My random number generator is printing duplicate numbers

Question

I'm working on a program that uses a function that would generate 2 random integers that I would later use to ask the user to add and figure out the sum. For the time being, I came across a problem in which it's duplicating the same number.

Here's the program thus far:

#include<iostream>
#include<ctime>
using namespace std;


/*
This function generates a random integer
between 0 and max_int
*/
int get_rand_int(int max_int){
    srand(time(0));
    return int(rand()) % max_int;
}


This program generates random addition problems
with integers between 0 and 100


int main(){
    int num1 = get_rand_int(101);
    int num2 = get_rand_int(101);
    cout << num1 << endl;
    cout << num2 << endl;

Please note this this is a homework problem my teacher assigned. He provided us with the skeleton and we're trying to fill in the blanks. I have a feeling the problem lies within the srand section of my function but that was part of my teacher's work so I'm not sure whether I need to touch that. Thanks in advance.

@EricJ.: While I agree, the link you've provided is about C#. A better choice might be http://stackoverflow.com/questions/7343833/srand-why-call-it-only-once — Bill Lynch, Apr 24 '15 at 03:58
Same exact issue. Most languages in fact share this issue. Feel free to vote to close as a duplicate of the one you provide though. — Eric J., Apr 24 '15 at 04:00
I apologize that this might be a duplicate question. I read those other questions and saw how srand is the issue. I just want to make sure that's the only problem with it duplicating since my professor was the one that provided that srand in the skeleton program. — trungnt, Apr 24 '15 at 04:09

score 4 · Accepted Answer · answered Apr 24 '15 at 03:55

4

Yes. srand initializes the random sequence with the result of time. However, time changes only once per second! This means that if you call your function twice in a one-second delay, you get the same number.

One solution is to call srand just once, at the beginning of your program (in the main function, for instance).

answered Apr 24 '15 at 03:55

zneak

134,922
42
253
328

I thought so. Guess I need to tell me teacher he messed up! Thanks for the help! – trungnt Apr 24 '15 at 03:59

score 2 · Answer 2 · answered Apr 24 '15 at 03:54

You must seed the random generator only once in your program,

srand(time(0)); // this must happen only once, at the beginning of main()

You now seed it inside the function get_rand_int, and call the function twice in a very short amount of time. The seed ends up being the same, so you get the same sequence of numbers.

score 1 · Answer 3 · answered Apr 24 '15 at 04:10

Call the srand(time(0)) in the main to set the current time only once to use random generation instantly.

int main(){
srand(time(0));
int num1 = get_rand_int(101);                          
int num2 = get_rand_int(101);
cout << num1 << endl;
cout << num2 << endl;

}

My random number generator is printing duplicate numbers

3 Answers3