Thread: reversing a string

Hi, all--
I'm writing a program to reverse a string word by word, e.g. "some words" becomes "words some", and I'm having some trouble getting it to ignore the null portion of the array. Here's my code:

Code:

int main()
{
	char *wordptr[81], s1[81], s2[81];
	int i = 0;
	printf("Input: ");
	fgets(s1, 80, stdin);
	wordptr[i] = strtok(s1," \n");
	i++;
	
	while(strtok(NULL, " \n") != NULL)
	{
		wordptr[i] = strtok(NULL, " \n");
		i++;
	}//break string into tokens
	
	printf("Reversed: ");
	
	for(i = i; i >= 0; i--)
	{
		printf("%s ", wordptr[i]);
	}//print words marked by pointers in wordptr in reverse
	
	printf("\n");
	
	return 0;

}//main

And here's the output I'm getting:

Code:

teyla:~ quasigreat$ ~quasigreat/a.out
Input: bibbity boppity boo
Reversed:  boo bibbity 
teyla:~ quasigreat$ ~quasigreat/a.out
Input: birds and bees
Reversed:  bees birds
teyla:~ quasigreat$ ~quasigreat/a.out
Input: now is the time for all good men to come to the aid of their country
Reversed:  (null) their aid to to good for the now

I'm not sure why it seems to be skipping every other word. Help is much appreciated.

Originally Posted by quasigreat

I'm not sure why it seems to be skipping every other word. Help is much appreciated.

Because you're calling strtok twice each time through the loop.

Code:

while(strtok(NULL, " \n") != NULL)
{
    wordptr[i] = strtok(NULL, " \n");
    i++;
}

Originally Posted by quasigreat

I'm having some trouble getting it to ignore the null portion of the array.

I'm guessing you mean this:

Code:

Reversed:  (null) their aid to to good for the now

Just decrement i by one in the initialization step of the for loop:

Code:

for(i = i - 1; i >= 0; i--)
{
    printf("%s ", wordptr[i]);
}

OK, thank you, that makes sense. I've changed it back to:

Code:

while(wordptr[i] != NULL)
	{
		wordptr[i] = strtok(NULL, " \n");
		i++;
	}

And the output is as follows:

Code:

teyla:~ quasigreat$ ~quasigreat/a.out
Input: bees and birds
Reversed: (null) (null) (null) (null) (null) (null) (null) (null) (null) birds and bees

How do I keep it from picking up the nulls at the end?

ETA: Ah, hadn't seen your edit when I replied. That seems like it should work, but I'm still getting the output above (with one less "null") after changing it. I thought "while(wordptr[i] != NULL)" would keep it from creating pointers to the null portion--was I wrong, or am I screwing up somewhere else?

Try something like this, works for me:

Code:

/* strtok example */
#include <stdio.h>
#include <string.h>

int main()
{
	char *wordptr[81], s1[81], s2[81];
	int i = 0;
	printf("Input: ");
	fgets(s1, 80, stdin);
	//wordptr[i] = strtok(s1," \n");
	
   char *result = NULL;
   result = strtok( s1, " \n");
   printf("Reversed: ");
   while(result != NULL) {
       wordptr[i] = result;
       result = strtok( NULL, " \n");
       i++;
   }
	
	for(i = i-1; i >= 0; i--)
	{
		printf("%s ", wordptr[i]);
	}//print words marked by pointers in wordptr in reverse
	
	printf("\n");

  return 0;
}

Also have a look at Elisia's strtok function, I haven't used it yet, but u might like it more.

don't use i, j and t. It makes the code that much harder to read and understand on first viewing. Descriptive names are ever so useful

QuantumPete

Edit: Interestingly, I remember someone putting this problem to me the other day: whether it's possible to reverse the order of words in a string in linear time. I'm pretty sure you can, but I've not given it a lot of thought yet.

Originally Posted by maverix

Agreed
but i thought having large variable names for loop index would make the lines too long.

Generally that's a good rule of thumb. However, when you have nested loops and those nested loops use the index of the parent loop to compute something, it just becomes confusing and readability wins over line-length. Something like word_idx and letter_idx are short enough still, I think. I'm not saying your indices should be named something like: this_is_the_index_to_the_letter_of_the_current_wor d_that_i_m_parsing

QuantumPete

> but i thought having large variable names for loop index would make the lines too long.
Yes but, they're used as counters in mathematics -- why not use them here? (ie, j, k, n, i, etc);