Shuffle words from a 1D array - arrays

Question

I've been given this sentence and I need to shuffle the words of it:
char array[] = "today it is going to be a beautiful day.";
A correct output would be: "going it beautiful day is a be to today"
I've tried many things like turning it into a 2D array and shuffling the rows, but I can't get it to work.

Answer 1

Your instinct of creating a 2D array is solid. However in C that's more involved than you might expect:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <time.h>
int main()
{
char array[] = "today it is going to be a beautiful day.";
char out_array[sizeof(array)];
char words[sizeof(array)][46];
int word_count = 0;
int letter_count = 0;
int on_word = 0;
int count = 0;
int i = 0;
int j = 0;
srand(time(NULL));
// parse words into 2D array
for (i = 0; i < sizeof(array); i++) {
if (array[i] == ' ') {
if (on_word) {
words[word_count++][letter_count] = '\0';
letter_count = 0;
on_word = 0;
}
} else if (array[i] == '\0' || array[i] == '.') {
break;
} else {
on_word = 1;
words[word_count][letter_count++] = array[i];
}
}
words[word_count++][letter_count] = '\0';
// randomly swap around words
for (i = 0; i < word_count; i++) {
char temp[46];
int idx = rand() % word_count;
if (idx != i) {
strcpy(temp, words[idx]);
strcpy(words[idx], words[i]);
strcpy(words[i], temp);
}
}
// output words into out_array
for (i = 0; i < word_count; i++) {
for (j = 0; words[i][j] != '\0'; j++) {
out_array[count++] = words[i][j];
}
out_array[count++] = ' ';
}
out_array[count - 1] = '\0';
printf("%s", out_array);
return 0;
}

Answer 2

You need two basic algorithms to solve this problem.
Split the input string into a list of words.
Randomly sample your list of words until there are no more.
1. Split the input string into a list of words.
This is much simpler than you may think. You don’t need to actually copy any words, just find where each one begins in your input string.
today it is going to be a beautiful day.
^---- ^- ^- ^---- ^- ^- ^ ^-------- ^--
There are all kinds of ways you can store that information, but the two most useful would be either an array of integer indices or an array of pointers.
For your example sentence, the following would be a list of indices:
0, 6, 9, 12, 18, 21, 24, 26, 36
To do this, just create an array with a reasonable upper limit on words:
int words[100]; // I wanna use a list of index values
int nwords = 0;
 
char * words[100]; // I wanna use a list of pointers
int nwords = 0;
If you do it yourself either structure is just as easy.
If you use strtok life is much easier with a list of pointers.
All you need at this point is a loop over your input to find the words and populate your list. Remember, a words is any alphabetic or numeric value (and maybe hyphens, if you want to go that far). Everything else is not a word. If you #include <ctype.h> you get a very handy function for classifying a character is “word” or “not-word”:
if (isalnum( input[n] )) its_a_word_character;
else its_not_a_word_character_meaning_we_have_found_the_end_of_the_word;
Now that you have a list of words, you can:
2. Randomly sample your list of words until there are no more.
There are, again, a number of ways you could do this. Already suggested above is to randomly shuffle the list of words (array of indices or array of pointers), and then simply rebuild the sentence by taking the words in order.
→ Beware, Etian’s example is not a correct shuffle, though it would probably go unnoticed or ignored by everyone at your level of instruction as it will appear to work just fine. Google around “coding horror fisher yates” for more.
The other way would be to just select and remove a random word from your array until there are no words left.
The random sampling is not difficult, but it does require some precise thinking, making this the actually most difficult part of your project.
To start you first need to get a proper random number. There is a trick to this that people are generally not taught. Here you go:
int random( int N ) // Return an UNBIASED pseudorandom value in [0, N-1].
{
int max_value = (RAND_MAX / N) * N;
int result;
do result = rand(); while (result >= max_value);
return result % N;
}
And in main() the very first thing you should do is initialize the random number generator:
#include <stdlib.h>
#include <time.h>
int main()
{
srand( (unsigned)time( NULL ) );
Now you can sample / shuffle your array properly. You can google "Fisher-Yates Shuffle" (or follow the link in the comment below your question). Or you can just select the next word:
while (nwords)
{
int index = random( nwords );
// do something with word[index] here //
// Remove the word we just printed from our list of words
// • Do you see what trick we use to remove the word?
// • Do you also know why this does not affect our random selection?
words[index] = words[--nwords];
}
Hopefully you can see that both of these methods are essentially the same thing. Whichever you choose is up to you. I personally would use the latter because of the following consideration:
Output
You can create a new string and then print it, or you can just print each word directly. As the homework (as you presented it) does not require generation of a new string, I would just print the output directly. This makes life simpler in the sense that you do not have to mess with another string array.
As you print each word (or append it to a new string), remember how you separated them to begin with. If you use strtok you can just use something like:
printf( "%s", words[index] ); // print word directly to stdout
 
strcat( output, words[index] ); // append word to output string
If you found the beginnings of each word yourself, you will have to again loop until you find the end of the word:
// Print word, character by character, directly to stdout
for (int n = index; isalnum( words[index+n] ); n++)
{
putchar( words[index+n] );
}
 
// Append word, character by character, to output string
for (int n = index; isalnum( words[index+n] ); n++)
{
char * p = strchr( output, '\0' ); // (Find end of output[])
*p++ = words[index+n]; // (Add char)
*p = '\0'; // (Add null terminator)
}
All that’s left is to pay attention to spaces and periods in your output.
Hopefully this should be enough to get you started.