Vectorization of For-If combination

Requesting guidance on the proper vectorization methods for combinations of if statements within for loops. An example of the format referenced is included below. Code is utilizing a painful combination of if statements nested within for loops and appropriate vectorization is the only way in which this will run efficiency

X = zeros(1000,1)
C = 1:10
for A = 1:10
    for B = 1:10
          if C = 1
               X(A*B*C) = 1; 
          elseif C = 2
               X(A*B*C) = 2;
          else
               X(A*B*C) = 3;
          end
    end
end

[David Hill]

Your code does not make sense. C is an array that does not change, your code will only execute the else statement.

X = zeros(1000,1)
C = 1:10
for A = 1:10
    for B = 1:10
          if C == 1%need == for comparison, this is always false
               X(A*B*C) = 1; 
          elseif C == 2%comparison C= [1 2 3 4 5 6 7 8 9 10], this will be false
               X(A*B*C) = 2;
          else
               X(A*B*C) = 3;%this will execute but X(1:10)=3 for for loop
          end
    end
end

[MAB2020]

% Correction, use the following code.  Trying to generalize a much more complex segment. 
% The overall objective is to gain a proper understanding of vectorization algorithms for
% matrix operations consisting of if statements nested within for loops

for A = 1:10
    for B = 1:10
        for C = 1:10
            if C == 1
               X(A*B*C) = 1; 
            elseif C == 2
               X(A*B*C) = 2;
            else
               X(A*B*C) = 3;
            end
        end
     end
end

Answer 1

[David Hill]

Can't think of any better way to do your problem. Only 1 loop now.

x=zeros(1000,1);
for i=1:10
 x(i:i:10*i)=1;
 x(2*i:2*i:20*i)=2;
 x(3*i:3*i:30*i)=3;
 x(4*i:4*i:40*i)=3;
 x(5*i:5*i:50*i)=3;
 x(6*i:6*i:60*i)=3;
 x(7*i:7*i:70*i)=3;
 x(8*i:8*i:80*i)=3;
 x(9*i:9*i:90*i)=3;
 x(10*i:10*i:100*i)=3;
end

[MAB2020]

Excellent, thank you very much for your input! I will implement this into my code and evaluate for relative performance improvement