从 C 编程中的文本文件打印特定行

Question

I was wondering if there was any way for me to print a specific line from a text file in C? Do I need to create an array?

Answer 1

#include <stdio.h>
#include <stdbool.h>
#include <string.h>
  
int main() {
    FILE *fin = fopen("1.txt", "r");
    FILE *fout = stdout; // for testing
  
    char line[1000];
    bool print_line = false;
  
    while (fgets(line, sizeof line, fin) != NULL) {
        if (print_line) {
            fputs(line, fout);
            print_line = false;
        }
        if (strstr(line, "PROLIST") != NULL)
            print_line = true;
    }
  
    fclose(fin);
    return 0;
}

Answer 2

To print a specific line from a text file in C, you can use an array but you do not have to.

The easiest method involves reading one line at a time while counting them and output the required line when the count is right. To read a line, you would define an array of char and use fgets() .

There are some potential problems if some of the lines are very long as arrays must be defined or allocated with a specific length and lines from the file can always be longer than this arbitrary length. This problem can be addressed by testing if the line read by fgets() does end with a newline.

You could use getline() , available on many systems, that reallocates the array to accommodate for the length of the line it reads from the file. It remains possible to run out of memory if the line is longer than available memory, which would be the case for /dev/zero .

An alternative approach where no array is needed is this: read one byte at a time, counting newlines and output the byte if the line count is right.

Here is a simple method using an array of char and fgets() :

#include <stdio.h>
#include <string.h>

#define LINE 12   // output line number 12

int main() {
    long line = 1;
    char line[256];  // arbitrary length

    while (line < LINE && fgets(line, sizeof line, stdin)) {
        if (line == LINE)
            fputs(line, stdout);
        // only increase the line count if line is complete
        if (strchr(line, '\n'))
            line++;
    }
    return 0;
}

Here is a simpler alternative without an array:

#include <stdio.h>

#define LINE 12   // output line number 12

int main() {
    long line = 1;
    int c;

    while (line < LINE && (c = getchar()) != EOF) {
        if (line == LINE)
            putchar(c);
        if (c == '\n')
            line++;
    }
    return 0;
}