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JavaScript JavaScript Basics (Retired) Working With Numbers The Random Challenge Solution

Posted by Joseph Carrillo
Joseph Carrillo
2,137 Points

The Random Challenge Solution

Hello All,

I was trying to create a random generator but i think i did it correctly. Please if you can verify that i did do it correctly.

I am taking in two inputs from the prompts.

Taking the strings and converting to integers.

Adding those two values together then dividing it to get one single value.

Then i am taking that value and working with the Math.random object.

Here is my code below:

document.write('<h1> This is a program that will take two number inputs from 1 to 10.</h1><br /><br />'); document.write('<h3> Then it will use the two numbers to return a random number from the two inputs.</h3><br /><br />');

var numOne = prompt('Please put the first number in from 1 to 10');

resultOne = parseInt(numOne);

var numTwo = prompt('Please put the second number in from 1 to 10');

resultTwo = parseInt(numTwo);

randomNum = (resultOne + resultTwo) / 2;

document.write(Math.floor((Math.random() * randomNum) + 1));

Thank you for your help!

JC

3 Answers

Michael Davis
PLUS
Michael Davis
Courses Plus Student 12,508 Points
Michael Davis
Michael Davis
Courses Plus Student 12,508 Points

Ok, so here is your code as I understand it.

document.write('<h1> This is a program that will take two number inputs from 1 to 10.</h1><br /><br />'); document.write('<h3> Then it will use the two numbers to return a random number from the two inputs.</h3><br /><br />');

var numOne = prompt('Please put the first number in from 1 to 10');

resultOne = parseInt(numOne);

var numTwo = prompt('Please put the second number in from 1 to 10');

resultTwo = parseInt(numTwo);

randomNum = (resultOne + resultTwo) / 2;

document.write(Math.floor((Math.random() * randomNum) + 1));

Now, if you are trying to get a random number between 1 and the average of the two numbers... you're close.

However, if you're wanting to get a random number between resultOne and resultTwo, a few changes are in order.

Math.random() will generate a number between 0 and 1, without including 1. The random number will never be 1. This is why we add 1 at the end. If you tried Math.random(10) you could get any number from 0 to 10, without ever reaching 10. It could be 9.99999999, but you cannot get 10. We use Math.floor() which will drop everything after the decimal. So if Math.random(10) produces 9.898989, Math.floor() turns it into 9.

If you wanted to get a random number between 10 and 30, then you would use

Math.floor( Math.random(21) + 10 )

// Possible numbers are 10 - 30

Formula: Math.floor(Math.random()*(max-min+1)+min);

Joseph Carrillo
Joseph Carrillo
2,137 Points

Michael,

Thanks for your reply. I see what I am doing now. If I am inputting 6 and it adds 6 then that is 12 / 2 which is 6. Then it will give me a random number between 1 and 6.

I am trying to get a number between 1 and 10 using the numbers I input. So I am assuming I would use the solution you advised.

I am still unclear about why (max - min + 1) + min. Could you explain a little further and dissect what this means.

I am thinking if it takes 10 and 5 as the inputs. Which would be the result 5 + 1 which is 6. Then it would add 5 which would be 11. So then would it give you a random number from 1 and 11?

Thanks, JC

Michael Davis
Michael Davis
Courses Plus Student 12,508 Points
Michael Davis
Michael Davis
Courses Plus Student 12,508 Points

You're close. Using you're example, here is some pseudo-code.

// Random Number between 5 and 10

var myRand = Math.floor(Math.random( (10 - 5) + 1) + 5);

Let's break this down:

// Let's follow a random number of 5 to 10.

Step 1: var a = Math.floor(Math.random((10 - 5) + 1) + 5);
Step 2: var a = Math.floor(Math.random(5 + 1 ) + 5);
Step 3: var a = Math.floor(Math.random(6) + 5);
Step 4: var a = Math.floor(5.9899 + 5);
Step 5: var a = Math.floor(10.9899);

Final:  var a = 10.

