Why are is the zeros or zero(size) function in a for-loop function?

This is what i am trying to answer:

Compute 𝑦 = cos 2 𝑥 for x-values from 0 to 𝜋 (at least 100 values of x). Do this (i) with a for-loop

There are two different ways to do the y output. Im confused on that part

% Compute y= cos2x, with 100 values of x between 0 and pi

% (i) for-loop

% Create vectors
x = linspace (0,pi,100);   % 100 values of x from 0 to pi

% Initialize the output vector y
y = zeros(1,100);

% y = zeros(size(x))^2

%Compute y-values using a for-loop
for i = 1:100
    y(i) = cos(x(i))^2;
end

Answer 1

[Dyuman Joshi]

"Why are is the zeros or zero(size) function in a for-loop function?"

y = zeros(1,100);

This is called Pre-allocation. It is done to improve the code execution time.

Read more here - Preallocation, Techniques to Improve Performance

Another method to obtain the output is to vectorize the code - Vectorization

x = linspace(0,pi,100); 

%Take the cos() of values in x and square every element.
y = cos(x).^2;