How to select range of values when using arc4random()

Question

Can I set a range of numbers when using arc4random()? For example 50-100 only.

Answer 1

To expand upon JohnK comment.

It is suggested that you use the following function to return a ranged random number:

arc4random_uniform(51)

which will return a random number in the range 0 to 50.

Then you can add your lower bounds to this like:

arc4random_uniform(51) + 50

which will return a random number in the range 50 to 100.

The reason we use arc4random_uniform(51) over arc4random() % 51 is to avoid the modulo bias. This is highlighted in the man page as follows:

arc4random_uniform(upper_bound) will return a uniformly distributed random number less than upper_bound. arc4random_uniform() is recommended over constructions like ``arc4random() % upper_bound'' as it avoids "modulo bias" when the upper bound is not a power of two.

In short you get a more evenly distributed random number generated.

Answer 2

In many situations 10 thru 30 would mean inclusive, (includes 10 and 30) ...

int fromNumber = 10;
int toNumber = 30;
toNumber ++;
int randomNumber = (arc4random()%(toNumber-fromNumber))+fromNumber;

Notice the difference toNumber - fromNumber is now 21 ... (20+1) which yields the possible results of 0 thru 20 (inclusive) which when added to fromNumber (10) results in 10 thru 30 (inclusive).

Answer 3

int fromNumber = 10;
int toNumber = 30;
int randomNumber = (arc4random()%(toNumber-fromNumber))+fromNumber;

Will generate randon number between 10 and 30, i.e. 11,12,13,14......29

Answer 4

You can use this code for generating random values with range:

//range from 50 to 100
int num1 = (arc4random() % 50) + 50; or
int num1 = arc4random_uniform(50) + 50;

//range from 0-100
int num1 = arc4random() % 100; or
int num1 = arc4random_uniform(100);

Answer 5

In Swift you can use this (inspired by answer of @Justyn)

func generateRandomKey(fromRange rangeFrom:Int, toRange rangeTo:Int) -> Int{

    let theKey = arc4random_uniform(UInt32(rangeTo - rangeFrom)) +  UInt32(rangeFrom)
    return Int(theKey)
}

Will always give you a random range Integer.

Answer 6

As pointed out in other posts below, it is better to use arc4random_uniform. (When this answer was originally written, arc4random_uniform was not available). Besides avoiding the modulo bias of arc4random() % x, it also avoids a seeding problem with arc4random when used recursively in short timeframes.

arc4random_uniform(4)

will generate 0, 1, 2 or 3. Thus you could use:

arc4random_uniform(51)

and merely add 50 to the result to get a range between 50 & 100 (inclusive).