Sine
series  working without the sine (or cosine) function
Four ways to code a sine/cosine series in Matlab

The
sine
function (usually expressed in programming code as sin(th),
where th
is an angle in radians)
is one of the
basic functions in trigonometry. In this article, we’re going to
explore a number
of ways to calculate the sine
series without actually using the sine (or
cosine) function.
We’re going to play with the concepts of sine series, iterations,
vectorizing
programs... among others. 
Sometimes we just can’t use a trigonometric
function
and we must use an alternative method to generate a table of values,
for
example.
The
formulas that we are
going to review and code are based
on a few of the ideas expressed at
and
.
Experiment
1. Let’s code the cosine series to avoid using the cos(x) function
In this
case the formula under study is the cosine series, defined as
We obviously
can’t use an infinite number of terms, but we can see how accurate is
our
approach with a few terms. Our proposed function is
function y =
cos1(x)
n = 0 :
100;
y =
sum((1).^n .* x.^(2*n) ./ factorial(2*n));
We’re
not using forloops (that’s vectorization), but we’re using some
builtin
functions such as sum and factorial.
We’re
assumming that x
is a scalar number, and our calculation includes only
100
terms. We’re just implementing the formula above... We must keep in
mind
that
this is an approximation only.
If we
test the code by comparing what the actual ‘cos’ function calculates
against
our result, then
x = 10;
cos(x)
y =
cos1(x)
x = 40;
cos(x)
y =
cos1(x)
We get
from Matlab...
ans
= 0.8391
y = 0.8391
ans
= 0.6669
y =
NaN
We can
see that our result is good enough for the first case, but it’s not
good for
the second case. We must take into account that the builtin factorial
function
uses an algorithm with double precision numbers, and since double
precision numbers
only have about 15 digits, the answer is only accurate for N <=
21. We must
be very aware of the limitations of our resources...
Experiment
2. We’ll implement the sine series accepting a whole vector now
We’re
going to produce a whole response now, not only a scalar. This function
will
receive a vector and will output another one. The sine series formula
that
we’re going to implement is
Our
proposed code in this case is the following
function y =
sin2(x)
n = 1 :
100;
c =
2*n1;
s =
(1).^(n1);
f =
factorial(c);
for i = 1 :
length(x)
y(i) = sum(s .* x(i).^c ./ f);
end
We are just coding the
formula... We calculate the three parts
of the vectors: the alternating sign (s), the part that contains x_{i} (within the
loop), and the
factorial part (f). Finally
we multiply, divide and sum to produce every response corresponding to
each
element in the input vector. We have to iterate along all of the
elements of
the input vector to obtain all of the elements of the output vector.
Again,
we’re using n = 100, which seems to be a good enough number for our
experiments...
We can fastly test the
code, like this
x = 0 :
.1 : 10*pi;
plot(x,sin(x),'b',
x,sin2(x),'ro')
We use horizontal values
from 0 up to 10π, and we’re
comparing the
regular sine function (bluesolid line) against our experiment (sine
series in red
circles).
The result is as follows
The
approximation seems to work fine. Of course, we know that we need only
values
(responses) from 0 to π/2 and that we can modify our algorithm to
repeat those responses or change the sign as necessary, but we want
also to
simplify the code.
Experiment
3. We’ll use a product instead of a sum to generate the sine wave
We’re
going to implement another formula, shown here
If we use n = 1000, we
can implement an approach of the
series in this way
function y =
sin3(x)
n = 1 :
1000;
for i = 1 :
length(x)
y(i) = x(i).*prod(1  x(i)^2 ./ (n*pi).^2);
end
and a
fast test is achieved in this way
x = 0 :
.1 : 4*pi;
plot(x,sin(x),'b',
x,sin3(x),'r')
The result
is shown in the following figure
If we used n = 100 only,
we’d get
The blue line is the
ordinary sine function; the red line is
our approximation. We can see that the higher the xvalue (angle) the
higher
number of n elements we have to have in order to obtain a good
approximation.
It’s all about accuracy in our response. It can deteriorate very
easily...
Experiment
4. We’ll use exponential functions to generate the sine wave
The
definition of the sine function can be extended to complex arguments
z, using the definition Another available formula is
This
formula is very easy to evaluate if we have access to exponential
functions and
complex (imaginary) numbers. e is
the
base of the natural logarithm and i
is the imaginary number.
The
suggested code is very straightforward
function y =
sin4(z)
y =
(exp(i*z)  exp(i*z))/(2i);
The
equation is exact, it’s not an approximation. To test it we create a
simple
script like this
x = 0 :
.1 : 20*pi;
plot(x,sin(x),'b',
x,sin4(x),'ro')
and our
Matlab result is
We show 10 full cycles to show the accuracy of this method. The
bluesolid line is
the sine itself, and the red circles
represent the implemented formula.
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