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JavaScript

Posted by Gavin Broekema
.a{fill-rule:evenodd;}techdegree
Gavin Broekema
Full Stack JavaScript Techdegree Student 22,443 Points

Math.random [0,1)

In the random number generator challenge, it is said that Math.random selects a random number between 0 and 1 (with 0 inclusive). This is what I came up with from the exercise:

var randInt = Math.floor(Math.random() * (userIntHigh - userIntLow + 1)) + userIntLow;

In the very rare case that Math.random actually chooses 0, wouldn't the outcome of var randInt become userIntLow? Isn't the intent of this challenge to generate a number between userIntLow and userIntHigh? PS I'm probably just overthinking this.

1 Answer

Steven Parker
Steven Parker
243,318 Points
Steven Parker
Steven Parker
243,318 Points

You're right that if the random function returns 0, you get your lower limit. And if it returns something close to 1 (it can never be exactly 1), you will get your upper limit.

The trick here is that the word between refers to a range that is inclusive of the limits, which is different from what it means in a strictly mathematical sense. In computers it's often used in the inclusive way, another good example is SQL (the common database language).

Gavin Broekema
.a{fill-rule:evenodd;}techdegree
Gavin Broekema
Full Stack JavaScript Techdegree Student 22,443 Points

So by adding 1 to (userIntHigh - userIntLow), I am ensuring not only that userIntLow can be generated, but also that userIntHigh can be generated as well? Which leads me to another question... Wouldn't this make the probability of generating userIntLow much lower than the probablity of generating userIntHigh? (seeing that a Math.random value of 0 is much less probable than a value that could be Math.floor--ed down into a userIntHigh equivalent)

Steven Parker
Steven Parker
243,318 Points
Steven Parker
Steven Parker
243,318 Points

But 0 is not the only value that will yield your low limit. Because of the decimal multiply, the chances of any of the possible results are the same. Consider this sample mapping of raw random values to five final function values:

|<--------------------------------------- random ---------------------------------------->|
0                0.2               0.4               0.6               0.8              max
-------------------------------------------------------------------------------------------
|     lowest      |       ---       |       ---       |       ---       |     highest     |
|<-------------------------------------- function --------------------------------------->|
Gavin Broekema
.a{fill-rule:evenodd;}techdegree
Gavin Broekema
Full Stack JavaScript Techdegree Student 22,443 Points

Aha! Thank you Steven, this was very helpful. My head sometimes goes to mush after staring at a screen for too long lol.