Constructing a matrix in matlab from values generated by for loop

Question

Possible Duplicates:
Matlab - building an array while looping
Matrix of unknown length in MATLAB?

How do you put all of the "a" values together to form a vector?

for i=1:3
    a=2+i
end

Also this is maybe a style question but when do you put a semicolon after the end in a for loop such as the one above, also is it proper to put a semicolon after the first line?

Answer 1

You need to index into a, like this:

for ii=1:3
  a(ii) = 2+ii;
end

I prefer to use ii as a loop variable to avoid clashing with MATLAB's built-in i. You should also pre-allocate a if you know the size before the start of the loop, like so:

N = 100;
a = zeros(1,N);
for ii=1:N
  a(ii) = 2 + ii; 
end

Personally, I never put any punctuation after the for ii=1:3 part, except when writing a one-liner FOR loop, like so:

for ii=1:N, a(ii) = 2 + ii; end

Answer 2

Note that you can construct this more efficiently as such:

a=1:3;
a=a+2;

The first line assigns a to be the vector (1,2,3), the 2nd line adds 2 to every element.

"Efficiency" doesn't matter much in such a small vector, but in general you'll get much better mileage out of matlab if you get used to thinking more like this.