Saving data from nested for loop

Question

Maaz Madha 2020-3-17

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/511420-saving-data-from-nested-for-loop

评论： Maaz Madha 2020-3-18

采纳的回答： Adam Danz

在 MATLAB Online 中打开

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

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Answer 1

Adam Danz 2020-3-17

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/511420-saving-data-from-nested-for-loop#answer_420631

编辑：Adam Danz 2020-3-17

在 MATLAB Online 中打开

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

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Adam Danz 2020-3-18

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 2020-3-18