Why does rand()%10+1 give me a number between 1-10?

Question

Im fairly new to programming

So i was actually trying to figure something out

Why does rand()%10+1 give us a number between 1-10 whereas 32767%10 is actually 7?

Because you add 1 afterward. `rand() % 10` is between `0` and `9`, so `rand() % 10 + 1` is between `1` and `10`. — Nathan Pierson, Oct 29 '20 at 01:00
Obligatory link to [rand() Considered Harmful](https://channel9.msdn.com/Events/GoingNative/2013/rand-Considered-Harmful) — user4581301, Oct 29 '20 at 01:02
I've edited the title to be consistent with the body of the question. I don't think that discrepancy was relevant to the information the OP is looking for. And I *think* I've managed to dig down to what the OP is actually confused about (though I may be wrong). — Keith Thompson, Oct 29 '20 at 01:18

score 3 · Answer 1 · answered Oct 29 '20 at 01:17

I think I see the source of your confusion.

The fact that you referred to rand()%10 in your title and rand()%10+1 in the body of your question made that difficult.

You asked:

Why does rand()%10+1 give us a number between 1-10 whereas 32767%10 is actually 7?

It's because the function N%10 is not monotonically increasing. As the value of N increases, the value of N%10 goes up and down.

I think you're assuming:

that 32767 is the maximum value returned by rand() (which it can be, but on my system it's 2147483647, but that doesn't affect the point); and
that if 32767 is the maximum value returned by rand(), then 32767%10, which is 7, must be the maximum value of rand()%10.

Your second assumption is wrong. For any value N (we'll ignore negative values), N%10 is the last digit of its decimal representation. If rand() returns 9, then rand()%10 will be 9 and rand()%10+1 will be 10 -- which is larger than the value of 32767%10+1.

score -3 · Answer 2 · answered Oct 29 '20 at 01:09

-3

#include <stdio.h>
#include <stdlib.h>

int main()

{
    int randomnumber;
    randomnumber = rand() % 10;
    printf("%d\n", randomnumber);
    return 0;
}

When you run this code it will generate a number from 0 to 10 because of number 10 behind it (Because you want to random to 10 so that is why is a 10) but it will generate from 0 to 9 only. That's why you need a +1 at the end to generate random from 1 to 10. That's all.

answered Oct 29 '20 at 01:09

Kayaba Zeref

1
1

This does not answer the question and is poorly worded. – Eugene Oct 29 '20 at 01:15
Should mention `srand()` too. – Eljay Oct 29 '20 at 01:59

Why does rand()%10+1 give me a number between 1-10?

2 Answers2