Saving data from nested for loop

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Maaz Madha
Maaz Madha am 17 Mär. 2020
Kommentiert: Maaz Madha am 18 Mär. 2020
Hi
I want to store data from a nested for loop and this is my code
%%Initialisation 1
delta_x=(2*pi)/100;
delta_y=delta_x;
H=2*pi;
L=4*pi;
%Mesh
n=(L/delta_x)+1; Converts rectangle to a mesh
m=(H/delta_y)+1;
x=[0:delta_x:n]';
y=[0:delta_y:m]';
for i=x(2):delta_x:x(end-1)
for j=y(2):delta_y:y(end-1)
pointer=[(j-1)*n+i];
end
end
Everytime the pointer runs it only gives me the latest iteration and not all the iterations like I want. I tried putting (i,j) before it but that was to no avail and now am at my wits end as to what to do. Any help would be much appreciated

Akzeptierte Antwort

Adam Danz
Adam Danz am 17 Mär. 2020
Bearbeitet: Adam Danz am 17 Mär. 2020
Give this a shot. It's usually best to set up for-loops to iterate over integer values 1 to n. Those integer values can be used to index input values and store the outputs.
iLoop = x(2):delta_x:x(end-1);
jLoop = y(2):delta_y:y(end-1);
pointer = nan(numel(iLoop), numel(jLoop));
for i = 1:numel(iLoop)
for j = 1:numel(jLoop)
pointer(i,j)=(jLoop(j)-1)*n+iLoop(i);
end
end
  3 Kommentare
Adam Danz
Adam Danz am 18 Mär. 2020
Good question!
This is known as preallocation. My example shows how to store scalar outputs (single value) at location (i,j) in variable 'pointer'. Instead of building that matrix as the loops iterate, the matrix is created with NaN values prior to the loops. This makes the code faster and cleaner.
The numel() function just returns the number of elements of a vector.
Maaz Madha
Maaz Madha am 18 Mär. 2020
aah i understand now. Thank you so very much :) have a great day

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