program help

I am writing a program that takes the alphabet and randomly mixes it up and compares it to the alphabet (you have to compile it to fully understand) i got as far as making a random mixed alphabet but sometimes letters are used more than once, what do i put in to make sure that no letter is used more than once?

here's what i got so far

Code:

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#include <iostream>
#include <stdlib.h>
#include <time.h>
#include <math.h>
using namespace std;

void crypt();
int rand(int n);


char *alphabet[26] = {"a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"};
int main() {
    int i, n;
    srand(time(NULL));
   
    while(1) {
          cout<< "type 1 start type 0 to exit"<<endl;
          cin>> n;
          if (n == 0)
          break;
       
        if ( n == 1) {
          for (i=1; i<=26; i++)
          crypt();
          cout<<endl;
          cout<< "a b c d e f g h i j k l m n o p q r s t u v w x y z"<<endl;
          }
       
          }
          return 0;
          }
         
          void crypt() {
              int a;
              a = rand(26);
             
              cout<< alphabet[a]<<" ";
             
              }
             
              int rand(int n)
              {
                  return rand() % n;
                  }

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Code:

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char[] Alphabet = "abcdefghijklmnopqrstuvwxyz";
std::random_shuffle(Alphabet, Alphabet + 26); // located in <algorithm>

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Besides, you should use <cstdlib>, <ctime> and <cmath> over their deprecated .h alter-egos.

Edit: Can I also mention that you aren't using anything out of both the math.h and stdlib.h header files ?

srand() and rand() are both in stdlib

You don't randomly mix up the letters at all, you just pick random letters out of them. You may very well get lots of repeats.

Example:

Code:

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[a, b, c, d, e]
rand(5) = 6 % 5 = 1 (a)
rand(5) = 17 % 5 = 2 (b)
rand(5) = 1 % 5 = 1 (a) //repeat!

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You'll need to make sure letters which have already been printed can never be choosen again. You could shuffle the array, as suggested; it's prolly the simplest way. You could also add tokens to letters (eg. a second list of booleans) if they have already been printed, and check letters for existing tokens before printing them again. Slow and hackish though.

(i'm pretty new to c++) Can somebody explain how to use the shuffle array thing?

An article will probably help. Here's something concise:
Using the random_shuffle() Algorithm to Randomize a Sequence of Elements

On many implementations, before you call random_shuffle you need to call srand once to seed the random number generator just like you would for rand(). You also have to #include <algorithm>. Then, you call random_shuffle just like Desolation showed, with the first argument being the start of the array and the last being one past the end.

If you call random_shuffle once, it will shuffle everything in your array. Then you can go about comparing it to the actual alphabet like you had planned.

And some other popular implementations of random_shuffle() doesn't use rand() and srand() at all ( e.g. the one that comes with g++ 3.3.5 ).
in that case you have to write your own RandomGenerator function object to be able to seed the random function.
e.g.

Code:

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#include <iostream>
#include <algorithm>
#include <cstdlib>
#include <ctime>

using namespace std;

struct rndgen {
    int operator ()( int num ) {
        static bool isinitialized = false;
        if ( ! isinitialized ) {
            srand(time(0)); isinitialized= true;
        }
        return rand() % num;   
    }
};

int main() {
    char alphabet[] = "abcdefghijklmnopqrstuvwxyz";
    rndgen r;
    random_shuffle(alphabet, alphabet+strlen(alphabet), r );
    cout << alphabet << endl;
}

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Kurt

The random_shuffle is probably better. If you wanted to write your own, you might want to just try picking 2 random numbers between 0 and 25 and just swapping the letters at those indexes.

Code:

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char alphabet[]="abcdefghijklmnopqrstuvwxyz";
char temp;

int random_num1=rand()%25;
int random_num2=rand()%25;

temp=alphabet[random_num1];
alphabet[random_num1]=alphabet[random_num2];
alphabet[random_num2]=temp;

rinse and repeat.....

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If you want to write it yourself the algorithm is pretty simple although it is easy to get wrong. You start from the end and pick a random number from 0 to the that index, swapping with the value at the random index. Then move to the next index down and continue until there is only one index left.

Daved: It's called the Knuth shuffle (or Fisher-Yates shuffle).