#6: Rounding numbers

#6: Notes on Rounding numbers

By Larissa Cury in R

April 30, 2023

Shall we start with notes on Rounding numbers?

Ps: Notes based on Jonas Schmedtmann’s course on Udemy – No credits intended

Rouding Integers

#1. Math.trunc()

Aim: Remove any decimals after the integral part of the number
Mutates the array? N/A
Input: a number
Returns: Returns the integral part of the a numeric expression, x, removing any fractional digits. If x is already an integer, the result is x.
Does type coercion? Yes!

// Countries' data:

const countriesPOP = [214.3, 5.857, 331.9, 59.11, 51.52, 45.81];

const countriesSUM = countriesPOP.reduce(
  (acc, country, i, arr) => acc + country,
  0
);

console.log(countriesSUM); // 708.4970000000001

// Parameters:

// 1. x — A numeric expression.

// In practice:

console.log(Math.trunc(countriesSUM)); // returns 708

// Other examples:

console.log(Math.trunc(20.89)); // returns 20
console.log(Math.trunc(20.39)); // returns 20

#2. Math.round()

Aim: Rounds to the next integer
Mutates the array? N/A
Input: a number
Returns: Returns a supplied numeric expression rounded to the nearest integer.
Does type coercion? Yes!

// Countries' data:

const countriesPOP = [214.3, 5.857, 331.9, 59.11, 51.52, 45.81];

const countriesSUM = countriesPOP.reduce(
  (acc, country, i, arr) => acc + country,
  0
);

console.log(countriesSUM); // 708.4970000000001

// Parameters:

// 1. x — The value to be rounded to the nearest integer.

// In practice:

console.log(Math.round(countriesSUM)); // returns 708

// Other examples:


console.log(Math.round(countriesSUM)); // returns 708


console.log(Math.round(20.89)); // returns 21
console.log(Math.round(20.39)); // returns 20
console.log(Math.round('20.39')); // returns 20

#3. Math.ceil()

Aim: To round up a number
Mutates the array? N/A
Input: a number
Returns: Returns the smallest integer greater than or equal to its numeric argument.
Does type coercion? Yes!

// Countries' data:

const countriesPOP = [214.3, 5.857, 331.9, 59.11, 51.52, 45.81];

const countriesSUM = countriesPOP.reduce(
  (acc, country, i, arr) => acc + country,
  0
);

console.log(countriesSUM); // 708.4970000000001

// Parameters:

// 1.@param x — A numeric expression.

// In practice:

console.log(Math.ceil(countriesSUM)); // returns 709

// Other examples:

console.log(Math.ceil(20.89)); // returns 21
console.log(Math.ceil(20.39)); // returns 21
console.log(Math.ceil('20.39')); // returns 21

#4. Math.floor()

Aim: To round down a number
Mutates the array? N/A
Input: a number
Returns: Returns the greatest integer less than or equal to its numeric argument.
Does type coercion? Yes!

// Countries' data:

const countriesPOP = [214.3, 5.857, 331.9, 59.11, 51.52, 45.81];

const countriesSUM = countriesPOP.reduce(
  (acc, country, i, arr) => acc + country,
  0
);

console.log(countriesSUM); // 708.4970000000001

// Parameters:

// 1.@param x — A numeric expression.

// In practice:

console.log(Math.floor(countriesSUM)); // returns 708

// Other examples:

console.log(Math.floor(20.89)); // returns 20
console.log(Math.floor(20.39)); // returns 20
console.log(Math.floor('20.39')); // returns 20

#5. Math.floor()

Aim: To round down a number
Mutates the array? N/A
Input: a number
Returns: Returns the greatest integer less than or equal to its numeric argument.
Does type coercion? Yes!

// Countries' data:

const countriesPOP = [214.3, 5.857, 331.9, 59.11, 51.52, 45.81];

const countriesSUM = countriesPOP.reduce(
  (acc, country, i, arr) => acc + country,
  0
);

console.log(countriesSUM); // 708.4970000000001

// Parameters:

// 1.@param x — A numeric expression.

// In practice:

console.log(Math.floor(countriesSUM)); // returns 708

// Other examples:

console.log(Math.floor(20.89)); // returns 20
console.log(Math.floor(20.39)); // returns 20
console.log(Math.floor('20.39')); // returns 20

Note: Math.trunc() x Math.floor()

They are equal when dealing with POSITIVE numbers. However, they aren’t equal when dealing with NEGATIVE numbers
With negative numbers, rounding works the other way around
So, the tip is to use Math.floor() instead of Math.trunc()

console.log(Math.trunc(-50.2)) // - 50
console.log(Math.floor(-50.2)) // - 51

Rouding floating point numbers (aka, decimals)

#6. .toFixed()

Aim: To round the decimals
Mutates the array? N/A
Input: a number
Returns: Returns a string representing a number in fixed-point notation.
Does type coercion? No!

// Countries' data:

const countriesPOP = [214.3, 5.857, 331.9, 59.11, 51.52, 45.81];

const countriesSUM = countriesPOP.reduce(
  (acc, country, i, arr) => acc + country,
  0
);

console.log(countriesSUM); // 708.4970000000001

// Parameters:

// 1. fractionDigits — Number of digits after the decimal point. Must be in the range 0 - 20, inclusive.


// In practice:

console.log(countriesSUM.toFixed()); // 708
console.log(countriesSUM.toFixed(1)); // 708.5
console.log(countriesSUM.toFixed(2)); // 708.50
console.log(countriesSUM.toFixed(3)); // 708.497
console.log((-25.3684555585).toFixed(3)); // -25.368


// To return a number:

console.log(+countriesSUM.toFixed()); // 708
console.log(+countriesSUM.toFixed(1)); // 708.5
console.log(+countriesSUM.toFixed(2)); // 708.50
console.log(+(-25.3684555585).toFixed(3)); // -25.368

🐕 Au-au! This section will be written soon!