Question

I’m new to C++. Only been programming for 2 days so this will probably look messy. The purpose of the program is that you enter a word, and then the program randomizes the placement of the letters in the word.

I have three questions.

1. Why, if the same string is entered twice, will the same “random” numbers be output?
2. How can I make sure no random number is picked twice. I already tried an IF statement nested inside the FOR statement but it just made things worse.
3. What will make this work?

The code:

#include <iostream>
#include <sstream>
#include <string>
#include <cstdlib>
#include <stdio.h>
#include <string.h>

using namespace std;

int main () {
    cout << "Enter word to be randomized: ";
    char rstring[30]; 
    char rstring2[30]; 
    cin >> rstring;
    strcpy(rstring2, rstring);
    int length;
    length = strlen(rstring);

    int max=length;
    int min=0;
    int randint;

    for (int rdm=0; rdm<length; rdm++) {
        randint=rand()%(max-min)+min;
        cout << rstring[rdm]; //This is temporary. Just a visualization of what I'm doing.
        cout << randint << endl; //Temporary as well.
        rstring2[randint]=rstring[rdm];
    }

    cout << endl << rstring2 << endl;
    return 0;
}

If you compile and run this you will notice that the same random numbers are output for the same text. Like “hello” outputs 24330. Why is this random generator generating nonrandom numbers?

Answer 1

You need to seed your random number generator to get different results with each run. Otherwise, (as you have noticed) you will get the same random numbers with each run.

Put this at the start of the program:

srand(time(NULL));

This will seed the random number generator with time – which will likely be different between runs.

Note that you’ll also need #include <time.h> to access the time() function.