Write a program to remove all trailing blanks and tabs from each line of input, and to delete entirely blank lines.
其实做这道题目有两种思路：
1.后向模式：利用getline()先将输入流中，每一行完全接收，然后从接收的line字符串中末尾，往前扫，直到发现第一个非空格和制表符字符；
2.前向模式：每接收一个字符，都要进行输出、判断。
/* K&R2 1-18 p31: Write a program to remove trailing blanks and tabs from each line of input, and to delete entirely blank lines. The program specification is ambiguous: does "entirely blank lines" mean lines that contain no characters other than newline, or does it include lines composed of blanks and tabs followed by newline? The latter interpretation is taken here. This implementation does not use any features not introduced in the first chapter of K&R2. As a result, it can't use pointers to dynamically allocate a buffer to store blanks that it has seen, so it must limit the number of blanks that are allowed to occur consecutively. (This is the value of MAXQUEUE, minus one.) It is intended that this implementation "degrades gracefully." Even though a particular input might have 1000 or more blanks or tabs in a row, causing a problem for a single pass, multiple passes through the file will correct the problem. The program signals the need for such an additional pass by returning a failure code to the operating system. (EXIT_FAILURE isn't mentioned in the first chapter of K&R, but I'm making an exception here.) */#include#include #define MAXQUEUE 1001int advance(int pointer){ if (pointer < MAXQUEUE - 1) return pointer + 1; else return 0;}int main(void){ char blank[MAXQUEUE]; int head, tail; int nonspace; int retval; int c; retval = nonspace = head = tail = 0; while ((c = getchar()) != EOF) { if (c == '\n') { head = tail = 0; if (nonspace) putchar('\n'); nonspace = 0; } else if (c == ' ' || c == '\t') { //能执行这个if，只能说明输入行中字符溢出了。 if (advance(head) == tail) { putchar(blank[tail]); tail = advance(tail); nonspace = 1; retval = EXIT_FAILURE; } //只要遇到空格和制表符，就先存起来 blank[head] = c; head = advance(head); } else { //一次性把前面积攒的空格和制表符输出来 while (head != tail) { putchar(blank[tail]); tail = advance(tail); } putchar(c); nonspace = 1; } } return retval;}
Chris Sidi writes:
Ben,I thought your solution to 1-18 was really neat (it didn't occur to mewhen I was doing the exercise), the way it degrades gracefully andmultiple passes can get rid of huge blocks of whitespace.However, if there is a huge block of non-trailing whitespace (eg "A",2000spaces, "B\n") your program returns an error when there's not a need forit. And if someone were to use your program till it passes it will loopinfinitely: $ perl -e 'print "A"," "x2000,"B\n";' > in $ until ./a.out < in > out; do echo failed, running another pass; cp out in; done failed, running another pass failed, running another pass failed, running another pass [snip]Below I have added a variable spaceJustPrinted to your program and checkto see if the spaces printed early are trailing. I hope you like theminor improvement. (Though I can understand if you don't give a [1] :))
[1] expletive deleted - RJH.
/* K&R2 1-18 p31: Write a program to remove trailing blanks and tabs from each line of input, and to delete entirely blank lines. The program specification is ambiguous: does "entirely blank lines" mean lines that contain no characters other than newline, or does it include lines composed of blanks and tabs followed by newline? The latter interpretation is taken here. This implementation does not use any features not introduced in the first chapter of K&R2. As a result, it can't use pointers to dynamically allocate a buffer to store blanks that it has seen, so it must limit the number of blanks that are allowed to occur consecutively. (This is the value of MAXQUEUE, minus one.) It is intended that this implementation "degrades gracefully." Even though a particular input might have 1000 or more trailing blanks or tabs in a row, causing a problem for a single pass, multiple passes through the file will correct the problem. The program signals the need for such an additional pass by returning a failure code to the operating system. (EXIT_FAILURE isn't mentioned in the first chapter of K&R, but I'm making an exception here.) */#include#include #define MAXQUEUE 1001int advance(int pointer){ if (pointer < MAXQUEUE - 1) return pointer + 1; else return 0;}int main(void){ char blank[MAXQUEUE]; int head, tail; int nonspace; int retval; int c; int spaceJustPrinted; /*boolean: was the last character printed whitespace?*/ retval = spaceJustPrinted = nonspace = head = tail = 0; while ((c = getchar()) != EOF) { if (c == '\n') { head = tail = 0; if (spaceJustPrinted == 1) /*if some trailing whitespace was printed...*/ retval = EXIT_FAILURE; if (nonspace) { putchar('\n'); spaceJustPrinted = 0; /* this instruction isn't really necessary since spaceJustPrinted is only used to determine the return value, but we'll keep this boolean truthful */ nonspace = 0; /* moved inside conditional just to save a needless assignment */ } } else if (c == ' ' || c == '\t') { if (advance(head) == tail) { putchar(blank[tail]); /* these whitespace chars being printed early are only a problem if they are trailing, which we'll check when we hit a \n or EOF */ spaceJustPrinted = 1; tail = advance(tail); nonspace = 1; } blank[head] = c; head = advance(head); } else { while (head != tail) { putchar(blank[tail]); tail = advance(tail); } putchar(c); spaceJustPrinted = 0; nonspace = 1; } } /* if the last line wasn't ended with a newline before the EOF, we'll need to figure out if trailing space was printed here */ if (spaceJustPrinted == 1) /*if some trailing whitespace was printed...*/ retval = EXIT_FAILURE; return retval;}