Chris Sidi writes: "instead of having an "inspace" boolean, you can keep track of the previous character and see if both the current character and previous character are spaces:"
Chris Sidi 写道：“我们可以不用‘inspace’这样一个布尔型旗帜变量，通过跟踪判断上一个接收字符是否为空格来进行过滤。”

#include <stdio.h>
 
/* count lines in input */
int
main()
{
        int c, pc; /* c = character, pc = previous character */
 
        /* set pc to a value that wouldn't match any character, in case
        this program is ever modified to get rid of multiples of other
        characters */
 
        pc = EOF;
 
        while ((c = getchar()) != EOF) {
                if (c == ' ')
                        if (pc != ' ')   /* or if (pc != c) */ 
                                putchar(c);
 
                /* We haven't met 'else' yet, so we have to be a little clumsy */
                if (c != ' ')
                        putchar(c);
                pc = c;
        }
 
        return 0;
}

3.利用循环进行过滤

Stig writes: "I am hiding behind the fact that break is mentioned in the introduction"!

#include <stdio.h>
 
int main(void)
{
        int c;
        while ((c = getchar()) != EOF) {
                if (c == ' ') {
                       putchar(c);
                       while((c = getchar()) == ' ' && c != EOF)
                               ;
               }
               if (c == EOF)
                       break; /* the break keyword is mentioned
                               * in the introduction... 
                               * */
 
               putchar(c);
        }
        return 0;
}