c - 打印文件每行的 m 到 n 列

Question

我正在从一本书上学习 C，其中一个练习如下:

Write a program that writes columns m through n of each line of a file to stdout. Have the program accept the values of m and n from the terminal window.

经过几个小时的尝试，我无法省略 n 之后的字符，然后转到下一行并开始搜索列号 m。我的代码的输出也不正确，我已经研究了两个多小时，但不知道出了什么问题或如何修复它。我的测试文件的内容是:

abcde
fghij
klmno
pqrst
uvwxyz

我得到的输出是

bc

我该怎么办？

我也不太喜欢我实现该程序的方式(有两个不同的 while 循环来测试 (c = getc(text)) != EOF。这一切似乎都太过分了让我很困惑，但我不知道我能做些什么来解决它

#include <stdio.h>
#include <stdlib.h> 

int main(int argc, char *argv[])
{
    // Ensure correct usage
    if (argc != 4)
    {
        fprintf(stderr, "Usage: ./program <file> <from> <to>\n");
        return 1;
    }

    FILE *text;

    int counter = 0, lines = 0, done = 0;
    int m = atoi(argv[2]);
    int n = atoi(argv[3]);
    char c;

    // Return if file is NULL
    if ((text = fopen(argv[1], "r")) == NULL)
    {
        fprintf(stderr, "Could not open %s.\n", argv[1]);
        return 2;
    }


    // Write columns m through n of each line
    while ((c = getc(text)) != EOF)
    {
        ++counter;

        if (c == '\n')
            ++lines;

        if (counter >= m && counter <= n && done == lines)
        {
            putc(c, stdout);
            ++counter;          

            if (counter == n)
            {
                ++done;

                while ((c = getc(text)) != EOF)
                {
                    if (c != '\n')
                        continue;

                    else
                    {
                        counter = 0;
                        ++lines;
                        putc(c, stdout);    
                    }
                }
            }                   
        }
    }

    return 0;
}

Answer 1

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    // Ensure correct usage
    if (argc != 4)
    {
        fprintf(stderr, "Usage: ./program <file> <from> <to>\n");
        return 1;
    }

    FILE *text;

    int counter = 0;
    int m = atoi(argv[2]);
    int n = atoi(argv[3]);
    char c;

    // Return if file is NULL
    if ((text = fopen(argv[1], "r")) == NULL)
    {
        fprintf(stderr, "Could not open %s.\n", argv[1]);
        return 2;
    }

    // Write columns m through n of each line
    while ((c = getc(text)) != EOF)
    {
        ++counter;

        if (counter >= m)
        {
            putc(c, stdout);
            if (counter == n)
            {
                while ((c = getc(text)) != EOF)
                {
                    if (c == '\n')
                    {
                        counter = 0;
                        putc(c, stdout);
                        break;
                    }
                    ++counter;
                }
            }
        }
    }
    putc('\n', stdout);
    return 0;
}