Selecting only half a row of an array

Hey guys,

Im fairly new to MATLAB and I'm just having a problem working out how to multiply ony half a row of my array.

I was hoping someone could help me out.

Basic example

a = 1x5

1

2

3

4

5

6

How would I select 2, 3, 4 and multiply it by 3

and how would I select 4, 5, 6 and multiply it by 3

Answer 1

[KSSV]

Read about MATLAB array indexing, instead of asking very simple basic questions.

If a is an narray of size 1x5.

a(1:2)  % gives first two arrays 
a(1)  % gives first 
a(end)   % gives last element 
a(2:4) 
iwant = a(1:3)*3 ;  % pick first three and multiply by 3 

[Jay Perks]

Thanks for the answer KSSV,

I had a look over indexing and I think I now know where I have gone wrong but comes another question.

I have an array of 201x1 with values of forces for a truss over a domain of -90 to 90 degrees.

When calculating the values, the values from 101 to 201 are to be negative to which I was going to multiply by -1.

when I select the range to be -1, how do I keep the first 101 values?

example

BA = cosd(DOMAIND); % BA = 201x1

BA =BA(101:201,1).*-1; % BA = 101x1

Do I need to add this onto my BA array or is there a better way of getting my full 201x1 array with half the values negative?

[KSSV]

BA = zeros(201,1) ; 
BA = cosd(DOMAIND); % BA = 201x1
BA =BA(101:201,1).*-1; % BA = 101x1

[Jay Perks]

Thanks KSSV!

In the end I used the below command to keep my 201x1 array without it changing to 101x1.

BA(101:201,1) = BA(101:201,1).*-1

Thankyou!