Programs to Compute Trig Functions in Python, JavaScript, & Perl (with Maclaurin Series)

Introduction

Have you ever wondered how your computer calculates certain mathematical functions, like division, or trigonometric functions like sine or cosine? Well, for some of these mathematical functions, there exist useful formulas to calculate very accurate results pretty easily. For sine and cosine, one commonly used formula looks like this:

Maclaurin Series of sin(x)

And for cosine:

Maclaurin Series of cos(x)

Note that the input of each function is in radians, not degrees.

The series used in both formulas is called a Maclaurin series (a type of Taylor series), and can be derived from the sine and cosine functions with a series expansion.

How the Programs Work

I've written programs to implement these two computations in three major scripting languages: Python, JavaScript, and Perl. These programs do not include any built-in trig functions or other utilities except the use of the in-built π constant in some cases. All code is CC0 licensed.

The approach I used creates a generalized function called computeSeries which takes in x as the number to calculate the sine or cosine of, a starting number in the series (x for sine and 1 for cosine), and the exponent and factorial base in the first term of the series (3 for sine and 2 for cosine).

In calculating each series, I found that only about 10 terms in the series were needed to get a decently accurate result.

The programs additionally include utility functions for sine and cosine functions in degrees. The end of each program also includes a few tests of each function, which work as expected.

In Python

Feel free to view the below code as a GitHub Gist.

1from math import pi
2
3# round a number (x) to nearest 10 digits
4def rounded(x):
5	return round(x, 10)
6
7# get the factorial of a number (x)
8# factorial(x) is the product of every number from 1 to N inclusive
9def factorial(x):
10	n = 1; # n is the result
11	# multiply n by every number from 1 to x inclusive
12	for i in range(2, x + 1):
13		n *= i
14	return n
15
16""" get the result of the cos and sin formulas
17    where the functions are sin(x radians) or cos(x radians),
18    n is the start value (n = x for sin, n = 1 for cos), and
19    i_start is the exponent and factorial base in the first term """
20def computeSeries(x, n, i_start):
21	iterations = 20 # iterations is twice the amount of terms to use
22	multiplier = 1
23	for i in range(i_start, i_start + iterations, 2): # i increases by 2 each term
24		multiplier *= -1 # alternates between addition and subtraction each term
25		next_term = (x**i) / factorial(i) # each term is (x^i) / i!
26		n += multiplier * next_term # add or subtract from final result
27	return n
28
29# get sin of x radians
30def sin(x):
31	return rounded(computeSeries(x, x, 3))
32
33# get cos of x radians
34def cos(x):
35	return rounded(computeSeries(x, 1, 2))
36
37# get sin of x degrees
38def sinDeg(x):
39	return sin(x * pi / 180)
40
41# get cos of x degrees
42def cosDeg(x):
43	return cos(x * pi / 180)
44
45# test the functions
46print(sin(pi / 6)); # 0.5
47print(sinDeg(45)); # 0.7071
48print(sinDeg(52)); # 0.78801
49
50print(cos(pi / 3)); # 0.5
51print(cosDeg(45)); # 0.7071
52print(cosDeg(52)); # 0.615661
53

In JavaScript

Feel free to view the below code as a GitHub Gist.

