C++ Homework please help

[multitasker451]

I am supposed to write a function that generates a random number between numbers that are parameters.  So when using this function, for example, if I want a number between 1 and 10, I would type

random(1,10).  

Can someone show me how to do this?  Any help is much appreciated

[Enderman]

http://www.cplusplus.com/reference/cstdlib/rand/

 

If your interval is n numbers (eg 4 to 10 inclusive, n=7) then you do modulus n, like this:

%n

Where n is (b-a+1)

 

Then since rand starts from 0, you want to add the minimum number (in this case 4) to the result, so that instead of 0-6 it goes 4-10.

 

The end result would be 

rand() %(b-a+1) +a

 

If you want it to not be inclusive, and only do what is between a and b, then just do

rand() %(b-a-1) +a+1

 

I think you get the idea

[Ithanul]

Yep, rand function.

Give me a bit, I got my C++ book laying about since I just finish up that class.

 

Better way to describe that function when doing a range.

 

lowerBound + rand()%(upperBound - lowerBound + 1)

 

Ex:

cout << 1 + rand()%(6 - 1 + 1);

displays a random integer from 1 through 6.

 

Hmmm, if reading your post right you need to build a value returning function named random that has the rand function in it.  That way you don't have to keep typing the rand with the range, correct?

 

If so, this is how you go about doing it.

//function definitions

int random (int lowerBound, int upperBound)

{

     int randNum = 0;

     randNum = lowerBound + rand()%(upperBound - lowerBound + 1);

     return randNum;

}  //end of random function

 

There you can just use that function without having to type that blasted rand function about.  Could probably build it as a void function as well.

 

[fizzlesticks]

This is C++ not C, rand is horrible and shouldn't be used.

http://www.cplusplus.com/reference/random/

[multitasker451]

Just now, fizzlesticks said:

This is C++ not C, rand is horrible and shouldn't be used.

http://www.cplusplus.com/reference/random/

I wouldn't use it if it wasn't required for this assignment

[Nineshadow]

The easiest way of doing it is definitely with modulo as @Enderman described, and you should probably stick to it.

 

Just for the sake of it and to enlighten our knowledge in generating random numbers in C++, let's cover other ways of doing it D:.

 

1.You could generate a random normalized value then scale it to your interval. To get a normalized random value you divide the result of your rand function by RAND_MAX ( the maximum rand() can return), then to scale it you multiply with the difference between the heads of the interval and add the left head of the interval to bring it into place. Pretty much :

(float)rand()/RAND_MAX*(dr-st+1)+st

This will generate a real number, keep in mind.

 

2.Let's assume you want just random integers. It's pretty obvious that rand()%n will not give you a uniform distribution for numbers in the range [0,n]. Not only that, but you don't know if the moduli of rand() are, indeed, random, they could maybe go 0,1,2,..., which might be uniform, but it's not random. So we can safely assume that rand() produces a Poission distribution. This implies that you can achieve a uniform distribution and scattered values for a finite set of values.

Anyway, using floating point numbers could be somewhat ok, but it suffers from rounding errors in principle. So to achieve this using only integer operations we could do something like this :

long random(long min, long max)
{
    long x;
    max=max-min;
    unsigned long num_bins=(unsigned long)max+1,num_rand=(unsigned long)RAND_MAX+1,bin_size=num_rand/num_bins,defect=num_rand%num_bins;    
    do x = rand();    while (num_rand - defect <= (unsigned long)x);
    return x/bin_size+min;
}

3. C++11 introduced standardized pseudo-random number generators in the form of <random>.

Not going too much in depth, code would look like this :

std::random_device rd;
std::mt19937 mt(rd());
std::uniform_real_distribution<int> dist(1.0, 10.0); // or whatever your interval is
//to get a random number you simply do :
dist(mt);

 

[Pinguinsan]

+1 for @Nineshadow's response. Don't be afraid of the standard library, it really is your friend when you start to get a handle one it.