for loop

Hi,I have created the following loop:

for i = 1:5 x = i*2 end

I wish to be able to record each value of the iteration i.e. 2,4,6,8,10 aswell as arrive at the final value of 10 which the above code dies.Could you tell me how to do this? Thanks

Answer 1

[Grzegorz Knor]

x = zeros(5,1);
for i = 1:5 
x(i) = i*2; 
end
disp(x)

Answer 2

[Andrei Bobrov]

for i1 = 5:-1:1, x(i1) = i1*2;end

for your case

x = (1:5)*2

or

x = 2:2:10

[Grzegorz Knor]

If I do it this way:

for i1 = 5:-1:1, x(i1) = i1*2;end

it doesn't have to define the variable x before the loop to preallocate memory?

[Andrei Bobrov]

Hi Grzegorz! I begin with last number 'x' and this way preallocate memory. Please read this http://www.mathworks.com/matlabcentral/answers/7039-how-to-multiply-a-vector-with-each-column-of-a-matrix-most-efficiently

[Grzegorz Knor]

nice trick :)

Answer 3

[mohammad]

x=[]; 
for i = 1:5 
x =[x;i*2]; %for column matrix 
end

OR:

x=[];
for i = 1:5 
x =[x,i*2]; %for row matrix 
end

[Andrei Bobrov]

Hi mohammad! Please read this http://www.mathworks.com/help/techdoc/matlab_prog/f8-784135.html#f8-793781

[Jan]

This is working and fast for tiny vectors. But it is really worth to pre-allocate arrays to get familiar with good programming methods. Without pre-allocation, this happens:

1. x is created as empty DOUBLE vector. This needs about 100 bytes for the internal header and 0 bytes for the data.

2. Inside the loop another vector x is created, again 100 bytes header, 8 bytes for the data. The former x is released.

3. In the 2nd iteration the next vector is created, again 100 bytes header, 16 bytes of data, 8 bytes of the formar array are copied. The former x is released.

4. etc.

If the loop runs until N, a total of N*100+prod(1:N)*8 bytes are allocated, filled with zeros, partially released later and prod(1:N-1)*8 bytes are copied.

In consequence you apporach is horribly slow for e.g. 1000 elements and might even crash with an out-of-memory error for larger vectors. Therefore *always* pre-allocate.

Answer 4

[Liam Mescall]

Thanks guys