Random Numbers within a range OTHER than 1-X

**Kaelin** · 02-15-2005

Hi there,

I have been trying to find a way to generate a random number within a range other than "1-X".

For example, the user will enter the low extreme and the high extreme - 20 and 80 perhaps. I have come up with a solution, but it doesn't seem very efficient. It bounces the number around until it satisfies the range.

I do have this working. What I'm wondering is if there is a better way to do it. Here's what I've got:

Code:

//l is for low extreme, h is for high extreme
int gen_rand(int l, int h) {
    srand(time(NULL));
    int my_rand = rand()%h +1;

    // (+/- 1) only - determines whether or not the number will
    // be reduced or increased in order to satisfy the given range
    int sign = 1;

    // if my_rand is lower than l (low range), add more - but if it goes above
    // h, subtract some.. repeat until my_rand is within the bounds of l and h.
    while (my_rand > h || my_rand < l) {
        if (my_rand > h) sign = -1; // we need to bring it down
        else if (my_rand < l) sign = 1; // bring it up
        my_rand += ((rand()%h)*sign) +1;
    }
    return my_rand;
}

I am a noob. I've tried multiple approaches to this and actually am lucky I was able to come up with even this. What I'm asking is whether or not there is a standard way of doing this (a common algorithm used "all the time") that I'm just missing.

I know that the code itself could probably stand some improvements, but I'm actually asking for help on the algorithm level, not the syntax level.

Thanks,
Kaelin

**The Brain** · 02-15-2005

If you want a number within a specified range, you could just do this:

Code:

//get user input for min
cin >> min;

//get user input for max
cin >> max;

//get random number
int rand_num = min + rand()%(max-min);

**bigtamscot** · 02-15-2005

You have a hi and lo number,

Code:

// subtract lo from hi (80 - 20 = 60)
// generate number in range up to difference (60)
random_num = rand() % (hi - lo);

// then just add the lo
random_num += lo;

say number generated was 42 adding lo would make it 62

**Scribbler** · 02-15-2005

For ease of comprehension, I typically refer to variables range and offset. offset being the minimum result.

Code:

    int range;
    int offset;
    int num;
    
    num = rand() % range + offset;

**bigtamscot** · 02-15-2005

Code:

int rand_in_range( int hi, int lo ) {
        srand(time(NULL));

        return ((rand() % (hi - lo)) + lo);
}

**cerin** · 02-15-2005

Is it possible to make a program that solves qaudratic equations?
I know it you have to make a recursive loop for powers, but I have high a high class graphing calculator and it has problems with negative exponents. So my math teacher told me not to trust the calcs on this kind of stuff.

**Kaelin** · 02-15-2005

Wow - after seeing the solution it's hard to believe I didn't get it in the first place. Thanks all, for your help

Alas, my biggest problem with C++ - not the syntax, but the creative thinking.

**7stud** · 02-15-2005

Alas, my biggest problem with C++ - not the syntax, but the creative thinking.

You can rest assured that most people didn't come up with that on their own--they read it in their beginning C++ text.

**bigtamscot** · 02-15-2005

cant remember ever reading Beginning C++, now Beginning C yes, but cant remember last time I picked it up though.

**major_small** · 02-15-2005

Originally Posted by 7stud

You can rest assured that most people didn't come up with that on their own--they read it in their beginning C++ text.

you can't come up with

Code:

return ((rand() % (hi - lo)) + lo);

on your own?

to slove a quadratic equation:

Code:

#include<cmath>

int main()
{
    double a,b,c;
    //for positive:
    a=((-b+sqrt(pow(b,2)-(4*a*c)))/2.0*a);
    //for negative:
    a=((-b-sqrt(pow(b,2)-(4*a*c)))/2.0*a);
}

you may want to look that over... I pumped it out real fast without double-checking...

edit: I'm guessing it's 7stud that likes to leave bad rep without a comment OR name...

**Kaelin** · 02-16-2005

^^^
???

To be honest, I'm not really sure where that post came from. I don't think there's any need to insult somebody for forgetting "the dreaded quadratic equation".

I know that some of us after high school made an oath never to use it again.. [laughs]

Though no, it wasn't me who reported on your post. I tend to leave comments.

**major_small** · 02-16-2005

Originally Posted by Kaelin

To be honest, I'm not really sure where that post came from. I don't think there's any need to insult somebody for forgetting "the dreaded quadratic equation".

that slight hostility came from

Originally Posted by 7stud

You can rest assured that most people didn't come up with that on their own--they read it in their beginning C++ text.

(no hard feelings)
and the quadratic bit came from

Originally Posted by cerin

Is it possible to make a program that solves qaudratic equations?
I know it you have to make a recursive loop for powers, but I have high a high class graphing calculator and it has problems with negative exponents. So my math teacher told me not to trust the calcs on this kind of stuff.

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