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A random integer from min to max

importance: 2

Create a function randomInteger(min, max) that generates a random integer number from min to max including both min and max as possible values.

Any number from the interval min..max must appear with the same probability.

Examples of its work:

alert( random(1, 5) ); // 1
alert( random(1, 5) ); // 3
alert( random(1, 5) ); // 5

The simple but wrong solution

The simplest, but wrong solution would be to generate a value from min to max and round it:

function randomInteger(min, max) {
  let rnd = min + Math.random() * (max - min);
  return Math.round(rnd);
}

alert( randomInteger(1, 3) );

The function works, but it is incorrect. The probability to get edge values min and max is two times less than any other.

If you run the example above many times, you would easily see that 2 appears the most often.

That happens because Math.round() gets random numbers from the interval 1..3 and rounds them as follows:

values from 1    ... to 1.4999999999  become 1
values from 1.5  ... to 2.4999999999  become 2
values from 2.5  ... to 2.9999999999  become 3

Now we can clearly see that 1 gets twice less values than 2. And the same with 3.

The correct solution

There are many correct solutions to the task. One of them is to adjust interval borders. To ensure the same intervals, we can generate values from 0.5 to 2.5, thus adding the required probabilities to the edges:

function randomInteger(min, max) {
  // now rnd is from  (min-0.5) to (max+0.5)
  let rnd = min - 0.5 + Math.random() * (max - min + 1);
  return Math.round(rnd);
}

alert( randomInteger(1, 3) );

An alternative way could be to use Math.floor for a random number from min to max+1:

function randomInteger(min, max) {
  // here rnd is from min to (max+1)
  let rnd = min + Math.random() * (max + 1 - min);
  return Math.floor(rand);
}

alert( randomInteger(1, 3) );

Now all intervals are mapped this way:

values from 1  ... to 1.9999999999  become 1
values from 2  ... to 2.9999999999  become 2
values from 3  ... to 3.9999999999  become 3

All intervals have the same length, making the final distribution uniform.