Use if / else inside a for-loop and change values of array depending on condition

Hi all,

I am stuck with the following step and I can't find the solution. The code I need is to change the columns of an array depending on the column index. For odd numbers, without change, for even numbers change the sign.

The size of the array should be variable by dimensions 1 and 2.

Here is what I tryied. Can someone help me please.

Thanks

Size_1 = 5;
Size_2 = 6;

array = ones(Size_1,Size_2);

for i=1:Size_2    
    if rem(i,2)==0        
        new_array_2 = array(:,i);
    else
        new_array_2 = -1*array(:,i);
    end    
end

new_array should be this

new_array =

1 -1 1 -1 1 -1

1 -1 1 -1 1 -1

1 -1 1 -1 1 -1

1 -1 1 -1 1 -1

1 -1 1 -1 1 -1

Answer 1

[madhan ravi]

No loops needed:

size1=5;
size2=6;
A=ones(size1,size2);
A(:,2:2:end)=-A(:,2:2:end)

[ARP]

Thanks!

I am still corious what I did wrong on my script. I will need to implement something like that later on. Specifically, to read if the folder number is even or uneven.

[madhan ravi]

See the answer below.

Reasons:

• You forgot to preallocate new_array_2.
• You forgot to index new_array_2.

Answer 2

[madhan ravi]

If you insist loop then:

Size_1 = 5;
Size_2 = 6;
A = ones(Size_1,Size_2);
for i=1:Size_2    
    if rem(i,2)==0        
       A(:,i)=-A(:,i); % if even change the sign
    else
       A(:,i)=A(:,i);
    end    
end

[ARP]

Brilliant.

thanks again