How to solve pre-allocating array?

Hello everyone,

I have a coding like this, where I need to extract the row which has same index number in column 5 of my data. Attached is my data,

%Example matrix
A = load('Ntxp0014.txt');

%Find indices to elements in first column of A that satisfy the equality
ind11 = A(:,5) == 11 ;
ind12 = A(:,5) == 12 ;
ind13 = A(:,5) == 13 ;
ind14 = A(:,5) == 14 ;
ind15 = A(:,5) == 15 ;
ind16 = A(:,5) == 16 ;
ind17 = A(:,5) == 17 ;
ind18 = A(:,5) == 18 ;
ind19 = A(:,5) == 19 ;
ind20 = A(:,5) == 20 ;
ind21 = A(:,5) == 21 ;
ind22 = A(:,5) == 22 ;

%Use the logical indices to index into A to return required sub-matrices
A11=A(ind11,:);
A12=A(ind12,:);
A13=A(ind13,:);
A14=A(ind14,:);
A15=A(ind15,:);
A16=A(ind16,:);
A17=A(ind17,:);
A18=A(ind18,:);
A19=A(ind19,:);
A20=A(ind20,:);
A21=A(ind21,:);
A22=A(ind22,:);

but it is not practical when deals with many data. How I would to solve this pre-allocating array? However, I've try using for ... end function but still have error.

%Example matrix
A = load('Ntxp0014.asc');

%Find indices to elements in first column of A that satisfy the equality
for i = 11:364
ind(i) = A(:,5) == i ;

%Use the logical indices to index into A to return required sub-matrices
A(i) = A(ind(i),:);
end

Answer 1

[Stephen23]

Using numbered variables is a sign that you are doing something wrong.

Accessing numbered variables is one way that beginners force themselves into writing slow, complex, buggy code that is hard to debug. Read this to know why:

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

You can easily avoid that bad code approach by splitting that data into a cell array, either using a simple loop or some other grouping function, for example using arrayfun:

>> A = dlmread('Ntxp0014.txt'); % better than LOAD
>> V = 11:22;
>> F = @(n) A(A(:,5)==n,:);
>> C = arrayfun(F,V,'UniformOutput',false);

Lets check the contents of the first cell (fifth column==V(1)==11):

>> C{1}
ans =
  1.9921e+013  1.84720  95.016  -36.904  11  14
  1.9921e+013  1.79460  95.034  -36.599  11  14
  1.9921e+013  1.74200  95.053  -36.346  11  14
  1.9921e+013  1.68940  95.072  -36.130  11  14
  1.9921e+013  1.63680  95.091  -35.879  11  14
... lots of lines
  1.9921e+013  0.216460  95.597  -28.778  11  14
  1.9921e+013  0.163860  95.616  -28.799  11  14
  1.9921e+013  0.111250  95.635  -28.807  11  14
  1.9921e+013  0.058644  95.653  -28.891  11  14
  1.9921e+013  0.006037  95.672  -28.904  11  14

[hanif hamden]

clc;clear all;close all;

% Input matrix
A = dlmread('Ntxp0014.txt'); 

% Identify and extract each row according to cycle
V = 11:364;
F = @(n) A(A(:,5)==n,:);
C = arrayfun(F,V,'UniformOutput',false);

% Reference LatLon of Cycle 11 
ref_latC1 = C{1}(:,2);
ref_lonC1 = C{1}(:,3);
latlon1 = [ref_latC1 ref_lonC1];

% LatLon of Cycle 12 to 364 
for j=22   %2-354 <-I have to run one by one 
obs_latC2 = C{j}(:,2);
obs_lonC2 = C{j}(:,3);
latlon2 = [obs_latC2 obs_lonC2];

%Extract minimum distance point by point
for k= 1:length(latlon2)
[d1km(k)]=lldistkm(latlon1(1,:),latlon2(k,:));
Dist1 = d1km';
end

B2 = [C{j} Dist1];
ind2=B2(:,7)== min(B2(:,7));
B2 = B2(ind2,:);

%Append the data with ref point
%Append1 = [C{1}(1,:) 0 ; B2]
Append1 = [B2]

    fid = fopen('txp0014_pt1.txt','a');
    fprintf(fid,'%17.2f \t %.6f \t %.6f \t %.3f \t %.0f \t %.0f \t %.4f\n',Append1.');
    fclose(fid);
end

Thank you sir for the previous solution. I've further my coding and look like this. I would to extract each cycle and append it all. since my coding works manually as I have to keep change the number of j=2, run, j=3, then run again. Could you help me sir if possible? if I put j=2:354, there will be an error.

[Stephen23]

Question continued here:

https://www.mathworks.com/matlabcentral/answers/500946-how-to-overcome-array-problem