Modulo gives impossible value

Question

I want a table of four values between 1 to 6.

I'm using: rand() % 6 + 1;

This should give values between 1 and 6.

Except if rand() generates the value 0.

I keep getting 7's. I don't want any 7's
What is the range of rand? How I prevent it from generation any 0 values?
Alternative solutions are quite welcome.
My teacher gave us the clue of using "random".
We use Borland C++ Builder 5 at school.
I am using Dev-C++ 5.3.0.3 at home.
I find there are a few differences to how they work, which I find strange..
I can't use random(), it gives me not declared in scope...
```
int main (){

    int I;
    int Fasit[3];

    srand (time(NULL) );

    for(I=0; I<4; I++) {
        Fasit[I]=rand() % 6 + 1;
    }

    std::cout << Fasit[0] << " " << Fasit[1] << " " << Fasit[2] << " " << Fasit[3] << " ";

    return 0;
}
```

Some values I get:

7 shouldn't be possible, should it?

PS: I know my print is ham fisted, I will make it a bit more elegant once the number generation works.

How often do you expect `rand() == 0`? – Alex Chamberlain Jan 30 '13 at 21:14 — Alex Chamberlain, Jan 30 '13 at 21:14

score 9 · Accepted Answer · answered Jan 30 '13 at 20:39

9

Consdier these lines:

int Fasit[3];
for(I=0; I<4; I++) {
  Fasit[I]

You declare an array of three entries, which you write to four times.

Try your program again, but with:

int Fasit[4];

answered Jan 30 '13 at 20:39

Robᵩ

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I was under the impression 3 meant: 0, 1, 2, 3 ? – DrOnline Jan 30 '13 at 20:39
1

@DrOnline No, 3 means the array has 3 elements. – melpomene Jan 30 '13 at 20:40
@DrOnline when you initialize an array array[n], n is the number of values in the array. When you access it via index, it is zero indexed. – Rapptz Jan 30 '13 at 20:40
Alright, the 7's disappeared. Thanks. I wonder why the size of the table had that effect. – DrOnline Jan 30 '13 at 20:40
4

@DrOnline, Because undefined behaviour is inexplicable. – chris Jan 30 '13 at 20:42
1

Thanks everybody. The program works now, and IBM just called wanting me to do some hardcore programming for them with my new skills. Sweet. ;) – DrOnline Jan 30 '13 at 20:46

score 2 · Answer 2 · answered Jan 30 '13 at 20:40

2

You only have 3 elements in Fasit[3]; When you write to Fasit[3], you are in the realm of undefined behavior, which in this case manifests it self with apparent contradiction.

answered Jan 30 '13 at 20:40

Aki Suihkonen

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Thanks! I had misunderstood the relationship between index and table size, I thought 3 means size 4 with, 0, 1, 2, 3... – DrOnline Jan 30 '13 at 20:42

Cyrille Ka · Answer 3 · 2013-01-30T21:05:29.103

2

int Fasit[3];

You are creating an array of size 3, which can be accessed with indexes 0, 1 or 2 only.

You are writing and reading Fasit[3], which has an undefined behaviour. When a behaviour is undefined, you are bound to obtain weird results. This is it.

edited Jan 30 '13 at 21:05

answered Jan 30 '13 at 20:41

Cyrille Ka

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Thank you. This my first experience with this type of issue. Program works now! – DrOnline Jan 30 '13 at 20:43

score 2 · Answer 4 · edited May 23 '17 at 10:31

2

Fasit[3] allows you to access only Fasit[0], Fasit[1], and Fasit[2].

Accessing Fasit[3], either for reading and writing, is undefined behavior. Your code is both writing and reading to Fasit[3] :-). The program is accessing the array out-of-bound. Fix it!

As to why 7 is printed, that is just coincidence. Note that Fasit[0-3] is always printed in the range 1-6 as you expected.

Modulo gives impossible value

4 Answers4