rand() in range returning numbers outside of the range

Question

In my program, I have to find two random values with certain conditions:

i needs to be int range [2...n]

k needs to be in range [i+2...n]

so I did this:

i = rand() % n + 2;
k = rand() % n + (i+2);

But it keeps giving me wrong values like

for n = 7

I get i = 4 and k = 11

or i = 3 and k = 8

How can I fix this?

It's a simple math. `rand() % n + 2` will give you `[0..n-1] + 2 = [2..n+1]`. I trust you can make it from here. — Eugene Sh., May 25 '17 at 19:13
`rand() % N + A` will give you a value in the range `[A, A + N[`, not `[A, N[`. What you want may be `rand() % (n - 2) + 2` for example — KABoissonneault, May 25 '17 at 19:13
That's not a C or even programming question, but basic mathematics. — too honest for this site, May 25 '17 at 19:32

Ian Gallegos · Accepted Answer · 2017-06-04T17:53:17.017

2

The exact formula that I use in my other program is:

i = min + (rand() % (int)(max - min + 1))

edited Jun 04 '17 at 17:53

answered May 25 '17 at 19:18

Ian Gallegos

score 1 · Answer 2 · answered May 25 '17 at 19:18

As the comments say, your range math is off.

You might find it useful to use a function to work the math out consistently each time. e.g.:

int RandInRange(int x0, int x1)
{
    if(x1<=x0) return x0;
    return rand() % (x1-x0+1) + x0;
}

then call it with what you want:

i = RandInRange(2,n);
k = RandInRange(i+2,n);

2 Answers2