如何使用命令行输入文件在 linux 上运行 C 代码？

Question

I'm trying to do some simple tasks in C and run them from the command line in Linux. I'm having some problems with both C and running the code from the command line with a given filename given as a parameter. I've never written code in C before.

Remove the even numbers from a file. The file name is transferred to the program as a parameter in the command line. The program changes this file.

How do I do these?

• read from a file and write the results over the same file
• read numbers and not digits from the file (ex: I need to be able to read "22" as a single input, not two separate chars containing "2")
• give the filename through a parameter in Linux. (ex: ./main.c file.txt)

my attempt at writing the c code:

#include <stdio.h>
int main ()
{
    FILE *f = fopen ("arr.txt", "r");
    char c = getc (f);
    int count = 0;
    int arr[20];
    while (c != EOF)
    {
        if(c % 2 != 0){
            arr[count] = c;
            count = count + 1;
        }
        c = getc (f);
    }


    for (int i=0; i<count; i++){
            putchar(arr[i]);
    }
    fclose (f);
    getchar ();
    return 0;
} 

Answer 1

I'd do that like this (removing extra declarations => micro optimizations)

  /**
   * Check if file is avaiable.
   */ 
  if (f == NULL)
  {
    printf("File is not available \n");
  }
  else
  {
    
    /**
     * Populate array with even numbers.
     */ 
    while ((ch = fgetc(f)) != EOF)
        ch % 2 != 0 ? push(arr, ch); : continue;

    /**
     * Write to file those numbers.
     */ 
    for (int i = 0; i < 20; i++)
        fprintf(f, "%s", arr[i]);
  }

Push implementation:

void push(int el, int **arr)
{
    int *arr_temp = *arr;

    *arr = NULL;
    *arr = (int*) malloc(sizeof(int)*(n - 1));

    (*arr)[0] = el;
    for(int i = 0; i < (int)n - 1; i++)
    {
        (*arr)[i + 1] = arr_temp[i];
    }
}

In order to write to the same file, without closing and opening it, you should provide both methods, w+ (writing and reading), and this method will clear it's content.

So, change the line where you open the file, for this.

FILE *f = fopen ("arr.txt", "w+");

You should look for ways of implementing dynamic arrays (pointers and memory management).

With this example you could simply go ahead and write yourself, inside the main loop, a temporary variable that stores a sequence of numbers, and stack those values

Something like this (pseudocode, have fun:)):

DELIMITER one of (',' | '|' | '.' | etc);

char[] temp;
if(ch not DELIMITER)
  push ch on temp;
else
  push temp to arr and clear it's content;

Hope this was useful.

Answer 2

Here's a complete program which meets your requirements:

• write the results over the same file - It keeps a read and write position in the file and copies characters towards the file beginning in case numbers have been removed; at the end, the now shorter file has to be truncated. (Note that with large files, it will be more efficient to write to a second file.)
• read numbers and not digits from the file - It is not necessary to read whole numbers, it suffices to store the write start position of a number (this can be done at every non-digit) and the parity of the last digit.
• give the filename through a parameter - If you define int main(int argc, char *argv[]) , the first parameter is in argv[1] if argc is at least 2.

#include <stdio.h>
#include <ctype.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
    if (argc < 2) return 1;   // no argument given
    FILE *f = fopen(argv[1], "rb+");
    if (!f) return 1; // if fopen failed
    // read, write and number position
    long rpos = 0, wpos = 0, npos = 0;
    int even = 0, c;  // int to hold EOF
    while (c = getc(f), c != EOF)
    {
        if (isdigit(c)) even = c%2 == 0;
        else
        {
            if (even) wpos = npos, even = 0;
            npos = wpos+1;    // next may be number
        }
        fseek(f, wpos++, SEEK_SET);
        putc(c, f);
        fseek(f, ++rpos, SEEK_SET);
    }
    ftruncate(fileno(f), wpos); // shorten the file
}