problems with the step size in a for loop

Dear readers,

I'm fairly new to Matlab so i beg your pardon for asking what will possible be a rather simple question:

First of all my code:

    R=150;
h(1) = 0.6 ;
for i=1:2:10
   Q(i) = sqrt(h(i)/R)
   h(i+2) = -Q(i)*dT+h(i);
end

My problem is that when i look at the values of Q, it contains a lot of zeros. I realised that those must be values of the even i numbers ( Q(2), Q(4), Q(6) etc. ). My questions are: Why are those filled in as a zero, while i am not defining them at all. and how can i fix this? What i want is that the even i numbers do not have a Q value at all.

[Manoj]

I am not sure how you are calculating Q in the first place, because both h and R are single values. How do you want them to change in the loop to get an array of Q values ?

Answer 1

[Guillaume]

When you assign a value to a matrix, using a non-existent index, matlab will automatically resize the matrix to that index and fill up all non-existent indices below with 0:

m = 6;     %a 1x1 matrix, m(1) = 6
m(5) = 5;  %resizes m to 1x5 matrix and fill missing values with 0
disp(m)    %m(1) = 6, m(2:4) = 0, m(5) = 5

Therefore, when you assign a value to Q(3), it automatically assign a value of 0 to Q(2).

There's no concept of having no value at all in matrices.

You could instead put NaN (not a number) in your matrix:

Q(2:2:end) = NaN;

Or you could use cell arrays which can have empty elements

Q{i} = sqrt(h(i)/R)

Or simply rescale your indices to 1:5

for i=1:5
   Q(i) = sqrt(h(i)/R)
   h(i+1) = -Q(i)*dT+h(i);
end

Answer 2

[Thorsten]

Allocate Q to be a vector of 10 NaNs (not a number) before doing the computations

Q = nan(1,10);