1// round a number (x) to nearest 10 digits
2const rounded = (x) => {
3	return parseFloat(x.toFixed(10));
4}
5
6// get the factorial of a number (x)
7// factorial(x) is the product of every number from 1 to x inclusive
8const factorial = (x) => {
9	let n = 1; // n is the result
10	// multiply n by every number from 1 to x inclusive
11	for(let i = 2; i <= x; i++) {
12		n *= i;
13	}
14	return n;
15}
16
17/* get the result of the cos and sin formulas
18   where the functions are sin(x radians) or cos(x radians),
19   n is the start value (x for sin, 1 for cos), and i_start
20   is the exponent and factorial base in the first term */
21const computeSeries = (x, n, i_start) => {
22	const iterations = 20; // iterations is twice the amount of terms to use
23	let multiplier = 1;
24	let i = i_start;
25	while(i < i_start + iterations) {
26		multiplier *= -1; // alternates between addition and subtraction each iteration
27		const next_term = (x**i) / factorial(i); // each term is (x^i) / i!
28		n += multiplier * next_term // add or subtract from final result
29		i += 2 // i increases by 2 each term
30	}
31	return n
32}
33
34// get sin of x radians
35const sin = (x) => {
36	return rounded(computeSeries(x, x, 3));
37}
38// get cos of x radians
39const cos = (x) => {
40	return rounded(computeSeries(x, 1, 2));
41}
42// get sin of x degrees
43const sinDeg = (x) => {
44	return sin(x * Math.PI / 180);
45}
46// get cos of x degrees
47const cosDeg = (x) => {
48	return cos(x * Math.PI / 180);
49}
50
51// test the functions
52console.log(sin(Math.PI / 6)); // 0.5
53console.log(sinDeg(45)); // 0.7071
54console.log(sinDeg(52)); // 0.78801
55
56console.log(cos(Math.PI / 3)); // 0.5
57console.log(cosDeg(45)); // 0.7071
58console.log(cosDeg(52)); // 0.615661
59

In Perl

Feel free to view the below code as a GitHub Gist.

1#!/usr/bin/perl
2use warnings;
3
4$pi = 3.14159265358979323;
5
6# get the factorial of a number (x)
7# factorial(x) is the product of every number from 1 to N inclusive
8sub factorial {
9	my ($x) = @_;
10	my $n = 1; # n is the result
11	# multiply n by every number from 1 to x inclusive
12	my @nums_to_multiply = (1..$x);
13	for(@nums_to_multiply){
14		$n *= $_;
15	}
16	return $n;
17}
18
19=begin
20get the result of the cos and sin formulas
21where the functions are sin(x radians) or cos(x radians),
22n is the start value (n = x for sin, n = 1 for cos), and
23i_start is the exponent and factorial base in the first term
24=cut
25sub computeSeries {
26	$ITERATIONS = 20; # iterations is twice the amount of terms to use
27	my ($x, $n, $i_start) = @_;
28	my $multiplier = 1;
29	$i = $i_start;
30	while($i < $i_start + $ITERATIONS) {
31		$multiplier *= -1; # alternates between addition and subtraction each term
32		$n += $multiplier * (($x**$i) / factorial($i)); # add or subtract ((x^i) / i!) from final result
33		$i += 2; # i increases by 2 each term
34	}
35	return $n;
36}
37
38# get sin of x radians
39sub mySin {
40	my ($x) = @_;
41	return computeSeries($x, $x, 3);
42}
43# get cos of x radians
44sub myCos {
45	my ($x) = @_;
46	return computeSeries($x, 1, 2);
47}
48# get sin of x degrees
49sub sinDeg {
50	my ($x) = @_;
51	return mySin($x * $pi / 180);
52}
53# get cos of x degrees
54sub cosDeg {
55	my ($x) = @_;
56	return myCos($x * $pi / 180);
57}
58
59# test the functions
60print(sin($pi / 6) . "\n"); # 0.5
61print(sinDeg(45)   . "\n"); # 0.7071
62print(sinDeg(52)   . "\n"); # 0.78801
63
64print(cos($pi / 3) . "\n"); # 0.5
65print(cosDeg(45)   . "\n"); # 0.7071
66print(cosDeg(52)   . "\n"); # 0.615661
67

Conclusion

I hope this helps in understanding how computers and languages would go about calculating trigonometric functions like sine and cosine. If you'd like to read more about how exactly mathematical formulas used to calculate the trig functions are derived, I would recommend taking a look at the videos on Taylor and Maclaurin series by Khan Academy.

These programs are all licensed under CC0, so feel free to use any of the code as you wish, without attribution.

Thanks for scrolling.

— Gabriel Romualdo, December 31, 2020