Reverses lines from a file given parameter - c

Question

I have to write a function void reverse_file(FILE* file) that reads lines from the parameter file one line at the time into a local character array buffer whose size is #defined as a preprocessor constant MAXLINE. For each line, this function calls the previous reverse_string function to reverse the line, then writes the reversed line in standard output.
I have the reverse_string method
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void reverse_String(char* str);
int main()
{
char str [256];
strcpy_s(str, "Hello");
reverse_String(str);
return 0;
}
void reverse_String(char* str)
{
int i, j;
char temp;
i=j=temp=0;
j=strlen(str)-1;
for (i=0; i<j; i++, j--)
{
temp=str[i];
str[i]=str[j];
str[j]=temp;
}
printf("%s",str);
}
I just need help with the reverse_file(FILE* file) part.
Any help is appreciated.

Answer 1

Maybe something like this?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void reverse_String(char* str)
{
int i, j;
char temp;
i=j=temp=0;
j=strlen(str)-1;
for (i=0; i<j; i++, j--) {
temp=str[i];
str[i]=str[j];
str[j]=temp;
}
printf("%s",str);
}
void reverse_File(FILE* file) {
char* line;
size_t len = 0;
ssize_t read;
while ((read = getline(&line, &len, file)) != -1) {
reverse_String(line);
}
}
int main(void) {
FILE * fp;
fp = fopen("Maze.txt", "r");
if (fp == NULL)
exit(EXIT_FAILURE);
reverse_File(fp);
}

Answer 2

You already have a fine answer, but there are a few subtle points to consider. If you are using line-oriented input methods (fgets or getline), you should remove the '\n' or '\r\n' (DOS/windows) line-ending read and included in the original line by either fgets or getline. (this is why you have an initial blank line in the output of the original file).
A simple way to remove the line-endings is to simply overwrite the first of either line-ending encountered with a nul-terminating character. To do this, your main program logic will look like:
while (fgets (str, MAXC, fp)) { /* read each line */
rmcrlf (str); /* trim \r or \n */
strrev (str); /* reverse string */
printf ("%s\n", str); /* output reverse */
}
Your rmcrlf (remove carriage-return line-feed) could look like the following:
/** stip trailing newlines and carraige returns by overwriting with
* null-terminating char. 's' is modified in place.
*/
void rmcrlf (char *s)
{
if (!s || !*s) return; /* validate not NUL and not empty */
char *p = strpbrk (s, "\r\n"); /* locate first line-ending char */
*p = 0; /* set to nul-terminating char */
}
(note: you could use strlen (or a pointer and loop) to find the end of the string and work backwards, but strpbrk allows you to search for either '\n' or '\r' in the forward direction without needing to find the end and work backwards)
You can then reverse your string in place in the same way (but using strrchr to find the end instead of strlen)
/** strrev - reverse string 's' in place.
* The original string is not preserved.
* If 's' is not valid, no action taken.
*/
void strrev (char *s)
{
if (!s || !*s) { /* validate s not NUL and not empty */
printf ("strrev() error: invalid string\n");
return;
}
char tmp;
char *begin = s, *end = strrchr (s, 0) - 1;
while (end > begin) {
tmp = *end;
*end-- = *begin;
*begin++ = tmp;
}
}
You can put all the pieces together as follows. This is simply another way to approach the same task. Note: the following code uses an enum instead of a #define to set define the constant (just to show the alternate method). Also note: that the code will read from the file given as the first argument (or from stdin by default, if no filename is given):
#include <stdio.h>
#include <string.h>
enum { MAXC = 256 };
void strrev (char *s);
void rmcrlf (char *s);
int main (int argc, char **argv) {
char str[MAXC] = "";
FILE *fp = argc > 1 ? fopen (argv[1], "r") : stdin;
if (!fp) { /* validate file is open */
fprintf (stderr, "error: file open failed '%s'.\n", argv[1]);
return 1;
}
while (fgets (str, MAXC, fp)) { /* read each line */
rmcrlf (str); /* trim \r or \n */
strrev (str); /* reverse string */
printf ("%s\n", str); /* output reverse */
}
if (fp != stdin) /* close file (if not stdin) */
fclose (fp);
return 0;
}
/** strrev - reverse string 's' in place.
* The original string is not preserved.
* If 's' is not valid, no action taken.
*/
void strrev (char *s)
{
if (!s || !*s) { /* validate s not NUL and not empty */
printf ("strrev() error: invalid string\n");
return;
}
char tmp;
char *begin = s, *end = strrchr (s, 0) - 1;
while (end > begin) {
tmp = *end;
*end-- = *begin;
*begin++ = tmp;
}
}
/** stip trailing newlines and carraige returns by overwriting with
* null-terminating char. 's' is modified in place.
*/
void rmcrlf (char *s)
{
if (!s || !*s) return;
char *p = strpbrk (s, "\r\n");
*p = 0;
}
Example Input
$ cat dat/strtorev.txt
abc def ghi jkl mno pqr
ABC DEF GHI JKL MNO PQR
Output
$ ./bin/strrev_line <dat/strtorev.txt
rqp onm lkj ihg fed cba
RQP ONM LKJ IHG FED CBA
(note: as Johnathan asked in the comment, you can either reverse the line as shown above, or you can reverse each word (e.g. cba fed ihg...) make sure you have correctly stated what you need to do, if not we can help further).
Let me know if you have any questions.