12.6. Combining Math
Methods¶
The Math
methods provide useful actions, but each one is fairly specific in
what it does (e.g. taking a square root). At first glance, this might seem to
limit how often we need to call on Math
. However, the methods can be
manipulated or combined to produce some clever results.
12.6.1. Random Selection From an Array¶
To select a random item from the array happiness = ['Hope', 'Joy', 'Peace',
'Love', 'Kindness', 'Puppies', 'Kittens', 'Tortoise']
, we need to randomly
generate an index value from 0 to 7. Since Math.random()
returns a
decimal number between 0 and 1, the method on its own will not work.
The Math.random appendix page describes how to
generate random integers by combining the random
and floor
methods. We
will use this functionality now.
Let's define a function that takes an array as a parameter. Since we might not
know how many items are in the array, we cannot multiply Math.round()
by a
specific value. Fortunately, we have the length
property…
Example
1 2 3 4 5 6 7 8 9 10  function randomSelection(arr){
let index = Math.floor(Math.random()*arr.length);
return arr[index];
}
let happiness = ['Hope','Joy','Peace','Love','Kindness','Puppies','Kittens','Tortoise'];
for (i=0; i < 8; i++){
console.log(randomSelection(happiness));
}

Console Output
Tortoise
Love
Kindness
Hope
Kittens
Kindness
Love
Hope
The happiness
array has a length of 8, so in line 2
Math.floor(Math.random()*arr.length)
evaluates as
Math.floor(Math.random()*8)
, which generates an integer from 0 to 7.
Line 3 then returns a random selection from the array.
12.6.2. Rounding to Decimal Places¶
The ceil
, floor
, and round
methods all take a decimal value and
return an integer, but what if we wanted to round 5.56789123 to two decimal
places? Let's explore how to make this happen by starting with a simpler
example.
Math.round(1.23)
returns 1, but what if we want to round to one decimal
place (1.2)? We cannot alter what round
doesit always returns an
integer. However, we CAN change the number used as the argument.
Let's multiply 1.23 by 10 (1.23*10 = 12.3) and then apply the method.
Math.round(12.3)
returns 12. Why do this? Well, if we divide 12 by 10
(12/10 = 1.2) we get the result of rounding 1.23 to one decimal place.
Combining these steps gives us Math.round(1.23*10)/10
, which returns the
value 1.2.
Let's return to 5.56789123 and step through the logic for rounding to two decimal places:
Step 
Description 


Evaluate the numbers in () first: 5.56789123*100 = 556.789123 

Apply the 

Perform the division 557/100 = 5.57 
The clever trick for rounding to decimal places is to multiply the original
number by some factor of 10, round
the result, then divide the integer by
the same factor of 10. The number of digits we want after the decimal are
shifted in front of the '.' before rounding, then moved back into place by the
division.
Decimal Places In Answer 
Multiply & Divide By 
Syntax 

1 
10 

2 
100 

3 
1000 

etc. 
etc. 
etc. 
12.6.3. Check Your Understanding¶
Question
Which of the following correctly rounds 12.3456789 to 4 decimal places?
Math.round(12.3456789)*100/100
Math.round(12.3456789*100)/100
Math.round(12.3456789*10000)/10000
Math.round(12.3456789)*10000/10000