Thread: Reversing a sentence.

Just wodering C code for reversing a sentence. I searched at this board but could not find it. Anybody knows where else can I find it?

For example.
If I input,

change is truth of life

output should be

life of truth is change

Please let me know.

Break it down into steps:

Read in the sentence
Identify were the words change
reverse them

functions that might help:
fgets()
strtok()

Other hints:
Using multiple arrays
Possibliy using a link list to store the location of each of the words after strtok()

Do a board search

Originally Posted by Prelude

He said he already did even though I know this question has be posed many times before. I've answered it quite a bit that I can remember.

I am aware that the OP claimed to have done a board search, but seriously, what were the search terms? They could not have been 'reverse a string' or 'reversing a string'



~/

Here you go...

Any changes/optimizations are welcome...

Code:

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


#define DELIM   ' '
#define NLINE	'\n'

#define EXCHANGE(a,b)	\
					a = a^b; \
					b = b^a; \
					a = a^b; 

void my_reverse(char *p,char *q);

                 
int main(void)                             
{
	char string[100];
	char *p,*walk;

	printf("Enter a string: ");
	fflush(stdout);

	if ( fgets(string,sizeof(string),stdin) != NULL )
	{
		if ( ( p = strchr(string,'\n')) != NULL )
			printf("\n<%s>",string);
		else
			exit(1);
	}
	else
	{
	printf("Error\n");
		exit(1);
	}

	my_reverse(string,(p-1));
	printf("\n<%s>",string);

	walk = string;
	while ( walk < p )
	{
		char *sep;
		if ( ( (sep = strchr(walk,DELIM)) != NULL ) || ( (sep = strchr(walk,NLINE)) != NULL ) )
		{
			my_reverse(walk,sep-1);
			while( sep++ == DELIM )
				;
			walk = sep;
		}
		else
			break;
	}
	printf("\n<%s>",string);
	return 0;	
}


void my_reverse(char *p,char *q)
{
	int i;
	
	i = (q-p);

	if (i%2)
		i = i/2+1;
	else
		i = i/2;

	while ( i-- )
	{
		EXCHANGE(*p,*q);
		p++;
		q--;
	}
}

is there any possible to solve it using recursion and without string.h?

i tried to solve using recursion, but failed.

#define EXCHANGE(a,b)\
a = a^b; \
b = b^a; \
a = a^b;

This a really bad way to do a swap. Search the boards and you'll find plenty of threads discussing it.

Meh I'm bored so here goes:
It uses an in place solution and pretty much does exactly what Prelude said, reverse the entire thing and then reverse each word. It avoids using an extra array and avoids using recursion since IMO recursion is not the best for this.

Code:

#include <stdio.h>
#include <string.h>

char * wordBasedStringRev(char *, int);
char * myStringRev(char *, char *);
void swap (char *, char *);

int main(void)
{
  char arr[] = "Hello my name is Thantos how are you today";
  puts("Before");
  puts(arr);
  wordBasedStringRev(arr, sizeof arr -1);
  puts("After");
  puts(arr);
  return 0;
}

char * wordBasedStringRev(char *str, int len)
{
  char *p=str, *q=NULL;
  myStringRev(str, str+len-1);
  while ( (q = strchr (p, ' ')) != NULL)
  {
    myStringRev(p,q-1);
    p = q + 1;
  }
  myStringRev(p, (str+len)-1);
  return str;
}

char * myStringRev(char *beg, char *end)
{
  int offset;
  for ( offset = 0; offset < ((end - beg) / 2)+1; offset++)
    swap (beg+offset, end-offset);

  return beg;
}

void swap (char *a, char *b)
{
  char c = *a;
  *a = *b;
  *b = c;
}