如何在行的开头而不是末尾插入一个字符？

Question

我是C的初学者，从本质上讲，我试图逐个读取char文件，并将字符回显到输出，但是，在每一行的开头都包含行号。我设法弄清楚如何计算行数，但是当我尝试插入行号时，我无法弄清楚如何将其插入下一行，而不是遇到换行符后立即插入。

这是我的代码：

int main() {
    int c, nl;

    nl = 1; 
    FILE *file;
    file = fopen("testWords.in", "r");

    if (file) {
        printf("%d. ", nl);
        while((c = getc(file)) != EOF) {
            if (c == '\n') {
                ++nl;
                printf("%d", nl);
            }
            printf("%c", c);
        }
        fclose(file);
    }
}

这是输出：

1. The Three Laws of Robotics:2
First: A robot may not injure a human being or, through inaction,3
   allow a human being to come to harm;4
Second: A robot must obey the orders given it by human beings5
   except where such orders would conflict with the First Law;6
Third: A robot must protect its own existence as long as7
such protection does not conflict with the First or Second Law;8
The Zeroth Law: A robot may not harm humanity, or, by inaction,9
    allow humanity to come to harm.10
    -- Isaac Asimov, I, Robot11

Answer 1

我相信您要在打印行号之前先打印换行符\n。您只需将打印字符行移至if语句上方即可解决此问题。

int main() {
    int c, nl;

    nl = 1; 
    FILE *file;
    file = fopen("testWords.in", "r");

    if (file) {
        printf("%d. ", nl);
        while((c = getc(file)) != EOF) {
            printf("%c", c);
            if (c == '\n') {
                ++nl;
                printf("%d", nl);
            }
        }
        fclose(file);
    }
}

无需过多更改，您可以通过记录以前的字符来防止打印多余的行号。等待打印行号，直到最后一个字符为\n并且您在新行上。这样，EOF将在打印无关的行号之前触发。

#include <stdio.h>

int main() {
    int c, nl, p;

    nl = 1;
    FILE *file;
    file = fopen("testWords.in", "r");

    if (file) {
        printf("%d. ", nl);
        while((c = getc(file)) != EOF) {
            if (p == '\n') {
                ++nl;
                printf("%d", nl);
            }
            p = c;
            printf("%c", c);
        }
        fclose(file);
    }
}

Answer 2

如何将其插入下一行，而不是遇到换行符后立即插入。

简单跟踪何时读取的字符是该行的第一个字符。

这很好地处理了没有行的文件以及最后一行不以'\n'结尾的文件。

    int nl = 0; 
    int start_of_line = 1;
    while((c = getc(file)) != EOF) {
      if (start_of_line) {
        printf("%d ", nl++);
      }
      printf("%c", c);
      start_of_line = (c == '\n');
    }