C 两个习题

练习 1 - 8 编写一个统计空格、制表符与换行符个数的程序。

/* ex08.c */
#include <stdio.h>

int main() {
    int c;
    long spacecnt = 0, tabcnt = 0, nlcnt = 0;
    
    while((c = getchar()) != EOF) {
        if ( c == ' ') {
            ++spacecnt;
        } else if (c == '\t') {
            ++tabcnt;
        } else if (c == '\n') {
            ++nlcnt;
        }
    }
    
    printf("spacecnt:\t%ld\n", spacecnt);
    printf("tabcnt:\t\t%ld\n", tabcnt);
    printf("newlinecnt:\t%ld\n", nlcnt);

    return 0;
}

编译运行结果如下：

$ ./ex08.out 
When you are old and    grey    and full of sleep,

And nodding by  the fire, take  down    this    book,


And slowly  read,   and dream       of the soft look,
spacecnt:   13
tabcnt:     12
newlinecnt: 6

练习 1 - 9 编写一个将输入复制到输出的程序，并将其中连续的多个空格用一个空格代替。

/* ex09.c */
#include <stdio.h>

int main() {
        int isspace = 0, c;

        while ((c = getchar()) != EOF) {
                if (c != ' ') {
                        isspace = 0;
                } else if (!isspace) {
                        isspace = 1;
                } else {
                        continue;
                } 
                putchar(c);
        } 

        return 0;
}

isspace 可以看成一个标志位，用来标识上一个字符是否是一个空格。
程序中用到了一个 continue 语句，while 循环中 continue 语句的执行意味着立即执行条件测试语句，而忽略当前循环中剩下的内容。
程序编译运行结果如下：

$ ./ex09.out 
When   you are old     and grey  and    full of    sleep,
When you are old and grey and full of sleep,

也可以声明两个变量，一个用来存储上一个字符的值，一个用来存储当前读入的字符，代码清单如下：

/* ex09_2.c */

#include <stdio.h>

int main() {
        int cur, pre;

        /* 为了便于第一个输入的字符比较，需要给 pre 赋非空格初值 */
        pre = EOF;
        while ((cur = getchar()) != EOF) {
                if (cur == ' ') {
                        if (pre == ' ') {
                                continue;
                        } 
                } 
                putchar(cur);
                pre = cur;
        }

        return 0;
}

编译运行结果：

$ ./ex09_2.out 
When you   are   old  and   grey  and   full of  sleep,
When you are old and grey and full of sleep,