42

Possible Duplicate:
Generating random integer from a range

I am trying to create a program where the computer guesses a number the user has in his/her mind. The only user input required is whether the guess was too high, too low, or correct. I'm having a problem generating a random number between two variables that store the min and max based on previous guesses. Here is my code:

 #include <iostream> #include <cstdlib> #include <ctime> using namespace std; int main() { srand(static_cast <unsigned int> (time(0))); int compGuess = rand() % 100 +1; //Generates number between 1 - 100 int highestNumber = 100; int lowestNumber = 1; char ready; char highLowSuccess; bool success; int tries = 0; cout << "Please pick a number between 1 - 100. I will guess your number. Don't tell me what it is!\n\n"; do { cout << "Are you ready? (y/n)\n\n"; cin >> ready; if (ready == 'y') { do { cout << "Is your number " << compGuess << "?\n\n"; cout << "High, Low or Success?"; ++tries; cin >> highLowSuccess; //User input telling the computer whether its too high, too low, or a success if (highLowSuccess == 'h') //Executes code if number guessed was too high. { highestNumber = compGuess - 1; //Stores variable indicating the highest possible number based on user input compGuess = rand() % highestNumber +1; //Generates a new random number between 1 and the new highest possible number success = false; } else if (highLowSuccess == 'l') //Executes code if number guessed was too low. { lowestNumber = compGuess + 1;//Stores variable indicating the lowest possible number based on user input compGuess = (rand() % highestNumber - lowestNumber + 1) + lowestNumber // <---- Not producing the desired result success = false; } else if (highLowSuccess == 's') //Executes code if the computer's guess was correct. { cout << "I guessed your number! It only took me " << tries << " tries!"; success = true; } } while (success != true); } else { continue; } } while (ready != 'y'); return 0; }

highestNumber is what the max should be and lowestNumber is what the min should be. I need an equation that lets me generate a random number while taking the highest and lowest possible numbers into account.

Forgive me if the answer is really simple, I'm a noob programmer. xD

Improve this question
3
  • Replace 100 with highestNumber and replace 1 with lowestNumber? Commented Sep 30, 2012 at 1:38
  • And besides being a dupe, it looks like you were on the right track anyway. You just have an order-of-operations problem on your line marked "not producing the desired result". Parenthesize correctly and you'll be good. There are many example answers here. Commented Sep 30, 2012 at 2:26
  • 2
    @CarlNorum : Sorry for the duplicate. =( Commented Sep 30, 2012 at 3:28

4 Answers 4

Reset to default
78

To generate a random number between min and max, use:

int randNum = rand()%(max-min + 1) + min;

(Includes max and min)

Improve this answer
Sign up to request clarification or add additional context in comments.

8 Comments

This answer is probably good enough for most uses, but the OP should be aware that this method has a bit of a bias towards the low end if the range of rand() isn't divisible by max - min + 1.
Could you tell me a little more about it? What does the "range of rand()" mean?
rand returns numbers between 0 and RAND_MAX. That's RAND_MAX + 1 possible input values to your modulo. There are max - min + 1 possible output values. If the ratio of inputs to outputs isn't an integer, then you're ending up with a small bias towards the lower numbers. Imagine writing out all RAND_MAX + 1 inputs: 0, 1, 2, ... RAND_MAX, right? Now write out underneath them the post-modulo ones (for min = 0 and max = 5), say: 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, etc. Now if RAND_MAX isn't divisible by 6, you get a partial cycle 0, 1, 2 ...
... or something like at the end. That means there are more 0, 1, 2 outputs than 3, 4, 5 outputs - giving your algorithm a bias towards the 0, 1, 2 answers.
Ah! I get your point. Thanks.
|
38

Really fast, really easy:

srand(time(NULL)); // Seed the time int finalNum = rand()%(max-min+1)+min; // Generate the number, assign to variable.

And that is it. However, this is biased towards the lower end, but if you are using C++ TR1/C++11 you can do it using the random header to avoid that bias like so:

#include <random> std::mt19937 rng(seed); std::uniform_int_distribution<int> gen(min, max); // uniform, unbiased int r = gen(rng);

But you can also remove the bias in normal C++ like this:

int rangeRandomAlg2 (int min, int max){ int n = max - min + 1; int remainder = RAND_MAX % n; int x; do{ x = rand(); }while (x >= RAND_MAX - remainder); return min + x % n; }

and that was gotten from this post.

Improve this answer

5 Comments

This answer is probably good enough for most uses, but the OP should be aware that this method has a bit of a bias towards the low end if the range of rand() isn't divisible by max - min + 1.
@CarlNorum, I updated my answer to reflect that.
Looks good. +1 for you.
@CarlNorum, also showed how to remove bias in normal C++ as well. :D
@Link : I'll keep the example for removing bias for future use since the bias isn't a big issue right now. Thanks a lot!
31

If you have a C++11 compiler you can prepare yourself for the future by using c++'s pseudo random number faculties:

//make sure to include the random number generators and such #include <random> //the random device that will seed the generator std::random_device seeder; //then make a mersenne twister engine std::mt19937 engine(seeder()); //then the easy part... the distribution std::uniform_int_distribution<int> dist(min, max); //then just generate the integer like this: int compGuess = dist(engine);

That might be slightly easier to grasp, being you don't have to do anything involving modulos and crap... although it requires more code, it's always nice to know some new C++ stuff...

Hope this helps - Luke

Improve this answer

1 Comment

Sadly, I'm not using a C++11 compiler. Sorry, I guess I should have mentioned my compiler in the OP
8 
rand() % ((highestNumber - lowestNumber) + 1) + lowestNumber
Improve this answer

1 Comment

This answer is probably good enough for most uses, but the OP should be aware that this method has a bit of a bias towards the low end if the range of rand() isn't divisible by highestNumber - lowestNumber + 1.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.