CODEWITHSUNDEEP
matlab
William Falcon
William Falcon

Reputation: 9823

What is this code doing? Machine Learning

I'm just learning matlab and I have a snippet of code which I don't understand the syntax of. The x is an n x 1 vector.

Code is below

p  = (min(x):(max(x)/300):max(x))';

The p vector is used a few lines later to plot the function

plot(p,pp*model,'r');

Upvotes: 0

Views: 65

Answers (3)

Robert Peruzzi
Robert Peruzzi

Reputation: 1

This code examines an array named x, and finds its minimum value min(x) and its maximum value max(x). It takes the maximum value and divides it by the constant 300.

It doesn't explicitly name any variable, setting it equal to max(x)/300, but for the sake of explanation, I'm naming it "incr", short for increment.

And, it creates a vector named p. p looks something like this:

p = [min(x), min(x) + incr, min(x) + 2*incr, ..., min(x) + 299*incr, max(x)];

Upvotes: 0

David
David

Reputation: 8459

Certainly looks to me like an odd thing to be doing. Basically, it's creating a vector of values p that range from the smallest to the largest values of x, which is fine, but it's using steps between successive values of max(x)/300.

If min(x)=300 and max(x)=300.5 then this would only give 1 point for p.

On the other hand, if min(x)=-1000 and max(x)=0.3 then p would have thousands of elements.

In fact, it's even worse. If max(x) is negative, then you would get an error as p would start from min(x), some negative number below max(x), and then each element would be smaller than the last.

I think p must be used to create pp or model somehow as well so that the plot works, and without knowing how I can't suggest how to fix this, but I can't think of a good reason why it would be done like this. using linspace(min(x),max(x),300) or setting the step to (max(x)-min(x))/299 would make more sense to me.

Upvotes: 1

rwong
rwong

Reputation: 6162

It generates an arithmetic progression.

An arithmetic progression is a sequence of numbers where the next number is equal to the previous number plus a constant. In an arithmetic progression, this constant must stay the same value.

In your code,

  • min(x) is the initial value of the sequence
  • max(x) / 300 is the increment amount
  • max(x) is the stopping criteria. When the result of incrementation exceeds this stopping criteria, no more items are generated for the sequence.

I cannot comment on this particular choice of initial value and increment amount, without seeing the surrounding code where it was used.

However, from a naive perspective, MATLAB has a linspace command which does something similar, but not exactly the same.

Upvotes: 1

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