5/30/08

C Example to Reverse a file

This an example how to reverse a file. It is based on recursion.
The idea is to read all the file and when you reach the end(EOF) print them.


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

void reverse(FILE * file) {
int fscanf_return_value;
char x;
/* read a char */
fscanf_return_value = fscanf(file,"%c",&x) ;
if(fscanf_return_value == EOF) { //fscanf returns EOF as the END OF FILE
return;
}
reverse(file); //Get the next char
//do something with the char e.g. print
putchar(x);
return;
}

int main(int argc, char *argv[]) {
int i;
FILE *fd;
if (argc!=2) {
printf("Usage : \n %s FILENAME\n",argv[0]);
exit(0);
}
if(!(fd=fopen(argv[1],"r"))) {
printf("Opening file error\n");
exit(1);
}
reverse(fd);
printf("\n\n\t---\tenoD - Done\t---\n");
close(fd);
exit(0);
}

To run it just give it argument the name of the file you want.
For example if the code above is in a file named reverse.c
and compiled with the command

gcc reverse.c -o reverse

just type in the konsole

./reverse reverse.c

and you will get the output bellow.
If you want to save it to another file the easiest way to do it
is to execute

./reverse reverse.c >> output.txt

Example of output is:

}
;)0(tixe
;)df(esolc
;)"n\---t\enoD - Donet\---t\n\n\"(ftnirp
;)df(esrever
}
;)1(tixe
;)"n\rorre elif gninepO"(ftnirp
{ )))"r",]1[vgra(nepof=df(!(fi
}
;)0(tixe
;)]0[vgra,"n\EMANELIF s% n\ : egasU"(ftnirp
{ )2=!cgra( fi
;df* ELIF
;i tni
{ )][vgra* rahc ,cgra tni(niam tni

}
;nruter
;)x(rahctup
tnirp .g.e rahc eht htiw gnihtemos od//
rahc txen eht teG// ;)elif(esrever
}
;nruter
ELIF FO DNE eht sa FOE snruter fnacsf// { )FOE == eulav_nruter_fnacsf(fi
; )x&,"c%",elif(fnacsf = eulav_nruter_fnacsf
/* rahc a daer */
;x rahc
;eulav_nruter_fnacsf tni
{ )elif * ELIF(esrever diov

>h.bildts< edulcni#
>h.oidts< edulcni#

--- enoD - Done ---

I think it is a simple to understand example.
Another way to do it is to use fseek(FILE,SEEK_END) to move the pointer in the file at the end and then start reading a char,do something with it,use fseek(FILE,-1,SEEK_CUR) to move the pos one char back and do this untill you reach the SEEK_SET.
i hope this was helpful. Please post any comments you have.
Del.icio.us

2 comments:

sanju said...

/* HOPE THIS CODE ALSO WORKS FINE:)*/
//http://www.facebook.com/sanju.uthaiah

#include
#include
#include
#include
int main(int argc,char **argv)
{
if(argc>1)
{
int fd,fil_size,j;
char c,filname;
clrscr();
fd=open(argv[1],O_RDONLY);//open file in read only mode

if(fd==-1)
{
printf("FILE NOT FOUND\n");
exit(-1);
}

if((fil_size=lseek(fd,SEEK_SET,SEEK_END))==0) //make the file pointer to point to last
{
printf("NOTHING TO DISPLAY\nTHE SPECIFIED FILE IS EMPTY\n");
exit(-1);
}

j=fil_size;
printf("FILE Pointer is in Last location(%d)\n\n",fil_size);//display the file size (that is the last location of file pointer gives the size of the file);
while(j!=-1)//traverse from last location of file to first
{
//printf("File Pointer is in %d location\n\n",s);
fil_size=lseek(fd,j-1,SEEK_SET);//each time set the file pointer to j-1th location
read(fd,&c,1);//read a character from a file
printf("%c",c); //print a char to std outpt
// printf("%d th character ::%c\n",j,c);
j--;
}
}
else
{
printf("\n::NO FILE NAME SUPPLIED::\n");
return -1;
}
getch();
return 0;
}

Anonymous said...

Thanks Andreas..
Great work Sanju...

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