reducing computational time by removing global variable

hello, i wanted to know whats the best way to get rid of the use of 'global" and still be able to run this code as it takes alot of time to get results because of the use of 'global'.

Lx = 101; % Defines the size of the lattice . 
Ly = 425;
pt =rand(425,101);
pfront=zeros(425,101);
IM = ones(Ly,Lx).*2; % Initialization of the lattice
% IM(i,j)=2
t =1;
tend =1; % Number of "time steps".
k = 1; % Counting index for front propagation . 
q = 1; % Counting index for image writing .
% Initial step. The invader starts from a "point" or a channel 
%IM(Ly,50)=1; % Point IP
IM(Ly,1:Lx)=1; % Channel IP
% Finds the indices of the largest element in a given row in pt , % and invades (=1) the corresponding element in IM
IM(Ly-1,pt(Ly-1,1:Lx)==max(pt(Ly-1,1:Lx))) = 1; 
while tend==1
if max(IM(2,1:end))==1 
    break
end
% Check if there are trapped clusters
IM=bwlabel(IM-1,4); % Labels defending clusters , by setting invader sites = 0.
IM=IM+1; % Keeps the labeling , and puts the invader sites = 1.
for i = 1:Ly
s = sort(IM(i,1:end)); % Sorts the i?th row in ascending order. 
if s(1)==1 % Tests if the i ? th row contains an invader site .
for j = 1:Lx
bin = front(i-1,j); % If the i?th row contains an invader site , all the elements in the (i?1)?th row are % being tested in the function front .m to % see if they are part of the front .
if bin==1
% pfront is the front matrix where the first column
% contains the probability and the second and third
% columns contain the corresponding indices for this front %site.
pfront(k,1) = pt(i-1,j); 
pfront(k,2) = i-1; 
pfront(k,3) = j;
k = k+1;
end
end
end
end
% Probability . % y?coordinate. %x?coordinate.
% Finds the row?index of the largest probability in pfront , % and invades (=1) the corresponding element in IM
e = find(pfront(:,1)==max(pfront(:,1))); 
IM(pfront(e,2),pfront(e,3)) = 1;
clear pfront 
k = 1;
% Put invader sites = 1, and defending sites = 0.
IM(IM ~= 1) = 0;
% The following lines saves the image for each iteration .
if ((t/1000)==q)
pic = label2rgb(IM(2:end,1:end)); number = int2str(q);
picloc = strcat('f:\',number,'.tif'); 
imwrite ( pic , picloc ,'tif') ;
q = q+1;
end
t = t+1;
end

function [bin] = front(a,b)
global IM
global Lx
global Ly
% The site is part of the front if one of the neighboring sites % is an invader.
if IM(a,b)~=IM(1,1) % Takes trapped clusters into account . If the site
% tested is not part of the large defending cluster it
% should not be counted as a front site .
bin=0;
elseif (b+1)==(Lx+1)
if (IM(a+1,b)==1)||(IM(a-1,b)==1)||(IM(a,b-1)==1) 
    bin = 1;
else
bin = 0;
end
elseif (b-1)==(1-1)
if (IM(a+1,b)==1)||(IM(a-1,b)==1)||(IM(a,b+1)==1)
    bin = 1;
else
bin = 0;
end
elseif (a+1)==(Ly+1)
if (IM(a-1,b)==1)||(IM(a,b+1)==1)||(IM(a,b-1)==1)
    bin = 1;
else
bin = 0;
end
elseif (a-1)==(1-1)
if (IM(a+1,b)==1)||(IM(a,b+1)==1)||(IM(a,b-1)==1) 
    bin = 1;
else
bin = 0;
end
else
if (IM(a+1,b)==1)||(IM(a-1,b)==1)||(IM(a,b+1)==1)||(IM(a,b-1)==1) 
    bin = 1;
else
bin = 0;
end
end
end

[Guillaume]

While global variables are indeed a very bad idea and are completely unnecessary in your code, have you actually established that they are the cause of the slow processing? I really doubt that they have any impact on the speed of your code.

It's extremely simple to get rid of the global variables, just pass IM, Lx and Ly as arguments, the same way you pass a and b

[Nkenna Amadi]

sorry could you show me an example of how to do this as when i do it it gives me an error. I want to see if im doing it right

[Nkenna Amadi]

and yes they are the cause of the slow processing

[Stephen23]

"and yes they are the cause of the slow processing"

How are you testing this?

[Nkenna Amadi]

i use the run and time which gives a profile at the end. i saw that the lines with global were responsible for the high computational time. with the 'global' removed now, what took me 30mins before takes less than 5mins.

Answer 1

[Guillaume]

function [bin] = front(a, b, IM, Lx, Ly)
    if IM(a,b)~=IM(1,1) 
    %... rest of code as it was
end

Then in your script instead of

bin = front(i-1,j);

use

bin =  front(i-1, j, IM, Lx, Ly);

[Nkenna Amadi]

thank you very much this really helped alot.