copy content of file in reverse order - c

Question

I need to write a program that is copying the content of a file to another file and reverses it.
I found an example and read it through to understand what is going on.
The problem is that my program has to use two functions:
void reverse(char line[]){
int i;
int length;
char tmp;
..
..
..
return;
}
(no further paramters or local variables)
The second function does the rest of the work(opens files, copies files, closes files)
The main program only reads the name of the files and calls the copy function.
#include<stdio.h>
#include<string.h>
void reverse(char line[])
{
int i;
int length;
char temp;
if (line == NULL)
return;
length = strlen(line);
for (i = 0 ; i < length / 2 + length % 2 ; ++i)
{
if (line[i] == line[length - i - 1])
continue;
temp = line[i];
line[i] = line[length - i - 1];
line[length - i - 1] = temp;
}
return;
}
int main()
{
FILE *src_fh, *dst_fh;
char src_fn[256+1], dst_fn[256+1];
printf("Enter Source File Name:\n");
fgets(src_fn, sizeof(src_fn), stdin); reverse(src_fn);
if( (src_fh = fopen(src_fn, "r")) == NULL )
{
printf("ERROR: Source File %s Failed To Open...\n",src_fn);
return(-1);
}
printf("Enter Destination File Name:\n");
fgets(dst_fn, sizeof(dst_fn), stdin); reverse(dst_fn);
if( (dst_fh = fopen(dst_fn, "w+")) == NULL )
{
fclose(src_fh);
printf("ERROR: Destination File %s Failed To Open...\n",dst_fn);
return(-2);
}
int ch;
while( (ch = fgetc(src_fh)) != EOF )
{
fputc(ch, dst_fh);
}
fclose(src_fh);
fclose(dst_fh);
return 0;
}

Answer 1

You only need to swap the first character with the last, the second with the pre-last, and so on.
You actually don't need the int temp variable, but since it seems to be required, here it is
void reverse(char line[])
{
int i;
int length;
char temp;
if (line == NULL)
return;
length = strlen(line);
for (i = 0 ; i < length / 2 + length % 2 ; ++i)
{
if (line[i] == line[length - i - 1])
continue;
temp = line[i];
line[i] = line[length - i - 1];
line[length - i - 1] = temp;
}
return;
}
This is an improved version, without int temp, instead we store the result of length / 2 + length % 2 so it's not recalculated on each iteration
void reverse(char line[])
{
int i;
int length;
int half;
if (line == NULL)
return;
length = strlen(line);
half = length / 2 + length % 2;
for (i = 0 ; i < half ; ++i)
{
if (line[i] == line[length - i - 1])
continue;
line[length] = line[i];
line[i] = line[length - i - 1];
line[length - i - 1] = line[length];
}
line[length] = '\0';
return;
}
just use the location of the terminating '\0' byte as the temp when swapping.
For the second function, read each line using fgets and write it to the file with fprintf, just remember to remove the newline character from the read strings, you can use the chomp function y posted for that, if you don't remove the newline, the reversed lines will have the newline at the beginning of the line.

Answer 2

The prameter name line in the prototype void reverse(char line[]) seems to give a hint, how the given exercise might be intended to be solved.
split the file in lines
reverse every line
reverse the order of the lines
Nevertheless you should watch out following this strategy, as there is still a really nasty gotcha involved, if your file may contain any data.
In this case you'll get in big trouble finding the end of line[] as '\0' termination might get confused with a literal '\0' in the line.
As a workaround you might try to replace any literal occurance of '/0' by the sequence '\0' 'x' and mark the end of your line by the sequence '\0' '-' or whatever before passing it to reverse() and reaversing the substitution after writing the reversed line to the file.
Unfortunately this attempt doesn't look too elegant, but maybe reversing a file the way it is meant to be done in the exercise isn't really elegant anyays.

Answer 3

the following code
1) incorporates proper error checking
2) outputs each input line, reversed, to the output file.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char* chomp(char* p)
{
int len;
if(!p) return(p);
if( (len=strlen(p))<=0 ) return(p);
if( p[len-1] == '\n' ) { p[--len] = '\0'; }
if( p[len-1] == '\r' ) { p[--len] = '\0'; }
return(p);
} // end function: chomp
int main()
{
/* Create Usable Variables */
FILE *src_fh = NULL;
FILE *dst_fh = NULL;
char src_fn[256+1] = {'\0'};
char dst_fn[256+1] = {'\0'};
char line[2048] = {'\0'};
/* Retrieve Source File Name From User */
printf("Enter Source File Name:\n");
if( NULL == (fgets(src_fn, sizeof(src_fn), stdin) ) )
{ // fgets failed
perror("fgets for input file name failed" );
exit(EXIT_FAILURE);
}
// implied else, fgets successful
chomp(src_fn); // remove trailing newline characters
/* Attempt Opening Source File For Reading */
if( (src_fh = fopen(src_fn, "r")) == NULL )
{
perror( "fopen failed" );
printf("ERROR: Source File %s Failed To Open...\n",src_fn);
return(-1);
}
// implied else, fopen source file successful
/* Retrieve Destination File Name From User */
printf("Enter Destination File Name:\n");
if( NULL == (fgets(dst_fn, sizeof(dst_fn), stdin) ) )
{ // then fgets failed
perror( "fgets for output file name failed" );
fclose(src_fh); // cleanup
exit( EXIT_FAILURE );
}
// implied else, fgets for output file name successful
chomp(dst_fn); // remove trailing newline characters
/* Attempt Opening Destination File For Writing */
if( NULL == (dst_fh = fopen(dst_fn, "w")) )
{
perror( "fopen for output file failed" );
fclose(src_fh); // cleanup
printf("ERROR: Destination File %s Failed To Open...\n",dst_fn);
return(-2);
}
// implied else, fopen for output file successful
int index;
/* Copy Source File Contents (reversed, line by line) to destination file */
while( NULL != (fgets(line, sizeof(line), src_fh) ) )
{
chomp(line); // remove trailing newline characters
index = strlen(line) - 1; // -1 because arrays start with offset 0
// and strlen returns offset to '\0'
// output reversed line to file
while( index >= 0 )
{
fputc( line[index], dst_fh );
index--;
} // end while
fputc( '\n', dst_fh );
} // end while
/* Close Files On Success */
fclose(src_fh);
fclose(dst_fh);
return 0;
} // end function: main