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# Simple Question about Optimization of a FOR loop

Asked by Mohsen on 14 Jun 2013

How can I optimize (vectorize?) the following code in order to avoid the for loop and make the code to run faster?

```    for i=1:55;
A(i)=B((i-1)*3+1,1);
end```

A is 1 x 55 and B is 165 x 1

Many thanks!

Matt Kindig on 14 Jun 2013

This loop will make A only 1x55, not 55x55.

Laila Kazemi on 14 Jun 2013

bc ur only assigning one raw to A

Mohsen on 14 Jun 2013

You are both right!

A is 1 x 55

Do you know how to optimize the for loop? Thanks!

## Products

Answer by Laila Kazemi on 14 Jun 2013

C= B*3 +1 A (1:55, 1:55)= C(1:55, 1)

but the dimensions of ur matrices dont match , if B was 55 by 1 , it would have worked

Laila Kazemi on 14 Jun 2013

C= B*3 +1

A (1:55, 1:55)= C(1:55, 1)

Laila Kazemi on 14 Jun 2013

actaully instead one make a matrix called D with D( , 1:165)=(1)

Mohsen on 16 Jun 2013

Thanks for your answer but it doesn't seem to be the right answer... Please note that the multiplication by 3 and the addition by 1 is used to access the content of each element and not to modify the content of the elements...

I found this answer and it worked:

A=B(1:3:55*3,1);

Thanks again.

Answer by Andrei Bobrov on 14 Jun 2013
Edited by Andrei Bobrov on 14 Jun 2013
```A(1,:) = B((0:size(A,2)-1)*3+1);
```

## 1 Comment

Mohsen on 16 Jun 2013