Explain this line of code (Index)

Hi, can someone explain to me what these line of code mean?

L = 2400

i = ceil(L*rand(1))

x = periodic_index(i, L, 1);

I understand what the code in the first two lines mean. My main concern is with the code in the last line.

Thanks in advance.

[Arif Hoq]

periodic_index is a user defined function i guess

Answer 1

[Image Analyst]

The second line does not always give 1. It gives a number between 1 and 2400 rounded up to the next higher integer. Look:

L = 2400 ;
i = ceil(L*rand(1)) 

i = 1778

The code

x = periodic_index(i, L, 1); 

basically picks out a value from column 2400 (which is L in the second index position) from a row randomly chosen from the range 1-2400 (which is i).

The code is poorly written for many reason:

1. No comments
2. Using i (the imaginary variable as a variable name)
3. No check that L does not exceed the number of rows in periodic_index
4. L and x are non-descriptive variable names (What the heck does L stand for?)

Be careful if you're taking programming advice from that person. Try this instead:

MATLAB Behavior: for » Loren on the Art of MATLAB

[Kai Yi Ng]

Thanks a lot for the answer! Really appreciate the comments for the code.

[Image Analyst]

OK, please click "Accept this answer" for the one, best answer. Thanks in advance. 🙂

Answer 2

[KSSV]

L = 2400 ;  % a number 
i = ceil(L*rand(1))   % ceil(rand(1)) always gives 1 
x = periodic_index(i, L, 1);  % From periodic_index array extract 2400x2400x1 element if it is array

[Kai Yi Ng]

I thought rand(1) gives a random value from 0 to 1. So why does the second line always give 1?

[KSSV]

Yes rand/ rand(1) will give you a random number lying betweem (0,1), as the function ceil is used, result will be 1. You can check it by yourself.

for i = 1:100
    ceil(rand)
end

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1

ans = 1