%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% handy data manipulation commands
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a=[1 2 3 4];

% fun with repmat:
A1=repmat(a,3,1);
A2=repmat(a,1,3);
A3=repmat(a,3,2);

%fun with reshape
B1=reshape(A1,12,1);
B2=reshape(A1,6,2);
B3=reshape(A1,4,3);

%Practice:
P1=reshape(A1',12,1);
P2=reshape(A1',2,6);
P3=repmat(reshape(A1',2,6),3,1);

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% indexing
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%Vector example
%%%%%%%%%%%%%%%%%
a=[11 2 6.7 0]
f1=find(a==0); %note the double-equal sign!
f2=find(a>2);
f3=find(isnan(a)); % find all elements that are "not numbers", 
%i.e. unidentified, such as might result from a divisin by 0.
f4=find(isinf(a)); %find all elements that are "infinity", 
% such as might result when you take the exponent of an already large number

b=a;% just to preserve the original a
b(f1)=[]; %delete all f1-indexed elements

b=a;
b=a(f2); %keep all f2-indexed elements

b=a;
b(f2)=15; %convert all f2-indexed elements to "15"

%direct indexing
b=a;
b(b==0)=[]
b=a(a~=0);
b=a(a>2);

%Matrix example
%%%%%%%%%%%%%%%%%%%%%%%%
A=[1 0 3 0; 0 11 0 0; 2 5 11 0]';
f1=find(A==0);

[r1,c1]=find(A==0); %returns row and column index for indexed elements
[r1 c1]; %makes explicit where exactly indexed elements are located

% deleting rows:
f=find(A(:,1)==0);
A(f,:)=[];

%practice:
M=3*[repmat((1:1:3)',2,1) [1 2 3 4 5 6]' ones(6,1)];
f=find(M(:,2)>9);
M1=M;
M1(f,2)=100;

M2=M;
M2(f,3)=100;

M3=M;
M3(f,:)=[];

M4=M;
M4(M4==3)=0