Random number between 1 to 10 using C++

Question

This code is supposed to generate random number between 1 to 10, but it returns 1 every time.

int random_integer;
int lowest=1, highest=10;
int range=(highest-lowest)+1;
random_integer = lowest + int(range*rand()/(RAND_MAX + 1.0));
cout << random_integer << endl;

What's wrong in the code?

C++11 have better [PRNG functionality](http://en.cppreference.com/w/cpp/numeric/random) built-in, including [classes](http://en.cppreference.com/w/cpp/numeric/random/uniform_int_distribution) to create numbers in a range. See the example in the last link. — Some programmer dude, Sep 25 '12 at 10:17

score 11 · Answer 1 · answered Sep 25 '12 at 10:18

11

If you want a random integer between lowest and highest, you'd better write

random_integer = lowest + rand() % range

answered Sep 25 '12 at 10:18

Anton Guryanov

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score 7 · Answer 2 · answered Sep 25 '12 at 10:18

7

You're subject to overflow here - range*rand().

Just use what regular folks use: rand() % 10 + 1.

answered Sep 25 '12 at 10:18

Luchian Grigore

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score 5 · Accepted Answer · edited May 23 '17 at 11:53

5

range * rand() / (RAND_MAX + 1.0)

does not do what you think. Introduce some parens:

range * (rand() / (RAND_MAX + 1.0))

(Note that this method gives skewed distributions, though.)

edited May 23 '17 at 11:53

Community

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answered Sep 25 '12 at 10:19

Fred Foo

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what is the easiest way to get a unskewed distribution between 1-10 . – rajat Sep 25 '12 at 10:25
@rajat: http://eternallyconfuzzled.com/arts/jsw_art_rand.aspx (or use the [C++11](http://en.cppreference.com/w/cpp/numeric/random/uniform_int_distribution) or [Boost](http://www.boost.org/doc/libs/1_51_0/doc/html/boost/random/uniform_int_distribution.html) libraries) – Fred Foo Sep 25 '12 at 10:27

score 2 · Answer 4 · edited Jan 05 '14 at 16:40

2

I agree with all the solution provided above .
now to get a different sequence every time you run your program you can use srand() function it will provide a seed to rand() function as follows:-

srand(time(NULL))  
random_integer = lowest + rand() % range

edited Jan 05 '14 at 16:40

BenMorel

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answered Sep 25 '12 at 11:18

birubisht

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score 1 · Answer 5 · answered Sep 25 '12 at 13:25

This two are always part of my programs

float randf(float lo, float hi) {
    float random = ((float) rand()) / (float) RAND_MAX;
    float diff = hi - lo;
    float r = random * diff;
    return lo + r;
}
int randi(int lo, int hi)
{
    int n = hi - lo + 1;
    int i = rand() % n;
    if (i < 0) i = -i;
    return lo + i;
}

score 0 · Answer 6 · answered Sep 25 '12 at 10:26

0

You seem to assume that rand() returns a value between 0 and 1.

This is not correct, it return value between 0 and RAND_MAX.

answered Sep 25 '12 at 10:26

Sebastian Mach

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Random number between 1 to 10 using C++

6 Answers6