So we had 10.9899, but we wanted a number between 1 and 10, right? Well, since we introduce our friend Math.floor() this is what happened.

Math.floor(10.9899);

// Becomes 10 because Math.floor() drops everything after the decimal.

I hope this helps!

Joseph Carrillo
Joseph Carrillo
2,137 Points

Michael will you see my revised post! I believe i did it correctly. (max - min) does matter if you put 1 in the first prompt or the second prompt. Or 10 in the first and 1 in the second. It will still do the calculations correctly.

Joseph Carrillo
Joseph Carrillo
2,137 Points

Thanks Michael! I think if I break it down further would be less confusing. It is so much easier to hard code the number than working with variables. I am a novice so it was hard understanding the formula. I get the concepts but the logic is not there for me.

Michael Davis
Michael Davis
Courses Plus Student 12,508 Points
Michael Davis
Michael Davis
Courses Plus Student 12,508 Points

Biggest thing to remember is that Math.random() will give a random number between 0, and just before 1. So anything you multiply it by will be between 0 and just before that number.

So If I did Math.random() * 40, I could get any number between 0 and 39.999999999.

Also, Math.floor() removes everything after the decimal. It does not round, it simply ignores anything after the decimal. So the same 39.99999999 then becomes 39.

So, given the above, my Math.random() * 40 is actually a 0 - 39 random. This is why we add 1 after the number is generated. This makes it a number between 1 and 40, inclusive.

Joseph Carrillo
Joseph Carrillo
2,137 Points
Joseph Carrillo
Joseph Carrillo
2,137 Points

I see what you mean now. I am going to dissect it to how i understand it.

So we have the formula

document.write(Math.floor(Math.random() * (max - min) + min));

Math.floor will round down to nearest whole number.

Math.random will give us a from 0 to 1 without calculating 1.

max - min which in my case would be 10 - 1, which would give me random number 0 - 9

the + min would add 1 to the random number which would give me 1 - 10 based upon the input.

Whatever you put in the parameters of the Math.random object would be the max number you would have a random number to ?

For example, Math.random(25) * (25 - 1) + 1

Revised Code:

document.write('<h1> This is a program that will take two number inputs from 1 to 10.</h1><br /><br />'); document.write('<h3> Then it will use the two numbers to return a random number from the two inputs.</h3><br /><br />');

var numOne = prompt('Please put the first number in from 1 to 10');

resultOne = parseInt(numOne);

var numTwo = prompt('Please put the second number in from 1 to 10');

resultTwo = parseInt(numTwo);

document.write(Math.floor(Math.random() * (resultTwo - resultOne) + resultOne ));

Michael Davis
Michael Davis
Courses Plus Student 12,508 Points
Michael Davis
Michael Davis
Courses Plus Student 12,508 Points

Your Revised code:

document.write('<h1> This is a program that will take two number inputs from 1 to 10.</h1><br /><br />'); document.write('<h3> Then it will use the two numbers to return a random number from the two inputs.</h3><br /><br />');

var numOne = prompt('Please put the first number in from 1 to 10');

resultOne = parseInt(numOne);

var numTwo = prompt('Please put the second number in from 1 to 10');

resultTwo = parseInt(numTwo);

document.write(Math.floor(Math.random() * (numOne - numTwo) + numOne ));

Everything is right on, except:

document.write(Math.floor(Math.random() * (numOne - numTwo) + numOne ));

This should be: 

document.write( Math.floor(Math.random() * (maxNum - minNum + 1)  + minNum ) );

Remember the formula: Math.floor(Math.random()*(max-min+1)+min);

Also, test it in the browser console.

Type:

Math.floor(Math.random() * (20 - 10 + 1) + 10);

It'll give you a random number between 10 and 20 every time.

Joseph Carrillo
Joseph Carrillo
2,137 Points
Joseph Carrillo
Joseph Carrillo
2,137 Points

I see if i do not add the + 1 to the max - min then i will only calculate up to 9.