如何從C中的文本文件中刪除特定行？

Question

例子：

喬治 50 40
石灰 30 20
凱倫 10 60

do {
    printf("\nInput player name:[1..10] ");
    fgets(name,10,stdin);
}

輸入名稱：石灰

然后文本文件將是：

喬治 50 40
凱倫 10 60

Answer 1

嘗試這個：

 /* C Program Delete a specific Line from a Text File
 */
#include <stdio.h>

int main()
{
    FILE *fileptr1, *fileptr2;
    char filename[40];
    char ch;
    int delete_line, temp = 1;

    printf("Enter file name: ");
    scanf("%s", filename);
    //open file in read mode
    fileptr1 = fopen(filename, "r");
    ch = getc(fileptr1);
   while (ch != EOF)
    {
        printf("%c", ch);
        ch = getc(fileptr1);
    }
    //rewind
    rewind(fileptr1);
    printf(" \n Enter line number of the line to be deleted:");
    scanf("%d", &delete_line);
    //open new file in write mode
    fileptr2 = fopen("replica.c", "w");
    ch = 'A';
    while (ch != EOF)
    {
        ch = getc(fileptr1);
        //except the line to be deleted
        if (temp != delete_line)
        {
            //copy all lines in file replica.c
            putc(ch, fileptr2);
        }
        if (ch == '\n')
        {
            temp++;
        }
    }
    fclose(fileptr1);
    fclose(fileptr2);
    remove(filename);
    //rename the file replica.c to original name
    rename("replica.c", filename);
    printf("\n The contents of file after being modified are as follows:\n");
    fileptr1 = fopen(filename, "r");
    ch = getc(fileptr1);
    while (ch != EOF)
    {
        printf("%c", ch);
        ch = getc(fileptr1);
    }
    fclose(fileptr1);
    return 0;
}

參考 - http://www.sanfoundry.com/c-program-delete-line-text-file/

Answer 2

有幾種方法可以刪除一行，一種簡單的方法是打開兩個文件，一個進一個出。

然后逐行復制，完成后跳過要刪除的行，刪除舊文件並將新文件重命名為舊名稱。

fopen()
fgets()
fputs()
rename()
unlink()

編輯：上述解決方案適用於小文件，但根據注釋，它不適用於大文件，因此這里有一個替代解決方案（GCC C99），它讀取整個文件，找到名稱然后在此之后移動行在緩沖區中向前行。

#include <stdio.h>
#include <sys/stat.h>
#include <stdlib.h>
#include <stdbool.h>

static size_t deleteLine( char*, size_t, const char* );

int main(int argc, char* argv[])
{
  char file[] = "yourfile.txt";

  if ( --argc )
  { 
    struct stat st;
    if ( stat( file, &st ) != -1 )
    {
      // open the file in binary format
      FILE* fp = fopen( file, "rb" );
      if ( fp != NULL )
      {
        // allocate memory to hold file
        char* buffer = malloc( st.st_size ); 

        // read the file into a buffer
        if ( fread(buffer, 1, st.st_size, fp) == st.st_size)
        {
          fclose(fp);

          size_t newSize = deleteLine( buffer, st.st_size, argv[1] );

          fp = fopen( file, "wb" );
          if ( fp != NULL )
          {
            fwrite(buffer, 1, newSize, fp);
            fclose(fp);
          }
          else
          {
            perror(file);
          }
        }
        free(buffer);
      }
      else
      {
        perror(file);
      }
    }
    else
    {
      printf( "did not find %s", file );
    }
  }
  return 0;
}

static size_t deleteLine( char* buffer, size_t size, const char* playerName )
{
  // file format assumed to be as specified in the question i.e. name{space}somevalue{space}someothervalue\n
  // find playerName
  char* p = buffer; 
  bool done = false;
  size_t len = strlen(playerName);
  size_t newSize = 0;
  do
  {
    char* q = strchr( p, *playerName ); // look for first letter in playerName
    if ( q != NULL )
    {
      if ( strncmp( q, playerName, len ) == 0 ) // found name?
      {
        size_t lineSize = 1; // include \n already in line size

        // count number of characters the line has.
        for ( char* line = q; *line != '\n'; ++line) 
        {
          ++lineSize;
        }

        // calculate length left after line by subtracting offsets
        size_t restSize = (size_t)((buffer + size) - (q + lineSize));

        // move block with next line forward
        memmove( q, q + lineSize, restSize );

        // calculate new size
        newSize = size - lineSize;
        done = true;
      }
      else
      {
        p = q + 1; // continue search
      }
    }
    else
    {
      puts( "no such name" );
      done = true;
    }
  }
  while (!done);

  return newSize;
}

Answer 3

您無需創建新文件。 您可以使用r+打開原始文件並將其內容存儲到數組中（按照以下步驟操作）。 然后您可以使用 for 循環掃描數組中要跳過的行，並刪除該行。 然后，您可以使用fseek(filename,0,SEEK_SET) （重置文件的位置指示符）並使用 for 循環和 fprintf 將內容從修改后的數組復制到文件中來覆蓋文件的內容。 （但是，使用此方法，您需要在最后一個 for 循環中輸入一個額外的空行以覆蓋原始文件的最后一行。）

Answer 4

考慮一個像數組這樣的文件。 如果我們想刪除數組索引 i 處的元素，我們將所有元素從 i+1 向左移動一步，然后邏輯上減小數組的大小。

我在文件上應用相同的原則。 最后 ftruncate() 用於減小文件的大小，從而刪除文件的末尾部分。此代碼也適用於大文件，因為在任何時候都只有一個字節被存儲在內存中。

#include<errno.h>
#include<unistd.h>
#include<stdio.h>
/*
 * Description: fdelete() deletes 'bytes' bytes of data from the stream pointed to by fp. 
 *              bytes will be deleted from the CURRENT FILE POSITION.
 *              File must be opened in read + write mode while passing file pointer 
 *              to this function.
 *              File position before calling the function and after the 
 *              function returns will be the same.
 * Return Values: returns 0 on success and errno on failure. Kindly use perror("") 
 *                to print error if non-0 return value returned.
 */
int fdelete(FILE* fp, int bytes) {
    
    // to store each byte/char from file
    char byte;
    long readPos = ftell(fp) + bytes, writePos = ftell(fp), startingPos = writePos;
    // start reading from the position which comes after the bytes to be deleted
    fseek(fp, readPos, SEEK_SET);
    while (fread(&byte, sizeof(byte), 1, fp)) {
        // modify readPos as we have read right now
        readPos = ftell(fp);
        // set file position to writePos as we are going to write now
        fseek(fp, writePos, SEEK_SET);
        
        // if file doesn't have write permission
        if (fwrite(&byte, sizeof(byte), 1, fp) == 0) 
            return errno;
        // modify writePos as we have written right now
        writePos = ftell(fp);
        // set file position for reading
        fseek(fp, readPos, SEEK_SET);
    }

    // truncate file size to remove the unnecessary ending bytes
    ftruncate(fileno(fp), writePos);
    // reset file position to the same position that we got when function was called.
    fseek(fp, startingPos, SEEK_SET); 
    return 0;
}