Question

Write a C program to print the contents of a text file line-by-line but backwards so that the last line is printed first. The filename should be specified as a command-line argument.

Answer 1

For this problem, we need to have knowledge of command line arguments and file handling. Following is the brief overview of the code which is meant for the same.

• First, the file name passed as cmd arguments should be checked whether it is correct or not.
• Then as file reading is done from top to down, but we want it in the reverse order so we can just call the function recursively and print the line by line data in reverse order.
• For the recursive function, the base case would occur when we are unable to read anything i.e. at the end of the file.
• Otherwise, we need to call this function recursively and then print the current line read.

Following is the code for the same.

#include<stdio.h>
#include<stdlib.h>
void printInReverse(FILE *fp){
   // defining the variables for using the getline function.
   int len = 0;
    int read = 0;
   char *line = NULL;
   // using the getline function for finding reading line by line
   read = getline(&line, &len, fp);
   // if it is the last line then return
   if(read == -1){
      return;
   }else{
      // calling recursively for next lines
      // and then print the line
      printInReverse(fp);
      printf("%s
", line);
   }
}
int main(int argc,char* argv[])
{
   // storing the file_name in this variable
   char *file_name;
    int counter;
    if(argc==1)
       // if no file name is given
        printf("
No File Name passed");
    if(argc==2)
    {
        // if it is given
        file_name = argv[1];
      // file pointer
      FILE *fp;
      // opening the file
      fp = fopen(file_name, "r");
      // checking if the file is not opened properly
      if(fp == NULL){
         printf("File Not Present or wrong file given");
      }
      // calling the function for printing the lines in reverse order
      printInReverse(fp);
      // last print statement
      printf("Done Reading ! ! !");
      // closing the file
      fclose(fp);
    }else{
       printf("Command line arguments should not be other than file name");
   }
    return 0;
} 

The file which is taken as the input is as follows:

This is the first line
This is the second line
This is the third line
This is the forth line
.
.
.
This is the second last line
This is the last line

The snippet of the sample Output:

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Thanks!