In c++ the rand() is generating different result when executing the program with and without debugging

Question

So i've been building a simple console game in c++ where I am using the rand() function from stdlib.h. I have recognized that i got different results from the rand function if I was executing the program in debug mode or without debugging.

Here is my code where I use the rand function:

void Game::SelectHiddenWord(Fstring &word) {
    std::vector<Fstring> WordList{ "plan","planet","planes","radios","gamers","images",
    "amigos","micros","macros","frogs","raids","roads","paris","smog","cars","macs","scam","some","farm","fair",
    "jam","rat","map","zig","zag","isogram","amorist","roaming","mirages","soaring","cargos","disarm","isobar"
};
    int32 Length = WordList.size();
    int32 randomNumber = std::rand()%Length;
    word = WordList.at(randomNumber);
}

So basically I have a wordlist with isograms and I am using the rand function to generate an integer which I will use as my index when selecting a word from the wordlist. I found that every time I was executing the program the randomNumber integer was always 14 which would then give me the word "smog" from the wordlist. First I thought that this maybe was a bug since I've read that the rand() function isn't the best to use to generate random numbers. So I started the program in debug mode and I found out that I the randomNumber integer now was 32 which would give me the word "disarm" from the wordlist. And this happend every time I was using debugmode.

So on startup without debugging I always get the word "smog" and on startup with debugging I always get the word "disarm" so I can really just play around with two words.

So is this some sort of bug with the rand() function that it will behave differently when executing the program in debug mode and without debug mode?

I'm trying to learn c++ and I couldn't find any information about the rand function behaving differently so I do not know how I can fix this issue. So if any of you guys know how to fix this issue, so that the rand() function will behave the same and generate the same number independently if I am using debug mode or without debugging. Or maybe I need to use another function to generate my random numbers?

Any help will be appreciated!

Edit:

I do not want to know why the rand() function generates the same value everytime. When I start the program without debugging the rand() always generates the number 14, and I want this to happen. But if I start the program in debug mode the rand() always generates the number 32. But I want the rand function to always generate the number 14, independently if I execute the program in debug mode or without debugging.

So the question is about why the rand() function is behaving differently and generating different numbers if it is executed with or without debugging. I don't even know if it's possible to answer this, maybe it could just be something with my machine.

My testing code:

void Game::Print() {
Fstring HIDDEN_WORD{}; //This MUST be an isogram
SelectHiddenWord(HIDDEN_WORD);

MyHiddenWord= HIDDEN_WORD;
std::cout << MyHiddenWord << std::endl; //This prints "smog" to the screen if I start the program without debugging, 
                                       //and if I debug the program it prints "disarm" to the screen

My main method:

Game MyGame;
int main() {
 MyGame.Print();
}

I am using Microsoft Visual C++ compiler.

Answer 1

Addressing why rand() behaves differently between in debug mode is a question for your compiler vendor. How it works internally is implementation-defined (i.e. up to the compiler's creators). From cppreference:

There are no guarantees as to the quality of the random sequence produced. [...] rand() is not recommended for serious random-number generation needs. It is recommended to use C++11's random number generation facilities to replace rand().

Clearly rand() is a bit old-fashioned, which I'll address toward the end of my answer. As to why you're seeing the same value during every run of the program, this is not a bug with rand(). rand() is a pseudo-random number generator that can generate seemingly random numbers, but in a completely deterministic manner. In order to get different results on every run, you need to seed the random number generator with something somewhat unpredictable, like, say, the current time. This is done as follows.

#include <cstdlib>
#include <ctime>
int main(){
    srand(time(0));
    // rand() will now behave differently on each run of the program
}

This is also explained here. It is also important to note that you should only call srand once at the start of your program. Calling srand multiple times within the same second will result in the same seed, and thus the same random sequence from rand, as explained here.

---

If you want to be most correct, C++11 now offers a good random number generation library that you should consider using. An example usage could look like:

#include <random>
std::random_device rd; // reliable random seed generator
std::default_random_engine randengine { rd() }; // global rng, seeded with random device

...

void Game::SelectHiddenWord(Fstring &word) {
    // NOTE: this vector can be static const if it never changes, to avoid
    // constructing and destructing it every function call
    static const std::vector<Fstring> WordList{ "plan","planet","planes","radios","gamers","images", "amigos","micros","macros","frogs","raids","roads","paris","smog","cars","macs","scam","some","farm","fair","jam","rat","map","zig","zag","isogram","amorist","roaming","mirages","soaring","cargos","disarm","isobar"};

    // define distribution as [ 0, WordList.size() )
    std::uniform_int_distribution<size_t> dist { 0, WordList.size() - 1 };
    word = WordList.at(dist(randengine));
}

---

I'm still a little confused by one thing you mentioned:

But I want the rand function to always generate the number 14 [...]

If this is really what you want, you can replace every rand() with 14 :-)