Question

Write a C program that reads in the file and outputs it to another file with its lines in reverse order. Use the following array to store the addresses of the lines as you read them in: char *lines[200]; Save each line so that it occupies only enough memory required by that line. Use recursion. Hint: On each call of the recursive function, read ONE line into a LOCAL buffer.

Answer 1

#include <stdio.h>

#include <string.h>

#include <stdlib.h>

#define bufSize 1024

int myfunction(FILE*,FILE*);

int main()

{

FILE* fp1;

FILE* fp2;

  

if ((fp1 = fopen("program.txt", "r")) == NULL)

{

printf("Error in opening the file");

exit(1);

}

  

  

if ((fp2 = fopen("save.txt", "w")) == NULL)

{

printf("Error in opening the file");

exit(1);

}

myfunction(fp1,fp2);

}

int myfunction(FILE *fp1,FILE* fp2)

{

static char lines[bufSize];

static char rstring[bufSize];

int lenbuf,lenstr,counter=0;

if (fgets(lines, sizeof(lines), fp1) != NULL)

{

lines[strlen(lines) - 1] = '\0';

lenbuf=strlen(lines);

  

for(int i=lenbuf-1;i>=0;i--)

{

rstring[counter]=lines[i];

counter++;

}

rstring[counter]='\0';

  

fputs(rstring,fp2);

fputs("\n",fp2);

  

counter=0;

  

myfunction(fp1,fp2);

  

}

else

{

printf("The data is successfully saved in reverse order");

fclose(fp1);

fclose(fp2);

return 0;

}

  

}

==========================================================================

OUTPUT:

The data in program.txt file

The sata in save.txt after reversing.

===========================================================================

DESCRIPTION:

First of all I have created a local buffer bufSize with the help of #define to define the size of the line, and defined it as 1024, you may increment and decrement the size according to your wish.

#define bufSize 1024

Then there is main function which is creating 2 file pointers, because we need 2 files, 1 is for reading and another is for writing in reverse order.

so for reading we are using the file program.txt.

make sure the name of your file is also program.txt and it is stored in the same location as that of your c program. otherwise you need to provide the full path of program.txt. I have saved some data in program.txt. you may see it in screenshot.

another file save.txt is used to save the data in reverse order. this file is empty for now.

in main function I am opening both the files using file pointers fp1 and fp2. program.txt is opened in read mode whereas save.txt is opened in write mode. then we are checking if the pf1 and fp2==NULL to make sure that file has been successfully opened else it will exit the program if file not present or unable to open.

then I am calling myfunction() which will perform the task.

in myfunction first we are using fgets function to read each line from the file fp1 into a character variable lines which is of buffer size.

if the line is read then at the end of lines variable we are putting '\0', because the end of a tring must be denoted by '\0'

then we are using the strlen() to find the length of the string line. and store the length in strbuf variable.

then using for loop starting we are storing each character into fp2 means save.txt in reverse order. when it is done we are again calling myfunction()(recursion) to read next line.

if we are not able to read the line that means no new lines are present so it will come out of the function. by closing all the files.