使用 c 文件處理在 txt 文件的第 n 行之后添加一行文本

Question

我想在文本文件的第五行之后添加一行文本。這是我的代碼：

#include <stdio.h> 

int main(){
  FILE *f;
  f = fopen("file1.txt", "r");

  int nCount = 0;
  char c = fgetc(f);
  while(nCount != 5){
    if(c == '\n'){
      nCount++;
    }
    // c = fgetc(f);
    fseek(f, 1, SEEK_SET);
  }

  c = fgetc(f);

  fprintf(f, "I have appended a line");
  
 
  return 0;
}

沒有附加行。出了什么問題？

Answer 1

怎么了？

如果不移動所有后面的行，就不能在文件中間插入數據。

---

要添加一行，請考慮形成一個新文件。

// Pseudo code
Open input file
Open output file
nth_line_found = false
while ((len = read_a_chunk(input, buffer)) > 0) 
  if (!nth_line_found) {
    Count lines in buffer
    while counting, 
      if (count == nth) {
        write first part of buffer
        write "I have appended a line\n"
        write last part of buffer
        nth_line_found = true
    if (nth_line_found still false) write buffer
  } else {
    write buffer
  }

  If desired, delete input file and rename output to input.

這種方法意味着使用fread()而不是fgets()讀取以避免fgets() () 的長度（和null 字符）限制。

Answer 2

這只是一個草圖！

將shole文件讀入memory

fopen();
int lineno = 0;
while (fgets()) {
    addlinetomemory();
    lineno++;
    if (lineno == 42) addextraline();
}
fclose();

現在以寫入模式重新打開

fopen();
for (int line = 0; line < lineno; line++) {
    fputs(); // write line
}
fclose();

瞧！

Answer 3

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

int main(int argc, char *argv[])
{
FILE *f_in,*f_out;

f_in=fopen("file1.txt", "rt");
if(!f_in) return -1;
f_out=fopen("file2.txt", "wt");
if(!f_out) return -1;

char *oneLine = (char*) malloc(LINE_MAX);
if(oneLine){

    int lineCounter=0;

    while(fgets(oneLine,LINE_MAX,f_in)){
        lineCounter++;
        fputs(oneLine,f_out);
        if(lineCounter == 5)
            fputs("a new line\n",f_out);
    }

    fclose(f_in);
    fclose(f_out);
    free(oneLine);
    unlink("file1.txt");
    rename("file2.txt","file1.txt");
    return 0;
} else
    return -1;
}

Answer 4

如果您的文本文件大小合理（以適合內存），則以下內容適用於一個打開的文件...我將其作為練習來處理 CRLF 的 Windows/DOS 約定，其中 LF 也可以。 （這只是一個粗略的切割，但它可能會導致你得到你想要的結果。）

它可以對返回值進行更多測試，尤其是 fwrite() 調用。

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

int my_main() {
    char *fname = "log.txt";

    FILE *fp = fopen( fname, "rb+" );
    if( fp == NULL ) {
        fprintf( stderr, "Cannot open %s r/w\n", fname );
        return 1;
    }

    // measure space needed
    fseek( fp, 0, SEEK_END );
    size_t size = ftell( fp );
    fseek( fp, 0, SEEK_SET );

    // get space
    char *iobuf = (char*)malloc( size );
    if( iobuf == NULL ) {
        fprintf( stderr, "Cannot allocate %ld byte buffer\n", size );
        fclose( fp );
        return 1;
    }

    // load entire file
    size_t nread = fread( iobuf, sizeof(char), size, fp );
    if( nread != size ) {
        fprintf( stderr, "Expecting %ld - Read %ld\n", size, nread );
        return 1;
    }

    int lnCnt = 5; // skip past 5 lines
    for( char *cp = iobuf; cp < iobuf + size; cp++ )
        if( *cp == '\n' && --lnCnt == 0 )
            break;
    cp += 1;
    // Up to you to handle situation where #lines < 5

    // position after 5th '\n'
    fseek( fp, cp - iobuf, SEEK_SET );

    // write the new string
    char *newText = "I wish I was a fish\n"; // Written after whatever line
    fwrite( newText, sizeof newText[0], strlen( newText ), fp );

    // then write back the rest
    fwrite( cp, sizeof *cp, size - (cp - iobuf), fp );

    fclose( fp );

    return 0;
}

Output：

This is line 1
This is line 2
This is line 3
This is line 4
This is line 5
I wish I was a fish
This is line 6
This is line 7