C++ rand generates the same number with srand time

Random number generation in C++ in the standard libraries is generally done using “ctime” (time.h) and “cstdlib” (stdlib.h). If you are experiencing an issue where calling rand() keeps generating the same number due to similar seeds, simply call it multiple times. (See below).

int main() {

//repeat as many times as you like to prove randomness
cout << GetRandomNumber(10) << "\n\r";
cout << GetRandomNumber(10) << "\n\r";
cout << GetRandomNumber(10) << "\n\r";
cout << GetRandomNumber(10) << "\n\r";
cout << GetRandomNumber(10) << "\n\r";

int pause;
cin >> pause;
return 0;
}

int GetRandomNumber(int upperlimit) {
    srand(time(NULL)); //use time which is accurate within seconds to generate random seed

rand();rand();rand(); //call rand multiple times to create more variance since subsequent calls within a short time frame will have a similar result

return rand() % upperlimit; //passing in 10 will generate from 0-10, 100 generates 0-100 etc.

}

References
Stackoverflow (blog), http://stackoverflow.com/questions/3032726/rand-generating-the-same-number-even-with-srandtimenull-in-my-main
LearnCPP, http://www.cplusplus.com/reference/clibrary/cstdlib/rand/

About Ronnie Diaz

Ronnie Diaz is a software engineer and tech consultant. Ronnie started his career in front-end and back-end development for companies in ecommerce, service industries and remote education. This work transitioned from traditional desktop client-server applications through early cloud development. Software included human resource management and service technician workflows, online retail e-commerce and electronic ordering and fulfillment, IVR customer relational systems, and video streaming remote learning SCORM web applications. Hands on server experience and software performance optimization led to creation of a startup business focused on collocated data center services and continued experience with video streaming hardware and software. This led to a career in Amazon Prime Video where Ronnie is currently employed, building software and systems which stream live sports and events for millions of viewers around the world.

Posted on June 3, 2011, in Programming & Development and tagged , , , , , , , , . Bookmark the permalink. Leave a comment.

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