Thread Subject:

How to reduce these loops into a line

Hi all,

Since loops elongate the run time, I was thinking how can I reduce these two loops and instead write them in a line using vector indexing perhaps.

noisycode=[];
    for j = 1:111
        for i=1:111
            noisycode = [noisycode, r(96+5*i , 96+5*j)];
        end
    end

The thing is that order is important here; I want to sweep the "r" matrix in a column-wise manner i.e. read column by column.
Do you guys have any suggestion of how to do it?

Thanks,
Elnaz

On 5/15/2012 5:31 PM, Elnaz wrote:
> Hi all,
> Since loops elongate the run time, I was thinking how can I reduce these
> two loops and instead write them in a line using vector indexing perhaps.
> noisycode=[];
> for j = 1:111
> for i=1:111
> noisycode = [noisycode, r(96+5*i , 96+5*j)];
> end end
>
> The thing is that order is important here; I want to sweep the "r"
> matrix in a column-wise manner i.e. read column by column. Do you guys
> have any suggestion of how to do it?
>
> Thanks,
> Elnaz

This looks like a reshape. Just use the RESHAPE function.
Pick out the section of r you need, then reshape it into a row vector.
Something like this:

n2 = reshape( r(96+5:5:96+555,96+5:5:96+555), 1, 111*111);

--
Idin Motedayen-Aval
The MathWorks, Inc.
zq=[4 2 5 -15 -1 -3 24 -57 45 -12 19 -12 15 -8 3 -7 8 -69 53 12 -2];
char(filter(1,[1,-1],[105 zq])), clear zq

On 5/15/2012 5:31 PM, Elnaz wrote:
> Hi all,
> Since loops elongate the run time, I was thinking how can I reduce these
> two loops and instead write them in a line using vector indexing perhaps.
> noisycode=[];
> for j = 1:111
> for i=1:111
> noisycode = [noisycode, r(96+5*i , 96+5*j)];
> end end
>
> The thing is that order is important here; I want to sweep the "r"
> matrix in a column-wise manner i.e. read column by column. Do you guys
> have any suggestion of how to do it?
>
> Thanks,
> Elnaz

I should also note that the killer in your code isn't so much the FOR
loops, but the fact that you're growing the noisycode vector inside the
loop. This code will be much more efficient:
noisycode=zeros(111,111); % pre-allocate your array
for j = 1:111
     for i=1:111
         noisycode(i,j) = r(96+5*i , 96+5*j);
     end
end

--
Idin Motedayen-Aval
The MathWorks, Inc.
zq=[4 2 5 -15 -1 -3 24 -57 45 -12 19 -12 15 -8 3 -7 8 -69 53 12 -2];
char(filter(1,[1,-1],[105 zq])), clear zq

That was greatly helpful; thanks.
What if I want to add 4 columns and rows of zeros between each element of a matrix just to separate the elements from each other with zeros in between, like this:

for i = 1:111
    for j = 1:111
        bits(5*i-4,5*j-4) = image(i,j);
    end
end

Can I do this without loops?

Elnaz

"Elnaz " <ebsadeghian@gmail.com> wrote in message <joupke$inc$1@newscl01ah.mathworks.com>...
> That was greatly helpful; thanks.
> What if I want to add 4 columns and rows of zeros between each element of a matrix just to separate the elements from each other with zeros in between, like this:
>
> for i = 1:111
> for j = 1:111
> bits(5*i-4,5*j-4) = image(i,j);
> end
> end
>
> Can I do this without loops?
>
> Elnaz

I have my own answer:

bits = zeros(551,551);
bits(1:5:551,1:5:551) = image;