Random Numbers within a range OTHER than 1-X

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:

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//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;
}

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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

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

Code:

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

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

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

---

You have a hi and lo number,

Code:

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// 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;

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say number generated was 42 adding lo would make it 62

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

Code:

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

---

Code:

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

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

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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.

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. :p

Quote:

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

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

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

Quote:

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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.

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you can't come up with

Code:

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

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on your own?

to slove a quadratic equation:

Code:

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#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);
}

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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...

^^^
???

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.

Quote:

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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".

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that slight hostility came from

Quote:

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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.

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(no hard feelings)
and the quadratic bit came from

Quote:

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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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